Diets of two human-subsidized predators, common raven and glaucous gull, on Alaska’s Coastal Plain...
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Diets of two human-subsidized predators, common raven and glaucous gull, on Alaska’s Coastal Plain Abby N. Powell, U.S. Geological Survey, Alaska Cooperative Fish and Wildlife Research Unit, University of Alaska Fairbanks Emily L. Weiser, University of Otago, Dunedin, New Zealand Stacia A. Backensto, National Park Service, Fairbanks, AK Study site Introduction Common ravens (Corvus corax) and glaucous gulls (Larus hyperboreus) are primary avian predators in northern Alaska. Populations of both species in the coastal plain region are considered human-subsidized. Populations of both species have increased in areas associated with oil development due to availability of structures for nesting ravens and food sources for both species. Many species of conservation concern nest in the same region. The impact of these human-subsidized predators on these species is unknown. We used analyses of regurgitated pellets combined with stable isotope analyses to determine relative amounts of garbage and natural prey items in diets of adults and chicks. Results (continued) Methods We collected pellets and food remains from within a 10-m radius of each common raven nest in 2004 and 2005, andfrom one glaucous gull colony (8-9 breeding pairs) in 2008-2009 and another colony (15 breeding pairs) in 2009. Pellets were dissected and food items were identified to the lowest possible taxonomic level. We opportunistically sampled mantle feathers from nearly-fledged ravens found dead at one nest in 2004. We captured live gull chicks and collected one mantle feather from each. Feathers were analyzed at the Alaska Stable Isotope Facility for carbon and nitrogen signatures. We corrected signatures using fractionation rates for carnivorous diets in birds along with an additional correction for δN for growing chicks. Acknowledgments Funding and other support were provided by the North Slope Borough Department of Wildlife Management, UAF Coastal Marine Institute, the U.S. Bureau of Land Management, UAF Angus Gavin Migratory Bird Research Grant, and ConocoPhillips Alaska, Inc. We thank the numerous technicians and volunteers who helped with this study. Results Conclusions Adult common ravens and glaucous gulls nesting close to the landfill used garbage as food, but it was more prevalent in gull diets. Both species prey on birds and mammals, including species of conservation concern. Young birds of both species likely consumed garbage, but gull chicks appeared to be fed a higher proportion of garbage than ravens. Other data suggest that breeding ravens forage close to their nests during chick rearing and are not using the landfill during this period, whereas gulls are frequent visitors. However, ravens remain in Prudhoe Bay year-round, while gulls leave the area after the breeding season. Management to reduce predation on species of special concern should include reduced access to garbage for gulls and limiting nest structure availability for ravens. Raven nests and gull colonies within 8 km of the Deadhorse, AK landfill, which services the oil fields (Kuparuk and Prudhoe Bay) in the region. FS3 2004 FS3 2005 GC1 2004 GC1 2005 LPC 2005 PM 2008 PM 2009 WC 2009 Common Raven Glaucous Gull 0 20 40 60 80 100 Garbage Birds Mammals Other % occurrence in samples Figure 1. Percent occurrence of each food group in pellets and food remains from Common Ravens at Glaucous Gulls at Prudhoe Bay, Alaska. “Other” includes fish, crustaceans, insects, and unidentified items. -31 -29 -27 -25 -23 -21 -19 -17 -15 0 2 4 6 8 10 12 14 16 Common Raven Glaucous Gull Potential Prey δ13C δ15N Marine fish Other marine Zooplankton Freshwater fish Birds Caribou Small mammals Garbage Figure 2. Stable carbon and nitrogen isotope signatures (mean ± s.d.) of food groups used in mixing models for estimating glaucous gull diets in northern Alaska; and stable isotope signatures of Common Raven and Glaucous Gull chick feathers. Percent occurrence of garbage in pellets ranged from 6-32% for ravens and 58-80% for gulls. Mammals were more prevalent (70-100%) in raven pellets than in gull pellets (13-40%). Birds were less prevalent than mammals as prey for both species (30-74% for ravens; 7-25% for gulls). Feather isotope values indicated that gull chicks were fed diets with higher proportions of garbage than raven chicks.
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Transcript of Diets of two human-subsidized predators, common raven and glaucous gull, on Alaska’s Coastal Plain...
