NESTING SEASON DIET OF GOLDEN EAGLES ON SANTA CRUZ …€¦ · a small number of Golden Eagles. In...

FINAL REPORT

NESTING SEASON DIET OF GOLDEN EAGLES ON

SANTA CRUZ AND SANTA ROSA ISLANDS,

SANTA BARBARA COUNTY,

CALIFORNIA

SANTA BARBARA MUSEUM OF NATURAL HISTORYTechnical Reports - Number 3

RECOMMENDED DOCUMENT CITATION:

Collins, P. W., and B. C. Latta. 2006. Nesting Season Diet of Golden Eagles on Santa Cruz and Santa Rosa Islands, Santa Barbara County, California. Santa Barbara Museum of Natural History Technical Reports - No. 3.

NESTING SEASON DIET OF GOLDEN EAGLES ON

SANTA CRUZ AND SANTA ROSA ISLANDS,

SANTA BARBARA COUNTY,

CALIFORNIA

Report submitted to

National Park ServiceChannel Islands National Park

1901 Spinnaker DriveVentura, California 93001

Report Prepared by

Paul W. CollinsSanta Barbara Museum of Natural History

2559 Puesta Del SolSanta Barbara, California 93105

and

Brian C. LattaSanta Cruz Predatory Bird Research Group

Long Marine LabUniversity of California, Santa Cruz

Santa Cruz, California 95060

24 January 2006

Nesting Season Diet of Golden Eagles on Santa Cruz and Santa Rosa Islands

Page i

TABLE OF CONTENTS

ABSTRACT .................................................................................................................................................... 1

INTRODUCTION............................................................................................................................................. 1

Food Habits of Golden Eagles ............................................................................................................. 3

Food Habits of Golden Eagles on the Channel Islands ....................................................................... 4

Objectives ........................................................................................................................................... 5

STUDY AREA ................................................................................................................................................. 5

Santa Cruz Island .............................................................................................................................. 5

Santa Rosa Island .............................................................................................................................. 6

Golden Eagle nests Excavated .......................................................................................................... 7

METHODS ....................................................................................................................................................... 9

Sample Collection .............................................................................................................................. 9

Sample Identification ....................................................................................................................... 10

Data Analysis .................................................................................................................................. 10

RESULTS ....................................................................................................................................................... 11

Percent Diet Composition ................................................................................................................ 11

Channel Islands Diet Composition .......................................................................................... 11

Island-specific Diet Composition ............................................................................................. 12

DISCUSSION ................................................................................................................................................. 19

Biases in Food Habits Determinations ............................................................................................ 19

Diet Composition ............................................................................................................................ 20

Invertebrates ............................................................................................................................ 21

Fish ........................................................................................................................................... 21

Amphibians and Reptiles ......................................................................................................... 21

Birds ......................................................................................................................................... 22

Land Mammals ......................................................................................................................... 23

Variation Between Islands ............................................................................................................... 25

Dietary Breadth ............................................................................................................................... 25


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Size Classes of Prey Selected .......................................................................................................... 26

Comparison of Golden Eagle and Bald Eagle Diets on the Channel Islands .................................. 27

CONCLUSIONS ............................................................................................................................................. 29

ACKNOWLEDGMENTS ............................................................................................................................... 30

LITERATURE CITED..................................................................................................................................... 30

APPENDICES ................................................................................................................................................ 37

Appendix 1. Proportion of major prey categories in breeding season diets of Golden Eagles inwestern North America .............................................................................................. 38

Appendix 2. Whole body weights used in calculating prey biomass ............................................ 39

List of Tables

Table 1. Composition of the diet of Golden Eagles nesting on Santa Cruz and Santa Rosa islands combined based on minimum number of individuals and on the biomass of individual prey species. ........................................................................................................................................... 13

Table 2. Diet of Golden Eagles nesting on Santa Cruz Island based on prey remains recovered from five nests. ....................................................................................................................................... 16

Table 3. Diet of Golden Eagles nesting on Santa Rosa Island based on prey remains recovered from four nests. ....................................................................................................................................... 17

Table 4. Proportion of prey categories in the nesting season diets of Golden Eagles and Bald Eagles on the northern Channel Islands, based on the proportion of individuals represented (%MNI) ..... 29

List of Figures

Figure 1. Golden Eagle nests on Santa Rosa and Santa Cruz islands excavated for this diet study .............. 8

Figure 2. Excavation of the Sierra Blanca nest in Laguna Canyon, Santa Cruz Island showing multiple layers of occupation ........................................................................................................................ 8

Figure 3. Proportion (%MNI) of prey classes in the diet of Golden Eagles nesting on Santa Cruz and Santa Rosa Islands ........................................................................................................................ 15

Figure 4. Proportion of prey in eight weight classes in the breeding season diets of Golden Eagles on Santa Cruz and Santa Rosa Islands ............................................................................................... 27

Figure 5. Comparison of relative frequency (%MNI) of prey classes between Bald and Golden Eagle diets on the Channel Islands ......................................................................................................... 28


ABSTRACT

Prey remains recovered during the excavation of 8 Golden Eagle (Aquila chrysaetos) nests on SantaCruz and Santa Rosa islands were used to reconstruct the nesting season diet of Golden Eagles on theChannel Islands. A total of 3,343 prey items representing 341 individuals of 34-36 species wereobtained from the excavated nests. The most frequently encountered prey categories based onminimum number of individuals were land mammals (43.4%), land birds (30.2%) and marine/aquaticbirds (21.1%) with the remaining prey categories accounting for only 5.3% of the eagle’s overall diet.The most common prey species that accounted for 89.8% of all prey items recovered were CommonRaven, Deer Mouse, Feral Pigs (piglets), Mule Deer fawns, cormorants (3 species), Mallard/Gadwall,Island Fox, gulls (3-4 species), Western Spotted Skunk, and Barn Owl. In terms of biomass,terrestrial mammals (54.9%) and birds (44.7%) dominated with land birds (22.8%) and marine/aquatic birds (21.9%) comprising almost equal proportions of the prey biomass. Feral Piglets,Common Ravens and Mule Deer fawns were the most important prey for eagles nesting on the islandsboth in frequency and biomass. There were island-specific differences both in the species and numberof prey recovered from nests and in the frequency and biomass that these prey represented for theeagle’s diet on each island. Feral Pigs comprised 59.1% of the eagle’s diet by biomass on Santa CruzIsland while Mule Deer fawns and Common Ravens made up 34.6% and 25.8%, respectively, of thebiomass of the eagle’s diet on Santa Rosa Island. Island Fox comprised 4.4% by frequency (%MNI)and 5.4% by biomass on Santa Cruz Island and 5.3% (MNI) and 8.5% (biomass) on Santa RosaIsland. Data obtained from this study clearly indicates that Golden Eagles nesting on Santa Cruz andSanta Rosa islands rely more heavily on terrestrial vertebrate prey than did Bald Eagles nestinghistorically on San Miguel Island. Also nonnative mammals such as Feral Pigs on Santa Cruz Islandand Mule Deer fawns on Santa Rosa Island were essential prey for Golden Eagles during the nestingseason on each island.

INTRODUCTION

Up until the early 1980s, Golden Eagles (Aquila chrysaetos) were an occasional visitor to the largerislands off the coast of southern California with most records coming from Santa Cruz Island (Jonesand Collins unpubl. ms). Historic records of Golden Eagles on the islands span the year with noobvious seasonality. By the late 1980s and early 1990s, sightings of Golden Eagles increasedsuggesting that they were becoming established as a year-round resident on Santa Cruz Island andwere probably nesting. The first confirmed breeding record of Golden Eagles on the Channel Islandscame in 1999 with the discovery of a nest at Coche Point on Santa Cruz Island (Latta et al. 2005,Roemer et al. 2001). Since then, eagle nests have been discovered at a series of sites on Santa CruzIsland and at two locations on Santa Rosa Island. Multiple nests within an eagle’s territory and nestswith a series of 2-5 distinct nest layers suggest that Golden Eagles have been nesting on both SantaCruz and Santa Rosa islands since at least the mid-1990s and possibly on Santa Cruz Island as earlyas the mid-to-late 1980s.

The recent colonization of the northern Channel Islands by Golden Eagles is possibly due in part to at leastthree factors. First was the extirpation of the resident population of Bald Eagles (Haliaeetusleucocephalus) on the Channel Island since the mid-20th century due in part to historical persecution byhumans (shooting, egg collecting, nest destruction, trapping, and poisoning), and reproductive failure


resulting from eggshell-thinning effects of DDE contamination (Kiff 1980). While it is possible that a densepopulation of territorial Bald Eagles may have discouraged transient Golden Eagles from establishingbreeding territories on the Channel Islands by aggressively defending their already established breedingterritories, this mechanism is currently unproven and has no supporting evidence. The Northern ChannelIslands appear to contain adequate breeding habitat for Bald Eagles and sufficient upland habitat to supporta small number of Golden Eagles. In addition, these two eagles tend to utilize different resources and for themost part forage and build their nests in different parts of the islands. Also a recent study of a reintroducedpopulation of White-tailed Eagles (Haliaeetus albicillis) and an established population of Golden Eagles onthe Isle of Mull in Scotland found that Golden Eagles were not displaced by the larger White-tailed Eagle(Whitfield et al. 2002). Second, the presence of suitable introduced terrestrial mammal prey (carcasses ofFeral Pigs [Sus scrofa] and sheep [Ovis aries] on Santa Cruz Island and Mule Deer [Odocoileushemionus] and Elk [Cervus elaphus] on Santa Rosa Island) represent prey sources not previouslyavailable to Golden Eagles visiting the islands prior to the initiation of ranching operations on the islands.The depauperate endemic terrestrial vertebrate fauna of the islands, prior to the arrival of Europeans, wouldhave provided little suitable prey for Golden Eagles which are known to feed primarily on a diet of terrestrialvertebrates. Transient Golden Eagles probably would not have been able to find sufficient terrestrial prey tosupport permanent residence on the islands prior to the ranching era. Third, the decline in available openscrub foraging habitat for eagles in coastal areas of southern California due to extensive residentialdevelopment may have resulted in floater eagles dispersing to the islands in search of uninhabited nestingterritories. It is unclear if only one or all three of the above mechanisms contributed to Golden Eaglesestablishing a breeding presence on the Channel Islands.

Several recent studies have implicated Golden Eagles in the catastrophic decline of Island Fox(Urocyon littoralis) populations on San Miguel, Santa Rosa and Santa Cruz islands (Coonan et al.2002, 2005a; Roemer et al. 2001, 2002). Between 1994 and 1999, Island Fox populations on thenorthern Channel Islands declined by nearly 95% (Coonan et al. 2005a, Roemer et al. 2001, Roemerand Donlan 2004). In an attempt to stem this population decline, the National Park Serviceimplemented a series of emergency recovery actions designed to save the Island Fox from extinctionon the northern Channel Islands. First, the Santa Cruz Predatory Bird Group (SCPBG) was hired totrap and relocate Golden Eagles from the islands to the mainland. Between fall 1999 and fall 2004, atotal of 37 Golden Eagles were live-captured and translocated to the mainland (Coonan et al. 2005b,Latta et al. 2005). Second, a series of on-island captive breeding facilities were established on eachisland to protect Island Foxes from eagle predation and to produce foxes that could, in the future, bereintroduced back on the islands. In January 2000, 14 of the 15 remaining wild Island Foxes on SanMiguel Island were captured and placed into a captive breeding facility on this island (Coonan 2003).All remaining Island Foxes on Santa Rosa Island (n=14) were captured in 2000 and placed into an on-island captive breeding facility (Coonan et al. 2005b). A captive breeding facility was established inFebruary 2002 on Santa Cruz Island and was initially stocked with 12 adult Island Foxes (Coonan etal. 2005b). Third, in 2004 the U.S. Fish and Wildlife Service listed 4 Island Fox subspecies asendangered, including the 3 found on the northern Channel Islands (USFWS 2004). Fourth, TheNature Conservancy and the National Park Service began a program to eradicate Feral Pigs fromSanta Cruz Island in early 2005. This program is expected to be successful at eradicating pigs fromSanta Cruz Island within 2-4 years. This action is meant to remove one of the eagle’s primary preyspecies which should help lead to the eagles disappearance from the islands. Finally, in 2002, the Institutefor Wildlife Studies began a 5-year feasibility study to reintroduce Bald Eagles to the northern ChannelIslands. Between 2002 and 2003, a total of twenty-two Bald Eagles were released from hacking towers onSanta Cruz Island (Dooley et al. 2005) and by spring 2005 there were 25 juvenile Bald Eagles on the


northern Channel Islands (Coonan et al. 2005b). As a result of these conservation measures, the NationalPark Service began releasing Island Foxes back onto Santa Cruz Island in 2002 and 2003, Santa RosaIsland in fall 2003, and San Miguel Island in fall 2004 (Coonan et al. 2005b). Despite the intensive eagletrapping efforts, Golden Eagles continue to maintain a presence on Santa Cruz and Santa Rosa islands.

Accurate food habit data for Golden Eagles on the Channel Islands is needed in order to understandwhat prey resources were critical for initially enabling Golden Eagles to become year-round residentson the Channel Islands. The lack of any quantitative food habits data for Golden Eagles nesting onthe Channel Islands has resulted in uncertainty regarding the composition of the eagle’s diet on theislands. Obtaining accurate quantitative data on the prey constituents of Golden Eagle diets on theislands is critical for understanding what species of terrestrial and marine vertebrates are beingimpacted by Golden Eagle predation and to what extent eagles are feeding on Island Foxes and othersensitive species of wildlife on the islands. Accurate food habit data will also help to identify the preyspecies that are likely to be more heavily impacted by eagle predation once Feral Pigs are eradicatedfrom Santa Cruz Island.

Food Habits of Golden Eagles

Golden Eagles are opportunistic predators which forage on a wide variety and sizes of prey (Kochertet al. 2002, Olendorff 1976). They also feed on carrion, primarily during the winter, and are knownto kill larger prey including a variety of ungulates (deer [Odocoileus spp.], bighorn sheep [Oviscanadensis], mountain goats [Oreamnos americanus], and pronghorn [Antilocapra americana]) anddomestic animals such as sheep, goats (Capra hircus), calves (Bos tarus), and pigs (Kochert et al.2002, Olendorff 1976). They generally take young ungulates but have also been known to kill adults(Deblinger and Alldredge 1996). Based on averaging the results from 18 food habit studies, the dietof nesting Golden Eagles in western North America is composed of 76.7% mammals, 20.5% birds,1.6% reptiles, and 0.5% fish (see Appendix 1). During the nesting season they feed primarily on smallto medium-sized mammals with leporids (hares and rabbits) and sciurids (ground squirrels, prairiedogs, and marmots) comprising between 49-94% of prey items recovered in food habit studiesthroughout western North America (Kochert et al. 2002). They are also known to feed lessintensively on birds, and occasionally on reptiles (snakes), and fish (see Appendix 1). Golden Eaglesare opportunistic specialists that prey upon whatever is most readily available in a particular regionand that is within a suitable prey size that they can handle. Thus, the relative importance of particularprey species in the eagle’s diet tends to vary by region. In the northern Great Plains, the principalprey are White-tailed (Lepus townsendii) and Black-tailed (L. californicus) jackrabbits, cottontails(Sylvilagus spp.), and White-tailed (Cynonyms leucurus) and Black-tailed (C. ludovicianus) PrairieDogs with Yellow-bellied Marmots (Marmota flaviventris), and several species of ground squirrels(Spermophilus spp.) important secondary prey (Lockhart et al. 1977, MacLaren et al. 1988,McGahan 1968, Reynolds 1969). In the Great Basin, Black-tailed Jackrabbits and cottontails are theprimary prey with Yellow-bellied Marmots and ground squirrels being the principal secondary prey(Arnell 1971, Beecham 1970, Bloom and Hawks 1982, Collopy 1983, Kochert 1972, USDI 1979).In the southwestern United States, Black-tailed Jackrabbits and cottontails are the principal prey with RockSquirrels (S. variegates) and prairie dogs being the principal secondary prey (Eakle and Grubb 1986,Lockhart and Phillips 1976, Mollhagen et al. 1972). In central California, California Ground Squirrels(Spermophilus beecheyi) and Black-tailed Jackrabbits are the principal prey (Carnie 1954).


While Golden Eagles are known to prey on a variety of birds, gallinaceous birds (pheasants, grouse, andpartridge) comprise the majority of birds eaten by eagles (Olendorff 1976). McIntyre and Adams (1999)reported ptarmigan (Logopus spp.) as an important secondary prey for Golden Eagles in central Alaskawhile Ring-necked Pheasants (Phasianus colchicus) and Chuckars (Alectoris chukar) are importantsecondary prey for eagles in the Great Basin (Arnell 1971, Beecham 1970, Hickman 1968, Kochert 1972,Marr and Knight 1983, USDI 1979). Reptiles make up only a small proportion (1.6%) of the overall dietof Golden Eagles in western North America (Appendix 1). The most frequently recorded reptile prey forGolden Eagles are Gopher Snakes (Pituophis spp.) and rattlesnakes (Crotalus spp.). A variety offreshwater fish (e. g. trout, perch, suckers, carp, squawfish, and whitefish) have been recorded in the preyremains of Golden Eagles in North America (Brown 1992, Carnie 1954, Marr and Knight 1983, USDI1979), however, fish are only rarely eaten by Golden Eagles as evidenced by the fact that they onlycomprise an average of 0.5% of the eagles overall diet (Appendix 1).

Golden Eagles take the majority of their prey on or near the ground. They obtain food by directcapture, scavenging of dead prey, or stealing food from other eagles, large birds such as corvids andraptors, or occasionally from other mammalian predators such as canids (Dekker 1985, Ladygin1994, Marzluff et al. 1994, Meinertzhagen 1959). Golden Eagles use a variety of strategies to searchfor prey such as soaring, still-hunting from perches, or hunting via low contour flights (Dekker 1985,Dunstan et al. 1978, Palmer 1988). The particular strategy that they use on any given day is oftendetermined by weather conditions, topography and a prey’s escape response (Dekker 1985, Watson1997). They soar more often on sunny or windy days and hunt from perches on overcast, calm, orrainy days. When foraging over open habitats they hunt by soaring while they use contour flightwhile hunting over broken topography or to surprise prey that are capable of escaping into burrows(Kochert et al. 2002). Golden Eagles also feed on carrion which they locate during high-soaringflight or by observing the activity of other scavengers such as corvids (Corvus spp.; Watson 1997).

Food Habits of Golden Eagles on the Channel Islands

Up until the current study, there was no quantitative data regarding the food habits of Golden Eaglesresiding on the Channel Islands. Rather the only food habitat date was anecdotal observations of preyobserved being fed upon by eagles, prey remains observed in Golden Eagle nests, signs left oncarcasses of prey thought to have been killed and fed upon by Golden Eagles (e. g. Island Foxcarcasses), and eagles seen feeding on carrion placed out for trapping purposes or left at feeding sitesfor Bald Eagles that are being reintroduced onto the Channel Islands. Roemer et al. (2001) andCoonan et al. (2002, 2005a) used evidence (e. g. talon holes with contusions in the integument andsurrounding tissue, degloved limbs, eviscerated carcasses, and damaged fragile bones such as ribs andvertebrae) found on Island Fox carcasses recovered from Santa Cruz and San Miguel Islands,respectively, to document that eagles were the primary cause of observed Island Fox mortalities.Additional observations of Golden Eagle food habits on the islands include: the observation of anadult Golden Eagle seen by C. Collins passing a snake to a begging juvenile eagle over LagunaCanyon, Santa Cruz Island, in November 1997 (Roemer et al. 2001); the capturing of a gull at theChina Pines Ridge sea cliff (Latta 2002); and Golden Eagles seen foraging on carrion left out to feed BaldEagles (Latta 2003, 2005). Prey remains observed in the Coche Point nest found in September 1999included Island Fox, Feral Piglets, gulls (Larus spp.), cormorants (Phalacrocorax spp.), and fledglingCommon Ravens (Corvus corax; Latta 2001, Roemer et al. 2001). Other prey remains observed in eaglenests on the islands have included various reptiles, Western Spotted Skunk (Spilogale gracilis), DeerMouse (Peromyscus maniculatus), Mule Deer fawns, Western Meadowlark (Sturnella neglecta),


California Quail (Callipepla californica), Barn Owl (Tyto alba), and Mallard (Anas platyrhynchos; Latta2005, Latta et al. 2005). These are some of the same prey remains that are analyzed in greater detail in thefollowing food habit study.

Objectives

The objectives of this study were to: (1) determine the composition of the diet of Golden Eaglesnesting on Santa Cruz and Santa Rosa islands; (2) determine how important introduced mammals,such as Feral Piglets and Mule Deer fawns, are in the diet of Golden Eagles nesting on the islands; (3)determine the extent to which sensitive species like Island Fox, Peregrine Falcon (Falco peregrinus)and Loggerhead Shrike (Lanius ludovicianus) are being preyed upon by Golden Eagles on theislands; and (4) compare and contrast the breadth of prey fed upon by Bald Eagles nesting historicallyat San Miguel Island with the diet of Golden Eagles nesting today on Santa Cruz and Santa Rosaislands. Quantitative food habits data from Golden Eagle nests on the Channel Islands should help toclarify which resources were critical in helping to establish and maintain a resident population ofGolden Eagles on the northern Channel Islands. This information will also help to identify whichspecies are likely to be adversely affected if Golden Eagles are allowed to maintain a residentbreeding population on the Channel Islands.

STUDY AREA

Santa Cruz and Santa Rosa islands, the two largest of the northern Channel Islands off the coast ofSanta Barbara, are located approximately 30 km (19 mi) south and 44 km (27 mi) southwest,respectively, from the adjacent mainland. Santa Cruz Island lies about 9 km (5.5 mi) east of SantaRosa Island and 7 km (4.5 mi) west of Anacapa Island. These two islands have a Mediterraneanclimate, with mild wet winters and warm, dry summers. Most rain falls between November and Apriland averages about 50 cm (19.7 in) on Santa Cruz Island (Junak et al. 1995) and 30 cm (11.8 in) onSanta Rosa Island (Clark et al. 1990). Wind and fog are dominant climatic components on bothislands. Strong northwest winds of 10 to 40 knots per hour drive moisture-laden marine air andsummer fogs across these islands. Most areas of these two islands are influenced by a coastal marinelayer which helps to keep the islands a few degrees cooler than adjacent mainland areas near thecoast. The summer fog also brings some measurable precipitation to the islands and the strongnorthwest winds influence the distribution and phenology of plant communities on both islands.

Santa Cruz Island

Santa Cruz Island is the largest and most topographically diverse of the Channel Islands. It encompasses anarea of 249 km2 (96 mi2) and is about 38 km (23.5 mi) long, 12 km (7.5 mi) wide at its widest point, andreaches a maximum elevation of 753 meters (2,470 ft). Santa Cruz Island is dominated by two east-westtrending ridges separated by an intervening Central Valley which is 20 km (12.5 mi) long. These ridgesdivide the island into four ecological zones: (1) coastal slopes, flats and canyons along the north side of theisland; (2) south-facing slopes and canyons facing the Central Valley; (3) north-facing interior slopes andcanyons on the south side of the Central Valley; and (4) south-facing coastal slopes, flats, and canyons onthe south side of the island (Junak et al. 1995). The north shore of the island is very rugged terrain withsteep dissected ridges and slopes that often exceed 30 degrees while the south side reaches a maximum


elevation of 460 m (1,500 ft) and has gentler slopes and wider drainages. Much of the coastline of SantaCruz Island consists of steep coastal cliffs with pocketed, isolated coves, and small, secluded sandybeaches.