Diets of two human-subsidized predators, common raven and glaucous gull, on Alaska’s Coastal PlainAbby N. Powell, U.S. Geological Survey, Alaska Cooperative Fish and Wildlife Research Unit, University of Alaska Fairbanks
Emily L. Weiser, University of Otago, Dunedin, New ZealandStacia A. Backensto, National Park Service, Fairbanks, AK
Study site
Introduction
Common ravens (Corvus corax) and glaucous gulls (Larus hyperboreus) are primary avian predators in northern Alaska. Populations of both species in the coastal plain region are considered human-subsidized.
Populations of both species have increased in areas associated with oil development due to availability of structures for nesting ravens and food sources for both species.
Many species of conservation concern nest in the same region. The impact of these human-subsidized predators on these species is unknown.
We used analyses of regurgitated pellets combined with stable isotope analyses to determine relative amounts of garbage and natural prey items in diets of adults and chicks.
Results (continued)Methods
We collected pellets and food remains from within a 10-m radius of each common raven nest in 2004 and 2005, andfrom one glaucous gull colony (8-9 breeding pairs) in 2008-2009 and another colony (15 breeding pairs) in 2009.
Pellets were dissected and food items were identified to the lowest possible taxonomic level.
We opportunistically sampled mantle feathers from nearly-fledged ravens found dead at one nest in 2004. We captured live gull chicks and collected one mantle feather from each. Feathers were analyzed at the Alaska Stable Isotope Facility for carbon and nitrogen signatures. We corrected signatures using fractionation rates for carnivorous diets in birds along with an additional correction for δN for growing chicks.
AcknowledgmentsFunding and other support were provided by the North Slope Borough Department of Wildlife Management, UAF Coastal Marine Institute, the U.S. Bureau of Land Management, UAF Angus Gavin Migratory Bird Research Grant, and ConocoPhillips Alaska, Inc. We thank the numerous technicians and volunteers who helped with this study.
Results
Conclusions Adult common ravens and glaucous gulls nesting close to
the landfill used garbage as food, but it was more prevalent in gull diets. Both species prey on birds and mammals, including species of conservation concern.
Young birds of both species likely consumed garbage, but gull chicks appeared to be fed a higher proportion of garbage than ravens.
Other data suggest that breeding ravens forage close to their nests during chick rearing and are not using the landfill during this period, whereas gulls are frequent visitors. However, ravens remain in Prudhoe Bay year-round, while gulls leave the area after the breeding season.
Management to reduce predation on species of special concern should include reduced access to garbage for gulls and limiting nest structure availability for ravens.
Raven nests and gull colonies within 8 km of the Deadhorse, AK landfill, which services the oil fields (Kuparuk and Prudhoe Bay) in the region.
FS3 2004
FS3 2005
GC1 2004
GC1 2005
LPC 2005
PM 2008
PM 2009
WC 2009
Common Raven Glaucous Gull
0
20
40
60
80
100Garbage Birds Mammals Other
% o
ccu
rre
nce
in s
am
ple
s
Figure 1. Percent occurrence of each food group in pellets and food remains from Common Ravens at Glaucous Gulls at Prudhoe Bay, Alaska. “Other” includes fish, crustaceans, insects, and unidentified items.
-31 -29 -27 -25 -23 -21 -19 -17 -150
2
4
6
8
10
12
14
16Common RavenGlaucous GullPotential Prey
δ13C
δ15N
Marine fish Other marine
ZooplanktonFreshwater fish
Birds
Caribou
Small mammals
Garbage
Figure 2. Stable carbon and nitrogen isotope signatures (mean ± s.d.) of food groups used in mixing models for estimating glaucous gull diets in northern Alaska; and stable isotope signatures of Common Raven and Glaucous Gull chick feathers.
Percent occurrence of garbage in pellets ranged from 6-32% for ravens and 58-80% for gulls.
Mammals were more prevalent (70-100%) in raven pellets than in gull pellets (13-40%).
Birds were less prevalent than mammals as prey for both species (30-74% for ravens; 7-25% for gulls).
Feather isotope values indicated that gull chicks were fed diets with higher proportions of garbage than raven chicks.