Santa Cruz Island’s size and topographic and geologic complexity combine to result in a widediversity of woodland, scrub and grassland plant communities. A total of ten plant communities havebeen described for the island (Philbrick and Haller 1977). Grasslands, island chaparral, oakwoodland, and coastal-sage and coyote-brush scrub comprise approximately 89% of the vegetativecover of this island (Minnich 1980). Remaining plant communities include beach dunes, coastal-bluffscrub, riparian woodland, island woodland, coastal marsh and estuary, closed cone pine forest, andgroves of several introduced trees. Island chaparral, oak woodland, and Bishop-pine forest are mostcommon on mesic north-facing slopes while coastal-sage scrub and coyote-brush scrub are mostcommon on dry, rocky slopes across the island especially on the south side, on south-facing slopes inthe Central Valley, and at the east end (Junak et al. 1995). Island chaparral is most common on thenorth-facing slopes in the Central Valley.

The vertebrate fauna of Santa Cruz Island is depauperate with three species of amphibians, fivereptiles, 44 resident species of birds, four native land mammals (Deer Mouse, Western Harvest Mouse[Reithrodontomys megalotis], Island Fox, Western Spotted Skunk), and one introduced mammal(Feral Pig). Sheep were present on this island until they were removed from the western two-thirdsof the island in the late 1980s by The Nature Conservancy (Schuyler 1993) and from the eastern thirdof the island in the late 1990s by the National Park Service (T. Coonan pers. comm.). A program toeradicate Feral Pigs from Santa Cruz Island was begun in 2005. Cattle (Bos tarus) and horses (Equuscaballus) were present on this island until they were removed in 1988-1989 (Junak et al. 1995).Inaccessible sea cliffs found along the north coast and offshore islets and rocks support moderatenumbers of breeding and roosting cormorants along with several other species of sea birds (WesternGull [Larus occidentalis] and Pigeon Guillemont [Cepphus columba]; Carter et al. 1992). Also awide variety of marine birds are known to visit nearshore waters around Santa Cruz Island duringmigration and in the winter (Jones and Collins unpubl. ms.). Introduced populations of CaliforniaQuail and Wild Turkey (Meleagris gallopavo) occur on Santa Cruz Island. Red-tailed Hawks (Buteojamaisensis) and Common Ravens are the largest native resident land birds on the island whileWestern Meadowlarks and Horned Larks (Eremophila alpestris) are widespread and abundant ingrasslands on the island.

Santa Rosa Island

Santa Rosa Island is approximately 217 km2 (84 mi2) in size; 23 km (14.3 mi) long and 16 km (9.9 mi)wide; and reaches a maximum elevation of 484 m (1,589 ft). This island is less topographicallydiverse than Santa Cruz Island. The topography of Santa Rosa Island is dominated by an east-westtrending central highlands region centered around Soledad Peak. Lateral ridges and intervening canyonsextend in all directions from this highlands region. The north side of the island has an extensive marineterrace that rises gently from steep coastal bluffs toward the central highlands region and is cut by a series ofwell developed canyons that drain to the north and northwest. The south side of the island has shorter,steeper, and narrower canyons that extend from the central highlands region south and southwest to theocean. The south-facing slopes of the island lack the broad marine terrace found on the north side of theisland (Clark et al. 1990). The coastline of Santa Rosa Island is dominated by rocky intertidal areas, withwell developed sandy beaches and dunes on the southwestern and northeastern shores (Clark et al. 1990).


A small tidal marsh is located on the eastern end of the island. The topographic diversity of Santa RosaIsland results in a wide diversity of microclimates that in turn support a variety of plant communities.

A total of eighteen distinct plant communities occur on Santa Rosa Island (Clark et al. 1990).Grasslands account for over 65% of the island’s surface, with mixed-oak woodlands comprising0.35% of the island, and shrublands, made up of chaparral and six other scrub communities, coveringabout 25% of the island with the remainder of the island (6.9%) being bare ground. Stands of oaks(Quercus spp.), two species of pines (Pinus spp.), ironwood (Lynothamnus floribundus), and toyon(Heteromeles arbutifolia) trees occur in the upper reaches of sheltered canyons while grasslands andcoastal scrub habitats dominate the marine terraces and lower canyon slopes. There are two standseach of Torrey Pines (Pinus torreyanna) and closed cone pines (Pinus muricata), nine groves ofironwood, and seventeen isolated groves of Island Oaks (Quercus tomentella) on this island. Thereare also several planted windrows of eucalyptus (Eucalyptus globules) and scattered MontereyCypress (Cupressus macrocarpa) trees in the vicinity of the ranch headquarters at Beechers Bay, andwillows (Salix lasiolepis) and cottonwoods (Populus trichocarpa) located in small isolated grovesalong several of the island’s larger drainages. Coastal sage scrub and chaparral are the most commonshrub communities and are widely distributed on the island. Coastal bluff scrub, coastal dune scrub,caliche scrub, and chaparral scrub are more limited in their distribution on the island.

The vertebrate fauna of Santa Rosa Island is depauperate with two species of amphibians, threereptiles, 31 species of breeding land birds, three native land mammals (Deer Mouse, Island Fox,Western Spotted Skunk), and two introduced herbivores (Mule Deer and Elk). Sheep were presenton this island until they were removed in the early 1960s. Feral Pigs were present until they wereeradicated by the Park Service in 1993 (Lombardo and Faulkner 2002) and cattle and horses werepresent until they were removed from the island in the 1990s. The inaccessible sea cliffs found alongthe north coast and offshore rocks support large numbers of breeding and roosting cormorants andseveral other sea birds (Western Gull and Pigeon Guillemont; Carter et al. 1992). A diverse array ofother marine birds (loons, grebes, shearwaters, scoters, mergansers, gulls, terns and alcids) occurseasonally in the shallow near-shore waters around Santa Rosa Island (Jones and Collins unpubl. ms.).Also a variety of waterfowl are known to occur on this island due to the occurrence of suitableponded wetlands (man-made impoundments and estuaries).

Golden Eagle Nests Excavated

Prey remains were collected from eight Golden Eagle nests in five breeding territories on Santa Cruz andSanta Rosa islands in 2003 (Figure 1). On Santa Rosa Island, the Trap Canyon territory contained oneprimary nest (Big nest) and two alternate nests (2003 nest and No. 3 nest) while the Trancion Canyonterritory contained a single nest (Trancion Canyon nest). All four of the known Golden Eagle nests on SantaRosa Island were excavated for this food habit study. The Big nest in Trap Canyon contained four nestlayers, the Trancion Canyon nest contained three nest layers, and the two alternate nests in Trap Canyoncontained two nest layers each (Latta 2005). Based on the presence of multiple nests and nests with anumber of nest layers, the Trap Canyon territory on Santa Rosa Island is thought to have been active fromat least 1996 until 2004 and produced at least seven young (Latta 2005). The occurrence of Island Foxbones in two nests associated with the Trap Canyon pair (Big nest and Nest No. 3), suggests that thesenests were both active prior to 1999 when the last Island Foxes were removed from the wild on Santa RosaIsland and placed in an on-island captive breeding facility. Based on the presence of three nest layers and


Figure 1. Golden Eagle nests on Santa Rosa and Santa Cruz islands excavated for this diet study.

Figure 2. Excavation of the Sierra Blanca nest in Laguna Canyon, Santa Cruz Island showing multiple layers of occupation.


on direct observations, the Trancion Canyon territory was active from at least 2001 until 2003 when it wasunsuccessful at fledging any young (Latta 2005).

As of fall 2004 there were five known Golden Eagle territories on Santa Cruz Island (i. e. LagunaCanyon/Sierra Blanca, Cascada/Red Peaks, Coche Point, Lady’s Harbor, and Christy Water Tank)with at least 29 large eagle-sized nest structures known within these five territories (Latta 2005). TheCoche Point territory, which has been active since at least 1995, has produced at least 10 young andwas last active in 2002 (Latta 2005). The Laguna Canyon/Sierra Blanca territory has been activesince at least 1997, has produced at least four young and was last active in 2004 (Latta 2005). TheCascada territory was active in 2003 and produced one young while the Lady’s Harbor territory wasdiscovered active in 2004 and produced 2 young (Latta 2005). It is unknown if the Christy WaterTank territory was successful at producing any young. For this food habits study, two nests wereexcavated in the Coche Point territory (Coche Pt. Traditional and Coche Pt. Alternate [Slide] nests)while single nests were excavated in the Laguna/Sierra Blanca and Cascada territories. The Lagunanest contained 5 nest layers (Figure 2), the Coche Point Traditional nest and the Coche PointAlternate nest contained 4 nest layers each, and the Cascada nest contained one nest layer (Latta2005). Based on the presence of multiple nest layers in the Laguna Canyon and two of the CochePoint nests, Golden Eagles have probably been nesting on Santa Cruz Island since at least the late1980s or early 1990s.

METHODS

Sample Collection

Prey remains were collected in 2002-2003 by carefully and completely dismantling eight GoldenEagle nests on Santa Cruz and Santa Rosa islands (Figure 1). All nests were situated in rock hollowsor ledges on cliffs. Each nest site was treated and excavated as if it were an archaeological site.First, prey remains visible on the nest surface were collected by hand and placed in bags labeled“Layer A”. Then loose surface sticks were set aside to uncover the first nest cup layer. Each nest cuplayer was then carefully excavated using trowels, shop brushes, and a 1/16-in. (1.59-mm) screen sieveto help expose and retrieve prey remains which were then placed in separate bags by layer (e. g. LayerB, Layer C,). After each nest was excavated, it was reconstructed as per B. Latta’s permit conditionsusing sticks and sifted nest cup material that were set aside during the nest excavation (Latta et al.2005).

During identification and preliminary data analysis, all faunal material from each nest site was kept separatedby subsection of the nest site from which it was recovered. The data related to all faunal material recoveredfrom each nest layer was later combined for each nest when it was determined that there were no noticeabledifferences in the composition of faunal material from the various subsections of each nest. Samples werealso later combined by island and then the islands as a whole to determine prey composition of the diet ofGolden Eagles on each island and then on the islands as a whole.


Sample Identification

Faunal material was initially sorted into six broad taxonomic groups (fish, amphibians, reptiles, birds,mammals, and invertebrates). The material was identified in the lab to the highest taxonomic level possible(class, order, family, genus, or species) by comparing diagnostic elements (bones, teeth, otoliths,exoskeleton fragments, or shell fragments) with identified specimens housed in research collections at theSanta Barbara Museum of Natural History. Taxonomic identifications of vertebrate prey remains wasaccomplished by comparing diagnostic bones to identified specimens in the comparative osteologycollection at the Santa Barbara Museum of Natural History. For reptiles, scales, lower mandibles, skullfragments, and vertebrae were used for species diagnosis. For mammals, a total of 16 elements (skullbones, mandible, teeth, scapula, humerus, radius, ulna, carpal/tarsal bones, metacarpal, pelvis, femur, tibia,fibula, calcaneus, astragalus, and metatarsal bone) were sufficiently diagnostic to permit taxonomicidentification to species. Nearly all of the mammal and reptile bone material could be assigned to a speciesin part to the fact that the island’s mammal and reptile fauna is depauperate. Fragmentary specimens thatdid not possess diagnostic processes were identified to unidentified reptile or mammal and were excludedfrom further identification. For birds, a total of 17 elements (crania, maxilla, lower mandible, pelvic bones,sternum, sacral vertebrae, humerus, ulna, radius, carpometacarpus, D4P=phalange of wing, coracoid,scapula, clavicle, femur, tibiotarsus and tarsometatarsus) were sufficiently diagnostic to permit identificationto species. Vertebrae, ribs, phalanges, and miscellaneous bird bone fragments were not identified tospecies. Rather these bones were listed as unidentified bird and excluded from further identification. All ofthe diagnostic bird bone elements recovered from the nests were identified to species except for bones fromBrandt’s and Double-crested Cormorants, and Western and Glaucous-winged Gulls. For each of thesespecies groups, their bones were too difficult to tell apart so they were lumped into these two speciesassemblages. A large amount of feather material was also collected during the nest excavations, howeverthese prey remains have not yet been analyzed and are not included in this report. Fish remains wereidentified to the most specific taxon possible. The only fish element identified to species in this sample wasan otolith. All other fish bones were classified to unidentified Teleost. Invertebrates were identified to thehighest taxonomic level possible (species, genus, family, or order) using identified invertebrates in theentomology and invertebrate research collections at the Santa Barbara Museum of Natural History.

Data Analysis

All faunal material was initially identified and quantified by the specific location within the nest whereit was recovered. Although smaller bone elements were dominant in screened samples and largerelements in the hand picked samples, comparison of the faunal composition of these samples, both interms of number of individuals of each species and in terms of species composition revealed no majordifferences between the samples within a given nest. As a result, samples from a nest were combined todetermine the overall diet composition for the eagles that nested at a given nest.

Diet composition was calculated in three ways. First, the number of individual specimens (NISP) wasdetermined by counting the total number of elements identified to a particular species, genus, family,order, or class. Second, the minimum number of individuals (MNI) for fish, amphibians, reptiles,birds, and mammals was determined to be equal to the greatest number of identical bones per taxon.If seven right and six left tibia were recorded, then seven MNI were recorded for the species. Forinvertebrates, MNI values were calculated using non-repetitive elements. For barnacles we used thenumber of whole or nearly whole shells, and for other invertebrates the number of non-repetitiveelements. Third, a weight value was assigned for each species identified in the prey sample by using


published weight data (see Appendix 2). To assess the relative importance of a given prey species to theoverall diet of eagles on the islands, a total biomass estimate was then calculated by multiplying a species’average body weight by the MNI recorded for that species. Since the sex of most prey remains could notbe determined, we used the mean of both sexes for the weight estimates of individual species when weightsfor each sex were presented separately in the literature. For larger prey species, the weights of immatureanimals (e.g. newborns) were taken from the literature if available. For Mule Deer fawns, we used theweight at birth recorded for the subspecies of Mule Deer found in northern Arizona (Anderson and Wallmo1984) which is the subspecies used to originally stock Santa Rosa Island in the 1930s. For elk we used theweight from birth to one week of age recorded for elk calves (Johnson 1951). For sheep we used theweight of lambs from birth to 6 months of age recorded for sheep on Santa Cruz Island (VanVuren andCoblentz 1984). For Feral Piglets, we used an estimate of 2.5 kg for the weight (e. g. birth to 1 month ofage) which represents the average maximum weight of a prey item that an eagle could be expected to carryback to its nest. Although eagles undoubtedly fed on piglets, lambs, deer fawns, and elk calves that wereslightly older (i. e. to heavy to be carried back whole to their nests), it was impossible to determine anaverage weight for these slightly older animals. Thus, the biomass estimates for these larger prey speciesmay be slightly lower than the actual weight of these prey deposited in eagle nests on the islands.

Percent diet composition was examined relative to MNI and biomass (average body weight). Forminimum number of individuals, percent diet composition was calculated as the minimum number ofall prey items in a given taxonomic group, divided by the total minimum number of all prey itemstaken, multiplied by 100. A similar method was used to calculate percent biomass using average bodyweights. Species that were probably incidental nest midden constituents (e. g. prey stomach and/orcrop contents, or remains of species that died naturally within a nest structure) and not golden eagleprey were excluded from the biomass calculations. The two calculation methods (MNI vs biomass)resulted in differences in percent diet composition. MNI tends to overestimate the importance ofsmall prey such as invertebrates, lizards, and mice.

Food niche breadth (diet diversity) was calculated for the Channel Island sample and separately for eachisland prey sample using Levins’ (1968) formula: B = 1/sum pi

2 where pi was the relative occurrence of preyi in the diet. Values of niche breadth range from 1 to n with 1 representing the narrowest value for foodniche breadth. Values obtained for diet diversity of Golden Eagles nesting on the Channel Islands werecompared with those for other Golden Eagle food habit studies in Scotland and across Europe and Asia(see Tables 7 & 9 in Watson 1997), and in North America (see Appendix 1).

RESULTS

Percent Diet Composition

Channel Islands Diet Composition. A total of 3,343 faunal elements (bones, fish otoliths, shell fragments,invertebrate exoskeletons) representing 34-36 prey species were collected from eight Golden Eagle nestson Santa Cruz (4 nests) and Santa Rosa (4 nests) islands (Table 1). Of the 341 individuals identified, 175(51.3% MNI) were birds, 148 (43.4%) were terrestrial mammals, 8 (2.3%) were reptiles, 6 (1.8%) wereinvertebrates, 3 (0.9%) were fish, and 1 (0.3%) was an amphibian (Table 1). Based on minimum number ofindividuals (MNI), the three most important prey classes in this sample were land mammals (43.4% of totalMNI), land birds (30.2%), and marine birds (21.1%) with all of the other prey classes combined accounting


for only 5.1% of the eagles overall nesting season diet. The most common mammals present included DeerMouse (n=41 individuals), Feral Pig (n=36; Santa Cruz Island only), Mule Deer fawns (n=32; Santa RosaIsland only), Western Spotted Skunk (n=18), and Island Fox (n=17, Table 1). Common Raven (n=80),cormorants (n=31), waterfowl (n=24; Mallard and Gadwall), and gulls (n=17) were the most commonavian prey (Table 1). The fish and barnacle remains identified in the faunal material recovered from theseeagle nests are probably crop contents from larger marine bird prey brought to the nest by eagles. Theslender salamander and terrestrial invertebrate remains were probably incidental nest midden constituents (e.g. prey stomach and/or crop contents from birds of prey, or remains of species that died within a neststructure) and not Golden Eagle prey.

The relative proportion of a prey category or species in the nesting season diet of eagles on theChannel Islands changed when a prey’s biomass (body weight) rather than MNI was used as themeasure. Larger (heavier) species increased in the proportion they represent in the eagles diet whilesmaller (lighter weight) species declined (Table 1). In terms of total biomass of prey recovered fromeagle nests on the islands, terrestrial mammals and birds accounted for 54.9% and 44.7% of theeagle’s overall nesting season diet, respectively, and reptiles comprised only 0.4% of the diet (Table1). By biomass, the most important species of mammals were Mule Deer fawns (23.2% of totalbiomass), Feral Piglets (19.4%), Island Fox (7.5%) and Western Spotted Skunk (2.2%, Table 1). Themost important birds were Common Raven (20.7%), Double-crested/Brandt’s Cormorant (8.9%),Mallard (5.2%), Western/Glaucous-winged Gull (2.5%), and Barn Owl (1.6%; Table 1). While DeerMice comprised 12.0% of the eagle’s diet based on MNI, they only comprised 0.2% of the eagle’sdiet based on their biomass. Many of the other prey recorded in this sample also showed a decline inthe percentage of the eagles overall diet that they comprised when biomass was used as the measure(e. g. Western Spotted Skunk, all of the land birds, all of the marine birds except for Brandt’s/Double-crested Cormorants, and reptiles; Table 1).

Island-Specific Diet Composition: There were island-specific differences in dietary breadth andcomposition of faunal remains found in eagle nests on each island which probably reflects the relativeavailability of prey species on each island as well as the specific prey preferences of individual eaglepairs (Figure 3 and Tables 2 and 3). Golden Eagles on Santa Rosa Island had the most diverse diet(7.59) with 24-25 species represented while eagles on Santa Cruz Island had a less diverse diet (4.70) ofonly 14-16 species (Appendix 1 and Table 2 and 3). Species that were only observed in prey remainsrecovered from nests on Santa Cruz Island included gulls (Herring, California and Western/Glaucous-winged Gulls), California Quail, Feral Pig (piglets only), European Mouflon Sheep (lambs and at least 1adult), and barnacles (Table 2). Species only found in eagle nest on Santa Rosa Island included PelagicCormorant (Phalacrocorax pelagicus), waterfowl (Mallard, Gadwall [Anas strepera]), raptors (Red-tailed Hawk, Peregrine Falcon, Barn Owl), Loggerhead Shrike, 3 small passerines, Southern AlligatorLizard (Elgaria multicarinata), Santa Cruz Gopher Snake (Pituophis catenifer), Mule Deer fawns, Elkcalf, scorpion, 3 species of insects, 1 fish, and Channel Island Slender Salamander (Batrachosepspacificus; Table 3). On Santa Cruz Island, Feral Piglets comprised 39.6% MNI of the food items (Table2) while Mule Deer fawns and Elk calves comprised 13.2% MNI of food items in eagle nests on SantaRosa Island (Table 3). Western Spotted Skunks comprised only 1.1% MNI of the food items on SantaCruz Island and 6.8% MNI on Santa Rosa Island. Island Foxes comprised similar proportions of theeagle’s diet on both Santa Cruz and Santa Rosa islands (i. e. 4.4% and 5.3% MNI, respectively; Tables 2and 3). On Santa Cruz Island, marine birds accounted for 27.5% MNI of the eagle’s diet while land birdsonly comprised 16.5% MNI (Table 2). On Santa Rosa Island, land bird comprised 35.2% MNI of theeagle’s diet while marine birds only made up 18.8% of their diet (Table 3). Gulls were a dominant marine


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posi

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of th

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Figure 3. Proportion (%MNI) of prey classes in the diet of Golden Eagles nesting on Santa Cruz and Santa Rosa Islands.

0

5

10

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20

25

30

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40

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Inverte

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Amphibi

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Fish

Marine B

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Waterfowl

Land

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Pigs

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SCISRI

bird prey category (18.7% MNI) in nests on Santa Cruz Island while they were absent in nests on SantaRosa Island having been replaced by waterfowl (9.6% MNI) (Tables 2 and 3). Eagles on Santa CruzIsland only preyed on two species of land birds (California Quail and Common Raven) while on Santa RosaIsland they preyed upon eight species with Common Raven (26.8% MNI) and Barn Owl (5.6% MNI)being the most abundant land birds. Reptiles were absent from the eagle’s diet on Santa Cruz Islandbut comprised 3.2% of the eagle’s diet on Santa Rosa Island.

As with MNI, there were also island-specific differences in the relative proportion that individual prey andprey categories comprised of the eagle’s diet on each island when biomass was used as the measure.Terrestrial mammals accounted for 69.5% and 47.8% (biomass) of the eagle’s diet on Santa Cruz andSanta Rosa islands, respectively, while birds made up 30.5% and 51.6% of their diet (Tables 2 and 3).Feral Pigs were the most important prey for eagles on Santa Cruz Island comprising 59.1% of the biomassconsumed by eagles during the nesting season (Table 2). Island Fox (5.4%) and Sheep (4.5%) were thenext most important land mammals (Table 2). Brandt’s/Double-crested Cormorants (10.3%), CommonRavens (10.2%), and Western/Glaucous-winged Gulls (7.5%) were the most important birds eaten byeagles on Santa Cruz Island (Table 2). Mule Deer fawns (34.6%) and Common Ravens (25.8%) were themost important prey for eagles on Santa Rosa Island (Table 3). Island Fox (8.5%) and Western Spotted


Tabl

e 2.

D

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f Gol

den

Eagl

es n

estin

g on

San

ta C

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Isla

nd b

ased

on

prey

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1314

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Skunks (3.1%) were the next most important land mammals while cormorants (14.2%), Mallards (7.7%),and Barn Owls (2.4%) were the next most important bird prey for eagles on Santa Rosa Island (Table 3).Reptiles were absent in the diet of eagles on Santa Cruz Island while they comprised 0.6% (biomass) of theeagle’s diet on Santa Rosa Island.

DISCUSSION

Biases in Food Habits Studies

A variety of approaches have been used to document Golden Eagle food habits including analysis ofeagle stomach contents (Woodgerd 1952), direct observation of food items delivered to nests(Collopy 1983), and investigation of prey remains and egested pellets collected from nests or perches(Arnell 1971, Boag 1977, Bloom and Hawks 1982, Carnie 1954, Eakle and Grubb 1986, Lockie 1964,MacLaren et al. 1988, Marr and Knight 1983, Olendorff 1976). Most recent studies of Golden Eagle foodhabits have relied on the identification of items in regurgitated food pellets and in prey remains collected atnests (see references cited in Kochert et al. 2002, Olendorff 1976 and Watson 1997). Each of thesereported techniques has biases. Conclusions based on prey remains/pellet analyses may be skewed towarditems with hard bony structures (e.g. birds, medium-mammals, and large, bony fish), and under representsoft bodied items such as small mammals, small fish, soft-bodied fish and large-bodied prey/carrion that aretoo big to carry back to the nest (Collopy 1983, Mersmann et al. 1992). Collopy (1983) found that therewas no difference in the frequency of prey species observed in prey remains and pellets combinedcompared with direct observation of prey delivered to nests. However, Collopy (1983) did find that thecollection of prey remains from nests “substantially underestimated the biomass of prey delivered” by eaglesto nests by 51-59%. Because no one technique accurately records all prey types eaten by Golden Eaglesthroughout the year, accurate assessment of an eagle’s diet requires one to make use of all availabletechniques and to interpret the data obtained in light of known sampling biases (Collopy 1983, Mersmann etal. 1992).

The faunal material recovered during excavation of eight Golden Eagle nests on Santa Cruz and Santa Rosaislands does provide a qualitative picture of the diversity and relative importance of various preycomponents of the eagles nesting season diet on the Channel Islands. As stated above, analysis of preyremains recovered from nests is biased and is known to omit or to underestimate the importance of someprey categories of food (e. g. small mammals, small fish, soft-bodied fish, and large prey/carrion not carriedback to the nest). Therefore, results obtained by analyzing the prey remains excavated from these eightnests should be viewed with the above biases in mind. The principal prey category that may beunderrepresented in this sample is probably flesh from carcasses of larger mammals such as Mule Deer,Sheep, Elk calves, and Feral Pigs. This type of material does not leave behind any remains (e. g. bone,teeth or hair) that can be used to document its importance in the eagle’s diet. Thus, larger animals will tendto be underestimated in terms of their relative importance to the eagles overall nesting season diet.

The methods used for excavating the eagle nests on the islands were sufficient to recover very small and insome cases fragile faunal material (e.g. small fish vertebrae and otoliths, variety of small invertebrates, asalamander, a mouse, reptile scales, lizard and snake bones, and small landbirds). When minimum numberof individuals is used as the measure, small, fragile remains document the diversity of prey consumed but donot provide an accurate estimate of the importance of these smaller prey species to the eagle’s overall diet.In contrast, the large quantity of bird and terrestrial mammal bone recovered from these nests does provide


a good qualitative and quantitative picture of the prey species consumed by Golden Eagles during thenesting season on the Channel Islands. This material also provides a reasonably accurate picture of therelative importance of these prey categories in the eagle’s overall diet. Although biases exist with preyremains data, the excavation of nests provides the best and only reliable method for reconstructing the dietof eagles nesting on the islands.

At least one Golden Eagle nest excavated for this study may have occasionally been used for nestingby another species (e. g. Common Raven, Red-tailed Hawk, or Barn Owl). If this is the case, thenthese species could be responsible for depositing some of the smaller prey items (e. g. mice, lizards,and small land birds) found at several of the nests. It is also possible that some of the small preyremains (small fish and marine invertebrates) represent crop or stomach contents of larger marine bird preybrought to the nest by eagles, or are remains of animals that died naturally in a nest structure (e. g.salamander and terrestrial invertebrates). Additionally, recent (2005) nest excavations revealed theexistence of several live species (e.g. deer mice, alligator lizards, scorpions, spiders, beetles)inhabiting the nest structures. While it is not possible to determine with certainty which prey itemswere brought to the nest site by species other than Golden Eagles, which prey are incidental remainsof animals that died in a nest, or which prey are crop or stomach contents of larger marine bird prey,it is important to keep in mind that some of the faunal remains recovered from these nest sites wereprobably not brought to a nest site by eagles as prey.

Diet Composition

The composition of prey constituents in the diet of Golden Eagles nesting on Santa Cruz and SantaRosa islands varied somewhat from the diets reported from other Golden Eagle food habit studies inwestern North America (see Appendix 1). On the islands, Golden Eagles relied more heavily on birds(51.3% MNI) than the average (20.5% MNI) for 18 other eagle food habits studies in western NorthAmerica where birds comprised between 3.9% and 62.7% of the eagle’s diet (Appendix 1). Threefood habit studies report birds comprising a similar high percentage of the eagle’s overall diet (Hatler1974, Knight and Erickson 1978, Marr and Knight 1983). In the Marr and Knight (1983) study, a total of21 species of birds were identified in prey remains from eagle nests with Chukar (Alectoris chukar), BlueGrouse (Dendragapus obscurus), and Black-billed Magpie (Pica pica) being the most common avianprey recorded. Land mammals comprised a smaller percentage of the eagle’s diet on the islands (43.4%MNI) than the 76.7% average recorded from eighteen other eagle food habit studies (see Appendix 1).Mammals ranged from comprising 50.8% (Marr and Knight 1983) to 96.0% (Arnell 1971) of the GoldenEagle’s diet in eastern Washington and central Utah, respectively (Appendix 1). Reptiles (2.3% MNI) andfish (0.9%) were close to the average of 1.6% and 0.5%, respectively, for these eighteen eagle food habitstudies (Appendix 1).

It is clear that in the absence of an abundant diurnally active mammal and avian prey base, like thatfound elsewhere in western North America (e.g. Sciurids, Leporids, and Gallinaceous birds), thatGolden Eagles on the islands have had to switch to other novel prey such as Feral Piglets, Mule deerfawns, Common Raven, Island Fox, Spotted Skunk, gulls, waterfowl, cormorants, and Barn Owls tohelp supplement their diet. All of these species are either scarce or lacking in Golden Eagle diets onthe mainland. Golden Eagles on the islands have adapted their foraging strategies and food habits toaccess and harvest the diurnally active terrestrial and aquatic prey that are found on the islands. Thisprey switching has had a dramatic adverse affect on the endemic Island Fox that has evolved in the absenceof an apex terrestrial-based avian predator like the Golden Eagle. As a result, Island Foxes on the northern


Channel Islands have been nearly driven to extinction by Golden Eagle predation. It is unclear if eagles arehaving a similar adverse affect on any of the other native terrestrial and marine vertebrates found in their dieton the Channel Islands.

Invertebrates. Based on eighteen Golden Eagle food habit studies, invertebrates are extremely rare inprey remains found in eagle nests. Two beetles (0.1%) were reported in prey remains recovered fromGolden Eagle nests in southwestern Idaho (USDI 1979), which is the only Golden Eagle food habitstudy to report invertebrates. Some of the invertebrate remains recovered from Golden Eagle nestson the islands are undoubtedly remains of invertebrates found in the stomachs of marine birds andraptors brought back to the nest by eagles as prey. It is also possible that some of the terrestrialinvertebrates are the result of insects dying in the nest structure and not the result of eagles transportinginsects as prey to the nest. Differentiating whether invertebrate remains recovered from eagle nests on theislands were actually eaten by eagles or were stomach or crop contents of eagle prey is difficult to assess.All invertebrate remains recovered from Golden Eagle nest on the islands are most likely incidental remainsnot the result of eagles directly preying on invertebrates.

Fish. Based on eighteen food habit studies, fish only comprised 0.5% of individual food items foundin Golden Eagle nests in western North America (Appendix 1). A number of food habit studiesreported no fish in the nesting season diet of Golden Eagles, while several studies reported that fishcomprised from 0.2% (Kochert 1972) to 3.2% of the eagle’s diet (Carnie 1954). Only freshwater fish(e. g. perch, carp, squawfish, suckers, whitefish, and trout) have been documented from Golden Eaglefood habit studies in western North America (Brown 1992, Carnie 1954, Marr and Knight 1983,USDI 1979). The only fish recovered from Golden Eagle nests on the islands appear to be remains ofmarine fish that are from stomach or crop contents of larger marine bird prey brought to the nests byeagles. It does not appear that Golden Eagles on the islands are actively feeding on any dead floatingmarine fish or on beach-cast carcasses of fish as Bald Eagles are known to do on the Channel Islands(Collins et al. 2004, 2005; Sharpe 2003; Sharpe and Dooley 2001).

Amphibians and Reptiles. Amphibians have not previously been recorded in the prey remains of anyfood habit studies of Golden Eagles in western North America (Appendix 1). The single slender salamanderbone (Batrachoseps pacificus) recovered from a nest on Santa Rosa Island is probably the result of ananimal dying in the nest rather than the result of an eagle preying on a salamander. Based on a review ofGolden Eagle food habits studies from western North America, reptiles comprised an average of 1.6% ofindividual food items found in eagle nests (see Appendix 1). When reptiles are present, they havecomprised from 0.2% (Arnell 1971) to 5.6% (Carnie 1954) of the eagles nesting season diet in central Utahand central California, respectively. A variety of snakes have been recorded in prey remains of GoldenEagle nests in western North America. The most common species recorded were Gopher Snake andseveral species of rattlesnakes (Crotalus spp.). Lizards have only been reported in two Golden Eagle foodhabits studies, both in southwestern Idaho. Collopy (1983) recorded Western Fence Lizard (Sceloporusoccidentalis) and Western Whiptail (Cnemidophorus tigris) remains while the USDI (1979) studyrecorded Long-nosed Leopard Lizard (Gambelia wislizenii) remains.

Reptiles comprised 2.3% of the prey remains recovered from Golden Eagle nests on the Channel Islandswhich is slightly higher than the average of 1.6% obtained from eighteen Golden Eagle food habit studies inwestern North America (Appendix 1). While Gopher Snakes are not a new prey species for GoldenEagles, Southern Alligator Lizards are. Gopher Snakes certainly represent prey captured by Golden Eaglesand brought back to the nest to feed their young. However, Southern Alligator Lizards found in eagle nests


on the islands could be the result of animals dying within a nest structure, or being captured by eagles andcarried back to the nest to feed their young. It is impossible to determine which mechanism is responsiblefor the occurrence of alligator lizard remains in Golden Eagle nests on the Channel Islands. Based on theseresults, reptiles are clearly a relatively unimportant prey category for Golden Eagles nesting on the ChannelIslands.

Birds. Golden Eagles are adept predators of birds as evidenced by the wide variety of birds that havebeen reported in their diets throughout North America (Olendorff 1976). Birds ranged from 3.9% of allprey remains recorded from a study (Arnell 1971) to 47.6% of prey items (Marr and Knight 1983). It isclear from our study that birds represent an extremely important prey category for Golden Eagles nesting onthe islands. Birds comprised 51.3% of the eagles overall diet which is the highest percentage for birdsrecorded for the nesting season diet of any population of Golden Eagles in North America (see Appendix1). What most Golden Eagle food habit studies show is that, when mammalian prey within the size rangethat Golden Eagles prefer (i.e. 0.5-4.0 kg) is scarce, Golden Eagles shift to prey more intensively onavailable medium and smaller sized birds and mammals. It would appear that this is what has happened onthe Channel Islands where there preferred Leporid and Sciurid prey are absent.

Gallinaceous birds (pheasants, grouse, partridge, and ptarmigan) are an important secondary prey forGolden Eagles in a number of areas of western North America (Kochert et al. 2002), Scotland andcontinental Europe and Asia (Watson 1997). On the Channel Islands the only gallinaceous birdsavailable to eagles are California quail (both islands) and Wild Turkey (only Santa Cruz Island). Thefact that only two California Quail were recorded in the prey remains from eagle nests on the islandssuggests that this species is not an important prey species for eagles on the islands. In the absence ofthis prey category, eagles have switched to alternate avian prey which are more abundant on theislands. Common Ravens and cormorants were the most important avian prey for Golden Eagles onthe islands comprising 20.7% and 12.9%, respectively of the prey biomass recovered from eagle nestson the islands (Table 1). Waterfowl, gulls, and Barn Owls were the next most important avian prey foreagles on the islands (Table 1). All three species of cormorants, Gadwall and at least three species of gullswere new prey species for the Golden Eagle in North America. Watson (1997) recorded cormorants asrare (0.6% MNI) at eagle nests in Scotland. Only three previous food habit studies have recordedCommon Ravens in prey remains at eagle nests in North America (Eakle and Grubb 1986, Olendorff 1976,USDI 1979). In all three studies, Common Ravens were rare in the eagle’s diet. Common Ravens are alsoan uncommon prey for Golden Eagles in Scotland (Watson 1997). Most of the avian bone recovered fromeagle nests on the islands was from adult and subadult birds. However, remains of nestling cormorants,Common Ravens and Barn Owls in eagle nests on the islands indicates that eagles are probably occasionallyacquiring chicks by nest-robbing.

Birds of prey (hawks, falcons, and owls) are generally an uncommon to rare prey category for GoldenEagles in North America (Olendorff 1976), Scotland and across continental Europe and Asia (Watson1997). The three species of raptors found in prey remains from eagle nests on the islands have allpreviously been recorded in other Golden Eagle food habit studies. Peregrine Falcon was the mostunusual raptor recorded in the island prey samples having only been reported in two previous eaglefood habit studies (VanZandt 1982, Watson 1997). It is unclear whether eagles on Santa Rosa Islandactively preyed on or just scavenged a carcass of a Peregrine Falcon. On the mainland Golden Eaglesare known to prey upon young Peregrine Falcons at reintroduction sites and on falconers’ birds (B.Latta pers. comm.). At Santa Rosa Island, a young Golden Eagle was flushed off the carcass of an adultPeregrine Falcon that earlier had been observed losing a fight to an adult Red-tailed Hawk (Charlie


McLoughlin pers. comm. to B. Latta). Eagles fed more intensively on Barn Owls at the islands (14individuals, 4.1% MNI) than has been reported for any other food habit studies of eagles on the mainland(Bloom and Hawks 1982, Carnie 1954, Collopy 1983, Olendorff 1976, USDI 1979). Barn Owls are animportant secondary prey species for Golden Eagles on the Channel Islands. The presence of juvenile BarnOwl bones in eagle nests suggests that eagles are occasionally acquiring chicks by nest-robbing.

Land Mammals. While terrestrial mammals constitute an important prey category for Golden Eaglesnesting on the Channel Islands, they comprise only 43.4% of the eagle’s overall nesting season diet onthe islands. The percentage of mammals in the eagle’s diet on the islands is lower than that reportedfor most other Golden Eagle food habit studies elsewhere in North America (76.7%, n=18 studies;Appendix 1), Scotland (67.8%, n=9 sites; Watson 1997), and across Europe and Asia (57.7%, n=24studies; Watson 1997). The lack of abundant diurnally active species of mammals on the islands suchas rabbits, squirrels and marmots, has forced Golden Eagles to prey more intensively on mammalssuch as Deer Mice, Island Foxes, Spotted Skunks, Mule Deer fawns and Feral Piglets. The numberof individuals and the overall proportion of the eagle’s diet on the islands that each of these mammalscomprise is quite high when compared to other food habit studies for Golden Eagles in NorthAmerica and in most other areas of the world (Watson 1997).

Carnivores are typically incidental prey for Golden Eagles with most occurrences resulting from eaglesscavenging on carcasses of carnivores rather than actively preying on live animals. In Scotland Red Fox(Vulpes vulpes) cub remains were not uncommon in eagle nests in the western Highlands, young otterswere taken occasionally by eagles in coastal areas and on islands off the west coast of Scotland, and stoatsand weasels were a frequent prey of eagles across continental Europe and Asia (Watson 1997). A varietyof terrestrial carnivores (Raccoon [Procyon lotor], Ringtail [Bassariscus astutus], weasels [Mustela spp.],Striped Skunk [Mephitis mephitis], Hognosed Skunk [Conepatus leuconotus], American Badger[Taxidea taxus], Coyote [Canis latrans], Red Fox, Kit Fox [Vulpes macrotis], Gray Fox [Urocyoncinereoargenteus], Bobcat [Felis rufus], and domestic cat [Felis domesticus]) have been recorded inprey remains from Golden Eagle nests and roosts in North America (Carnie 1954, Hatch 1968, Mason2000, Olendorff 1976, Sumner 1931, USDI 1979). Most of these species were probably scavenged ascarcasses and as such were incidental to the overall diet of the population of eagles being studied. In mostfood habit studies of Golden Eagles in North America, carnivores comprise less than 1.0% of the eagle’soverall diet. However, a few studies have reported carnivores as being slightly more important dietconstituents. Carnivores comprised 5.4% of the overall diet of Golden Eagles in California (Carnie 1954),13.2% of the eagle’s diet in Arizona (Eakle and Grubb 1986), and at one completely dismantled nest inMongolia, Ellis et al. (1999) reports that mammalian predators, primarily adult Red Fox, Corsac Fox(Vulpes corsac), Siberian Polecat (Mustela eversmanni), and Pallas’ Cat (Felis manul), constituted 41%MNI of the total prey remains recovered. On the islands, carnivores (Island Fox and Western SpottedSkunk) comprised 10.3% of the eagle’s overall diet. Remains of Island Foxes were recovered from 3 of 4nests on Santa Cruz Island and from 2 of 4 nests on Santa Rosa Island. Western Spotted Skunk remainswere found in only one eagle nest on Santa Cruz Island but were present in all four eagle nests excavated onSanta Rosa Island. The heavier reliance on terrestrial carnivores on the Channel Islands is undoubtedly adirect result of the depauperate nature of available diurnally active terrestrial vertebrate prey on the islands,the fact that foxes and skunks are within eagle’s preferred prey size, and that both of these carnivoresexhibit some diurnal activity which makes them available to foraging eagles. In Scotland, Watson (1997)reported that Pine Martens shifted their foraging activity patterns due to increased eagle predation. While itis very clear that Golden Eagles have helped to drive Island Fox populations on the Northern ChannelIslands to the brink of extinction, it is unclear if the remaining Island Foxes and Spotted Skunks on the


Channel Islands have adapted to this increased predation pressure by shifting their activity patterns andhabitat preferences. If these two carnivores are to coexist with Golden Eagles on the islands, it is clear thatthey will have to adapt by shifting to a more nocturnal activity pattern and by spending more time foragingunder the protective cover of dense scrub and woodland habitats where they will be less accessible toforaging eagles.

Golden Eagles are known to occasionally kill large ungulates such as Mountain Goat, Bighorn Sheep,Dall’s Sheep (O. dalli), Caribou (Rangifer spp.), deer, and Pronghorn but typically feed on youngungulates (see review in Kochert et al. 2002, Olendorff 1976). They also prey on domestic livestockincluding sheep, goats (Capra hircus), cattle and horses (Kochert et al. 2002, Olendorff 1976, Watson1997). Ungulate and domestic livestock remains found in Golden Eagle nests include both carrionand eagle kills. Of 7,094 prey remains identified in Golden Eagle nests in North America, hoofedmammals and domestic livestock remains accounted for 1.4% and 4.4% of the remains, respectively(Olendorff 1976). Deer fawns are recorded frequently in Golden Eagle nests in Scotland and acrosscontinental Europe and Asia (Watson 1997). In Scotland deer comprise 1.2% to 22.3% of the eagle’sdiet while in continental Europe and Asia they comprise 0.1% to 14.9% of the diet (Watson 1997). InNorth America, deer have comprised between 0.1% and 12.7% of an eagle’s nesting season diet(Watson 1997).

On Santa Rosa Island Mule Deer fawns and Elk calves accounted for 13.2% of the eagle’s diet whileon Santa Cruz Island feral sheep accounted for 3.3% of the diet (Tables 2 and 3). Mule Deer fawnswere the most important (34.6% biomass) prey in the eagle’s diet on Santa Rosa Island and wererecovered from all four nests excavated on this island. All of the Mule Deer and Elk bones found ineagle nests on Santa Rosa Island were from very young animals (fawns and calves). There were nobones of adult or subadult deer or elk found in eagle nests on this island. Deer fawns are one of thefew terrestrial vertebrate prey on Santa Rosa Island that would be readily available to Golden Eagles duringthe eagle’s breeding season and that are within the size range for prey that are preferred by eagles (0.5-4.0kg). Based on the remains recovered from eagle nests on Santa Rosa Island, it is unclear whether eagleswere relying more on carcasses of newborn deer and elk or actively preying on these young animals. Onlythree bones from feral sheep were found associated with eagle nests on Santa Cruz Island. Two of thesebones were from newborn lambs and one weathered bone was from an adult. It is unclear if the sheepbones found in the Santa Cruz Island eagle nests were the result of eagles transporting sheep remains backto their nest while sheep were still present on the island or were incidental bones found on the ground andtransported to the nest sometime after sheep had been eradicated from the island. The relative lack ofsheep bone in eagle nests on Santa Cruz Island is probably a result of eagles first establishing a breedingpresence on this island just prior to or just following the sheep eradication project in the late 1980s. Weknow that Golden Eagles were nesting while sheep were still extant on Santa Cruz Island because the activeCoche Pt. nest was discovered in 1999 during the East End sheep round up and is thought to have beenactive for at least four years prior to discovery. It is also likely that the relative lack of sheep remains ineagle nests on Santa Cruz Island is due in part to the offset in timing between island lambing season (Winter)and eagle nestling rearing (late Spring and Summer). Also the majority of lambs available to eagles duringrearing of nestlings would have already been heavier than the eagle’s preferred prey weight class.

There are no previous published records of Feral Pig remains having been found at Golden Eaglenests in North America (Kochert et al. 2002, Olendorff 1976, Watson 1997). However, pig remainswere found recently in at least one Golden Eagle nest in the vicinity of Livermore California (B. Latta pers.comm.). Pig remains have been reported in 6 of 24 Golden Eagle food habit studies from continental


Europe and Asia where they comprised from 0.3 to 2.7% of an eagle’s diet (Watson 1997). Because of thedepauperate nature of the diurnally active terrestrial vertebrate fauna on Santa Cruz Island, Golden Eagleshave increased their reliance on introduced mammals such as Feral Piglets (39.6% MNI) and on feral sheep(3.3%) until they were eradicated from the island. Feral Piglets were the most important (59.1% biomass)prey in the eagle’s diet on Santa Cruz Island having been recovered from all four of the eagle nests that wereexcavated on this island. All remains of this species recovered from eagle nests on Santa Cruz Island werefrom very young pigs (newborn to 2 or 3 months of age). Very young pigs (0-2 months of age) are withinthe eagle’s preferred prey size (0.5-4.0 kg), are usually abundant, and thus are readily available to eaglesduring their spring breeding season. We found no subadult or adult pig bones in any of the prey remainsrecovered from nests on Santa Cruz Island. This could be due to the fact that eagles were only bringingback muscle and organ tissue from carcasses of older pigs and were not transporting bones from thesecarcasses to feed their young. It could also be that eagles had a preference during the nesting season toprey upon piglets because they could more easily transport a whole or partial piglet carcass back to the nestto feed their young. It is clear that Feral Pigs are the most important terrestrial vertebrate prey for eagles onSanta Cruz Island and as such are probably the principal reason why eagles were able recently tosuccessfully establish and maintain a breeding presence on the Northern Channel Islands. The absence ofFeral Pig remains in eagle nests on Santa Rosa Island suggests that eagles began nesting on this island afterFeral Pigs had been eradicated from the island in the early 1990s. If pigs were still present on Santa RosaIsland when eagles began nesting on the island, then we would have expected them to prey on piglets likethey have done on Santa Cruz Island.

Variation between Islands

A strong case can be made for the effect that preferred prey species have on the diet of Golden Eagles in adepauperate island setting. On Santa Cruz Island, Feral Pigs represent 59.1% of the prey biomass while onSanta Rosa Island, where pigs are absent, Golden Eagles switched to Mule Deer fawns (34.6%). Preyspecies diversity was lowest (12-14 species) on Santa Cruz Island where intake of a single prey species(Feral Pig) was highest (Table 2). In the absence of piglets, eagles on Santa Rosa Island, where Mule Deerfawns are a relatively less abundant food source by an order of magnitude, were forced to supplement theirdiet with a wider variety of aquatic and terrestrial birds (Table 3). Thus, as the proportion of an introducedherbivore (Mule Deer) in the eagle’s diet on Santa Rosa Island declined, the breadth of the eagle’s dietincreased. Also, as a result of the depauperate nature of the available diurnally active terrestrial vertebratefauna on the islands, atypical prey such as Mule Deer fawns, Island Fox, Western Spotted Skunk, CommonRaven, cormorants, gulls, waterfowl and Barn Owls accounted for a large portion (77.1%) of the total preybiomass (Table 1). Our data clearly shows that when a preferred prey species is readily available, eaglesbecome specialist hunters and their diet narrows, but when preferred prey is scarce they seek alternative,often less abundant, prey and as a result become generalists.

Dietary Breadth

Dietary breadth offers a quantitative measure of how specialized an eagle’s diet is. Watson (1997)examined dietary breadth of Golden Eagle populations across the world. Dietary breadth was mostconsistently restricted in North America (Appendix 1) where eagles showed a dependence on just oneor two prey families (Leporidae, or Sciuridae) while it was somewhat higher across Scotland andcontinental Europe where preferred prey tended to come from two or three of the four primary prey families(Leporidae, Sciuridae, Tetraonidae, and Phasianidae; Watson 1997). Golden Eagles exhibit a moregeneralized diet when their preferred prey are scarce or absent. Eagles in western Scotland had a more


generalized diet than the comparatively narrow diet exhibited by eagles in the eastern highlands of Scotland(Watson 1997). Steenhof and Kochert (1988) observed that, in Idaho, Golden Eagle’s diet was morevaried in years when their preferred prey of Black-tailed Jackrabbits was scarce.

On the Channel Islands, where rabbits, squirrels and large-sized Gallinaceous birds are lacking,Golden Eagles have developed a more diverse diet (8.73 dietary breadth) than exhibited by nearly allother eagle populations in North America (Appendix 1), Scotland and across continental Europe(Watson 1997). In the absence of their normal prey (rabbits and squirrels), Golden Eagles on theChannel Islands have filled the gap with a greater dependence on a wider diversity of prey and onspecies not typically preyed upon by eagles elsewhere in North America such as Feral Pig, CommonRaven, Mule Deer, Island Fox, Western Spotted Skunk, cormorants, gulls, waterfowl, and Barn Owls(Table 1). Eagles on Santa Cruz Island exhibit a slightly more specialized diet (4.70 dietary breadth)than do eagles on Santa Rosa Island (7.59) which is probably a result of the eagle’s heavier reliance atSanta Cruz Island on Feral Piglets (59.1% of prey biomass), an abundant alternative prey. In theabsence of this abundant prey on Santa Rosa Island, eagles have had to supplement their diet byincreasing the diversity of prey species eaten and shifting to alternative prey such as Mule Deer fawns,Common Ravens, Island Foxes, and Western Spotted Skunks. On islands off of Scotland, Watson(1997) observed that Golden Eagles had a moderately diverse diet (3.47-5.14) comprised of three orfour dominant prey families. However, Golden Eagle populations on the island of Gotland in Swedenand on Macedonia exhibit narrow diets comprised of unusual prey species. On the island of Gotland,where rabbits are rare, eagles have shifted to preying on Hedgehogs (Hogstrom and Wiss 1992) whiletheir principal prey on Macedonia are tortoises (Grubac 1987). Watson (1997) suggests that GoldenEagles on these two islands were able to shift to utilizing these unusual prey species because bothspecies fall within Golden Eagle’s preferred size range for prey and both species were available insufficient quantity. These are probably also the same reasons why Golden Eagles on the ChannelIslands have shifted to preying on species that are not normally eaten by Golden Eagles elsewhere in NorthAmerica.

Size Classes of Prey Selected

Another way to examine the diet of Golden Eagles on the islands is to look at the proportion ofdifferent size classes of prey taken by eagles. Based on biomass estimates assigned to prey recordedin the eagle’s diet on the islands (Appendix 2) and on the numbers of each prey species recorded inprey remains from nests on each island (Tables 2 and 3), we assigned prey to eight weight categories(see caption to Fig. 4). Size classes of prey taken by eagles on the islands were biomodal on bothislands with one peak in size classes 1 and 2 (<63 g and 64-125 g respectively, Fig. 4) and a secondlarger peak in size classes 5 through 7 (501 g – 4 kg, Fig. 4). No prey were recovered for size classes3 and 4 (126-500 g) and only one prey item was recorded for size class 8 (> 4 kg). The absence ofprey in these weight classes is probably a result of the depauperate nature of available diurnally activevertebrate prey in these weight classes on the islands. Ground squirrels and cottontails, importantprey for Golden Eagles in western North America, are absent from the fauna of the islands. The biomodalpattern observed in the proportion of prey of different size classes taken by eagles on theislands is different than the proportion of prey size classes recorded for Golden Eagles on the ScottishIslands, in Scotland, in continental Europe, and elsewhere in North America (Watson 1997). Againthis difference is due largely to the depauperate nature of available diurnally active prey in someweight classes on the islands. There are also island-specific differences in the proportion of prey sizeclasses represented in the diet of eagles on each island. Eagles on Santa Cruz Island showed lower


proportions of prey in size classes 1 and 6 but higher proportions in size classes 5 and 7 than eagles onSanta Rosa Island (Fig. 4). This is due in part to eagles feeding more intensively on Feral Piglets (size class7) on Santa Cruz Island and less intensively on Deer Mice and medium-sized birds and terrestrial mammals.The higher proportion of size classes 1 and 6 in the diet of eagles on Santa Rosa Island is due to eaglesfeeding more intensively on Deer Mice (size class 1) and on Common Raven, Mallard, and cormorants (allsize class 6). These observed differences in proportion of prey size classes in the diet of eagles on theislands is due in part to differences in the diversity and abundance of available prey in each size class oneach island and to the depauperate nature of this available prey base.

0

10

20

30

40

50

60

1 2 3 4 5 6 7 8Weight classes

%

SCI %SRI%

Figure 4. Proportion of prey in eight weight classes in the breeding season diet of Golden Eagles on Santa Cruz and Santa Rosa islands. Prey were allocated to the

following weight classes: 1 = <63g, 2 = 63-125 g, 3 = 126-250 g, 4 = 251-500 g, 5 = 501-1,000 g, 6 = 1-2 kg, 7 = 2-4 kg, and 8 = >4kg.

Comparison of Golden Eagle and Bald Eagle Diets on the Channel Islands

Our results differ significantly from food habit studies conducted on Bald Eagles nesting today on SantaCatalina Island (Garcelon et al. 1994a, 1994b; Sharpe 2003; Sharpe and Dooley 2001; Sharpe andGarcelon 1999, 2000), and historically on San Miguel Island (Table 4 and Figure 5; Collins et al. 2004,2005). The overall diet of Bald Eagles on Santa Catalina Island was composed of a much higherpercentage of fish (85.7% to 93.3%) and a lower percentage (1.5 to 10.8%) of birds than was the diet ofGolden Eagles on Santa Cruz and Santa Rosa islands where fish made up only 0.9% and birds made up51.3% of the eagle’s diet. The diet of Bald Eagles nesting historically on San Miguel Island was comprisedof a much higher diversity and abundance of marine prey (marine invertebrates [35+ species, 14.1% MNI],marine birds [39-43 species, 55.8%], fish [13+ species, 19.5%], and marine mammals [3 species, 0.9%])and a much lower percentage of terrestrial prey such as land mammals and land birds than was recorded inour analysis of the diet of Golden Eagles on Santa Cruz and Santa Rosa islands (Table 4 and Figure 5).Golden Eagles preyed more intensively on land mammals (7 species, 43.4% MNI) and land bird (9 species,30.2%) than on marine invertebrates (2 species, 0.6%), fish (2 species, 0.9%) or marine birds (7-9 species,21.1%; Table 4 and Figure 5). While the overall proportion of birds represented in the diet of Bald and


Figure 5. Comparison of the relative frequency (%MNI) of prey classes in the diets of Bald and Golden Eagles on the Channel Islands.

43.4

14.3

19.5

59.3

51.3

2.30.30.9

1.8 5.41.5

00

10

20

30

40

50

60

70

Inverte

brates

Fish

Amphibi

ans

Reptiles

Birds

Mammals

Perc

ent M

NI

Golden EagleBald Eagle

Golden Eagles on the Channel Islands was similar (i. e. 59.3% vs 51.3%), the species mix and diversity wasquite different. Bald Eagles preyed on 5 species of shorebirds (1.3% MNI), 6 species of land birds (2.2%)and 39-43 species of marine birds (55.8%; Collins et al. 2005). Golden Eagles preyed on 9 species of landbirds (30.2%) and 7-9 species of marine/aquatic birds (21.1%) but did not prey on any shorebirds (Table1). Many of the marine bird groups represented in the Bald Eagle’s diet on San Miguel Island (e. g. loons,grebes, shearwaters/fulmars, marine ducks, and alcids) were absent from the diet of Golden Eagles on theislands (Collins et al. 2005a). Apparently Golden Eagles are not preying on beach-cast carcasses of marinebirds like Bald Eagles did on San Miguel Island (Collins et al. 2004). Common Ravens were veryimportant in the Golden Eagle’s diet (n=80, 23.5% MNI; Table 1) but were only incidental in the BaldEagle’s diet on San Miguel Island (n=3, 0.6% MNI; Collins et al. 2005).

Terrestrial mammals were more important in the Golden Eagle’s diet than they were in the Bald Eagle’s dieton the islands (Table 4 and Figure 5). Feral mammals were an important element of the diet of both speciesof eagles on the islands but were of less importance in the Bald Eagle’s diet. Sheep (carrion) comprised2.9% (n=14 individuals) of the Bald Eagle’s diet (Collins et al. 2004) but only 0.9% (n=3 individuals) of theGolden Eagle’s diet (Table 1). Feral Piglets and Mule Deer fawns made up 10.6% and 9.4%, respectively,of the Golden Eagles diet (Table 1) but were absent from the Bald Eagle’s diet. Golden Eagles preyed uponIsland Foxes more readily (n=17, 5.0%) than did Bald Eagles on San Miguel Island (n=2, 0.4%). WesternSpotted Skunks (n=18, 5.3%) were only preyed upon by Golden Eagles (Table 1). Reptiles were of littleimportance in the overall diet of both eagles comprising 2.3% of the Golden Eagle’s diet and 1.5% of theBald Eagle’s diet (Table 4). Marine mammals were absent from the Golden Eagle’s diet while theycomprised only 0.9% of the Bald Eagle’s diet (Table 4). It is clear from both of these food habit studies on


the Channel Islands that Golden Eagles are a terrestrial predator that feeds almost exclusively on diurnallyactive terrestrial vertebrates while Bald Eagles feed more intensively on nearshore-marine vertebrates. Thisdifference helps to explain why Golden Eagles have been so detrimental to the diurnally active Island Foxwhile Bald Eagles do not appear to have adversely affected Island Fox populations on the Channel Islands.

CONCLUSIONS

Except for occasional anecdotal observations recorded of prey remains found in eagle nests, or of eaglesobserved foraging on carcasses or on live prey, no quantitative food habits data are available for GoldenEagles currently residing on the Channel Islands. This paper analyzes prey remains excavated from eightGolden Eagle nests on Santa Cruz and Santa Rosa islands. Of the 3,343 faunal elements recovered duringthis excavation, 3,335 (99.8% NISP) were from vertebrates and 8 (0.2%) were from invertebrates. Basedon minimum number of individuals, the percent diet composition was 51.3% birds, 43.4% land mammals,2.3% reptiles, 1.8% invertebrates, 0.9% fish, and 0.3% amphibians. Based on the biomass of individualprey species, the percent diet composition was 54.9% land mammals, 44.7% birds, and 0.4% reptiles.Remains of 175 birds (16-18 species), 148 mammals (7 species), 8 reptiles (2 species), 6 invertebrates (6species), 3 fish (1-2 species), and 1 amphibian (1 species) were identified. Land birds comprised 30.2%by MNI or 22.8% by biomass of the eagles nesting season diet while marine birds comprised 21.1% byMNI or 21.9% by biomass. Common Raven, cormorants, waterfowl, gulls and Barn Owls comprised mostof the bird remains. Feral Piglets on Santa Cruz Island (10.6% MNI or 19.4% biomass) and Mule Deerfawns on Santa Rosa Island (9.4% MNI or 23.2% biomass) were the most abundant terrestrial mammalprey with moderate numbers of Deer Mice, Island Fox and Western Spotted Skunk also recorded.Southern Alligator Lizard and Gopher Snake were the only reptiles recorded. Fish and marine invertebrateremains were probably incidental remains brought to the nests in the stomachs of larger marine bird prey.

Table 4. Proportion of prey categories in the nesting season diets of Golden Eagles and Bald Eagles on the Northern Channel Islands, based on the proportion of individuals represented (%MNI).

Golden Eagle Bald Eaglea

Prey Category No. Species MNI %MNI No. Species MNI %MNIINVERTEBRATES 6 6 1.8 36+ 66 14.3 Marine Invertebrates 2 2 0.6 35+ 65 14.1 Terrestrial Invertebrates 4 4 1.2 1 1 0.2VERTEBRATES 28-30 335 98.2 66-73 396 85.7 Fish 2 3 0.9 13+ 90 19.5 Amphibians 1 1 0.3 Reptiles 2 8 2.3 1 7 1.5 Birds 16-18 175 51.3 45-48 274 59.3 Marine Birds 7-9 72 21.1 39-43 258 55.8 Shorebirds 5 6 1.3 Land Birds 9 103 30.2 6 10 2.2 Mammals 7 148 43.4 7 25 5.4 Marine Mammals 3 4 0.9 Land Mammals 7 148 43.4 4 21 4.5TOTAL 34-36 341 100 102-105 462 100

a. Data from an historic Bald Eagle nest that was excavated on San Miguel Island (Collins et al. 2005).


Sensitive wildlife recorded in this faunal sample included Island Fox, Peregrine Falcon, and Channel IslandLoggerhead Shrike. The Island Fox comprised 5.0% MNI or 7.5% of the biomass of the eagles overallnesting season diet on the islands while the other two sensitive species each comprised only 0.3% MNI ofthe eagles diet. Data obtained from this study clearly indicates that Golden Eagles nesting on Santa Cruzand Santa Rosa Island rely more heavily on terrestrial vertebrate prey than did Bald Eagles nestinghistorically on San Miguel Island. Also nonnative mammals such as piglets on Santa Cruz Island (39.6%MNI, 59.1% biomass) and Mule Deer fawns on Santa Rosa Island (12.8% MNI, 34.6% biomass) werecritical prey for Golden Eagles during the nesting season on each of these islands.

ACKNOWLEDGMENTS

We are thankful to T. Coonan and K. Faulkner of Channel Islands National Park for helping to procurefunds for this project. Logistical support during the nest excavation work was provided by L. Laughrin (UCNatural Reserve System, Santa Cruz Island Reserve) and D. Mills (U. S. Navy) while on Santa Cruz Islandand by the National Park Service and the Vail Vickers Company while on Santa Rosa Island. We wouldalso like to thank D. Driscoll, G. Doney, N. Todd, A. Grant, P. Andreano, and D. Haines for assistance withsample collection; Marie Holmes and Terri Sheridan for helping with the many interlibrary loan requests;and Paul Valentich Scott and Michael Caterino for help with identification of marine and terrestrialinvertebrate prey remains. Finally we thank T. Coonan for providing constructive comments on an earlierdraft of this manuscript.

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APPENDICES


App

endi

x 1.

Pro

porti

on o

f maj

or p

rey

cate

gorie

s in

bree

ding

seas

on d

iets

of G

olde

n Ea

gles

in w

este

rn N

orth

Am

eric

a.

Pe

rcen

t of T

otal

Pre

y It

ems

No.

D

ieta

ryL

ocat

ion

Inve

rts

Fish

Rep

tiles

Bir

dsM

amm

als

Nes

tsB

read

thSo

urce

A

lask

a62

.739

.21

2.76

Har

tler (

1974

)A

lask

a4.

996

.1?

1.40

Mur

ie (1

944)

Ala

ska

1.5

4.5

9417

2.33

Ritc

hie

and

Cur

atol

o (1

982)

Ariz

ona,

cen

tral

2.6

18.4

78.9

94.

52Ea

kle

and

Gru

bb (1

986)

Cal

iforn

ia, I

nner

Coa

st R

ange

3.2

5.6

13.5

77.3

175.

30C

arni

e (1

954)

Cal

iforn

ia, N

E &

Nev

ada,

NW

2.0

6.0

88.0

119

1.36

Bloo

m a

nd H

awks

(198

2)Id

aho,

sout

hwes

tern

1.2

2.1

26.5

70.2

61Be

echa

m (1

970)

Idah

o, so

uthw

este

rn0.

21.

116

.182

.628

1.83

Koc

hert

(197

2)Id

aho,

sout

hwes

tern

0.1

2.0

3.6

20.7

73.1

605.

36U

SDI (

1979

)Id

aho,

sout

hwes

tern

4.8

16.9

78.4

81.

86C

ollo

py (1

983)

Mon

tana

, sou

th-c

entra

l0.

412

.486

.938

1.97

McG

ahan

(196

8)N

ew M

exic

o0.

64.

094

.541

2.09

Mol

lhag

en e

t al.

(197

2)O

rego

n2.

917

.573

.87

4.45

Thom

pson

et a

l. (1

982)

Uta

h, c

entra

l0.

23.

996

.019

1.60

Arn

ell (

1971

)W

ashi

ngto

n, e

aste

rn0.

31.

047

.650

.874

4.92

Mar

r and

Kni

ght (

1983

)W

ashi

ngto

n0.

31.

046

.851

.874

4.35

Kni

ght a

nd E

ricks

on (1

978)

Wyo

min

g, so

uthw

este

rn0.

68.

789

.843

3.63

Mac

Lare

n et

al.

(198

8)C

anad

a, A

lber

ta17

.084

.01

1.40

Boag

(197

7)A

vera

ge o

f eig

htee

n st

udie

s0.

10.

51.

620

.576

.7C

alifo

rnia

, Cha

nnel

Isla

nds

1.8

0.9

2.3

51.3

43.4

98.

73Th

is st

udy

CA

: San

ta C

ruz

Isla

nd2.

21.

143

.952

.75

4.70

This

stud

y

C

A: S

anta

Ros

a Is

land

0.8

1.2

3.2

54.0

43.2

47.

59Th

is st

udy


App

endi

x 2.

Who

le b

ody

wei

ghts

use

d in

cal

cula

ting

prey

bio

mas

s.

Fres

hC

omm

on N

ame

Scie

ntifi

c N

ame

Wei

ght (

g)Sa

mpl

e co

mpo

sitio

nSo

urce

REP

TIL

ES

REP

TIL

IA

Sout

hern

Alli

gato

r Liz

ard

Elga

ria

mul

ticar

inat

a25

Sexe

s com

bine

dEs

timat

ed

Sant

a C

ruz

Gop

her S

nake

Pitu

ophi

s cat

enife

r pum

ilus

305

Sexe

s com

bine

dEs

timat

ed

BIR

DS

AV

ES

D

oubl

e-cr

este

d/Br

andt

's C

orm

oran

tPh

alac

roco

rax

auri

tus/

peni

cilla

tus

1962

Ave

rage

, spe

cies

and

sexe

s com

bine

dW

alla

ce a

nd W

alla

ce (1

998)

, Dun

ning

(199

3)

Pe

lagi

c C

orm

oran

tPh

alac

roco

rax

pela

gicu

s18

68A

vera

ge, s

exes

com

bine

dD

unni

ng (1

993)

Gad

wal

lAn

as st

repe

ra92

0A

vera

ge, s

exes

com

bine

dD

unni

ng (1

993)

Mal

lard

Anas

pla

tyrh

ynch

os11

39A

vera

ge, s

exes

com

bine

dD

rillin

g et

al.

(200

2)

Hee

ring

Gul

lLa

rus a

rgen

tatu

s11

35A

vera

ge, s

exes

com

bine

dD

unni

ng (1

993)

Cal

iforn

ia G

ull

Laru

s cal

iforn

icus

607

Ave

rage

, sex

es c

ombi

ned

Dun

ning

(199

3)

W

este

rn/G

lauc

ous-

win

ged

gull

Laru

s occ

iden

talis

/gla

uces

cens

875

Ave

rage

, spe

cies

and

sexe

s com

bine

dD

unni

ng (1

993)

, Pie

rotti

et a

l. (1

995)

Red-

taile

d H

awk

Bute

o ja

mai

cens

is11

26A

vera

ge, s

exes

com

bine

dD

unni

ng (1

993)

Pere

grin

e Fa

lcon

Falc

o pe

regr

inus

768

Ave

rage

, sex

es c

ombi

ned

Dun

ning

(199

3)

C

alifo

rnia

Qua

ilC

allip

epla

cal

iforn

ica

173

Ave

rage

, sex

es c

ombi

ned

Dun

ning

(199

3)

Ba

rn O

wl

Tyto

alb

a52

4A

vera

ge, s

exes

com

bine

dD

unni

ng (1

993)

Logg

erhe

ad S

hrik

eLa

nius

ludo

vici

anus

47.4

Ave

rage

, sex

es c

ombi

ned

Dun

ning

(199

3)

C

omm

on R

aven

Cor

vus c

orax

1199

Ave

rage

, sex

es c

ombi

ned

Dun

ning

(199

3)

W

hite

-cro

wne

d Sp

arro

wZo

notr

ichi

a le

ucop

hrys

25.5

Ave

rage

, sex

es c

ombi

ned

Dun

ning

(199

3)

W

este

rn M

eado

wla

rkSt

urne

lla n

egle

cta

100.

7A

vera

ge, s

exes

com

bine

dD

unni

ng (1

993)

Brow

n-he

aded

Cow

bird

Mol

othr

us a

ter

43.9

Ave

rage

, sex

es c

ombi

ned

Dun

ning

(199

3)

MA

MM

AL

SM

AM

MA

LIA

D

eer M

ouse

Pero

mys

cus m

anic

ulat

us20

Ave

rage

, SC

I sex

es c

ombi

ned

Col

lins e

t al.

(197

9)

Isla

nd G

ray

Fox

Uro

cyon

litto

ralis

2036

Ave

rage

, SC

I & S

RI se

xes c

ombi

ned

Col

lins (

1982

)

Wes

tern

Spo

tted

Skun

kSp

iloga

le g

raci

lis a

mph

iala

560

Ave

rage

, SC

I sex

es c

ombi

ned

Cro

oks (

1994

)

Fera

l Pig

Sus s

crof

a25

00A

vera

ge, n

ewbo

rn to

1 m

onth

old

pig

lets

Estim

ated

Eu

rope

an M

ouflo

n Sh

eep

Ovi

s ari

es23

00A

vera

ge, n

ewbo

rn to

6 m

onth

old

lam

bsV

anV

uren

and

Cob

lent

z (1

984)

Mul

e D

eer

Odo

coile

us h

emio

nus

3365

Ave

rage

wei

ght a

t birt

h of

O. h

. hem

ionu

sA

nder

son

and

Wal

lmo

(198

4)

El

kC

ervu

s can

aden

sis45

00A

vera

ge, n

ewbo

rn to

1 w

eek

old

calv

esJo

hnso

n (1

951)

NESTING SEASON DIET OF GOLDEN EAGLES ON SANTA CRUZ …€¦ · a small number of Golden Eagles. In...

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