Hornero 14: 193-203, 1997

AVIAN SEASONALITY AT A LOCALITY IN THECENTRAL PARAGUAYAN CHACO

DANIEL M. BROOKSTexas A&M University: Department of Wildlife and Fisheries Sciences; Texas Cooperative Wildlife Collections:

College Station, Texas 77843. USA. E-Mail: Ecotropix@aol.com

ABSTRACT. Abundances of different species of birds were recorded in the centralParaguayan Chaco from August 1989 to August 1990 to investigate seasonal varia-tion at the guild level. Species were grouped into guilds based upon primary diet orwater dependence. The number of species (abundant : rare) in each guild is as fol-lows: insectivores (21:35), granivores and foliovores (20:6), faunivores (14:13), hy-drophil ic species (4:28), detrivores (3:1), nectarivores (1:1), and frugivores (0:5).Insectivores show the strongest seasonafity (SD = 1.63) followed by hydrophilic spe-cies (SD = 1.43), nectarivore (SD = 1.41), faunivores (SD = 1.33), granivores andfoliovores (SD = 1.20), and detrivores (SD = 0.50). Chi-square tests indicated thatdifferences between numbers of abundant versus rare insectivores (P < 0.01), granivores(P < 0.005), and hydrophilic species (P « 0.005) were highly significant. Results areintrepreted in light of ecological and evolutionary processes.

Key words: seasonality, resources, avian community, Chaco, Paraguay

Estacionalidad en las aves del Chaco Paraguayo central

RESUMEN. Registre la abundancia de especies de aves en el Chaco Paraguayo cen-tral, Agosto de 1989 hasta Agosto de 1990, para investigar la variacion estacional alnivcl de grupo funcional o gremio (guild). Las earacteristicas usadas para definirgrupos funcionalcs fueron la dependicia del agua o la dicta primaria. El numcro deespecies (abundancia : raro) por grupo funcional son: inscctivoras (21:35), granivo-ras and folivoras (20:6), faunivoras (14:13), especies asociadas al agua (4:28), dctri-tivoras (3:1), nectarivoras (1:1), y frugivoras (0:5). El grupo con la variacion estacio-nal mayor fue las insectivoras (SD = 1.63), siguiendoles las especies asociado alagua, (SD = 1.43), las nectarivoras (SD = 1.41), faunivoras (SD = 1.33), granivorasand folivoras (SD = 1.20), y dctritivoras (SD = 0.50). Pruebas de chi cuadrado indicanquo la diferencia cntre especies abundantcs y raras son muy significativas para las insec-tivoras (P < 0,01), granivoras (P < 0.005), y especies asociadas al agua (P « 0.005). Losrcsul tados sc mlerprelan en te rminus tic proccsos.de eeologia y cvoiucion.

Putubra.i clave: estacionalidad, recursos, comumdad de aves, Chaco, Paraguay

INTRODUCTION (including sub-humid forest, pantanal, trop-ical savannah, and pampas) interdigitate in

The Chaco is a mosaic of xeric habitats the areas of southeastern Bolivia, westernin the central portion of South America Paraguay, and northern Argentina. The areawhere several different neotropical biomes is characterized by low avian endemism bio-

Rccibido cl 20/02/96. Accptado c! 29/01/97

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AVIAN SEASONALLY IN THE PARAGUAYAN CHACO...

geographically (Short 1975) and is an ef-fective barrier to forest birds, but not wood-land or grassland birds (Nores 1992, Hayes1995).

The Chacoan avifauna was documentedby Short (1975), who concentrated on bio-geographic aspects. Since then several stud-ies have been undertaken on birds in theParaguayan Chaco (e.g., Short 1976; Short1980; Contreras & Mandelburger 1985;Contreras & Gonzalez-Romero 1989;Gonzalez-Romero & Contreras 1989; Hayeset al. 1990; Peris 1990; Peris et al. 1987;Neris & Colman 1991; Hayes et al. 1991;Brooks 1991, 1995, 1997; Hayes 1995). Al-though some of these studies have investi-gated seasonal variation for specific assem-blages such as shorebirds and waterbirds,none have attempted to investigate season-al variation for an entire avian communityat the guild level (Hayes pers. comm.). Theobjective of this paper is to determine therole of limiting resources in influencing sea-sonality of birds from a site in the semi-xe-ric Paraguayan Chaco.

Seasonal variation can be defined asvariation in annual abundance. Many tropi-cal environments are sharply seasonal, andassociated with rainfall rather than temper-ature variations. These changes affect habi-tat structure and food supplies, and onewould expect the bird species to respond(Wiens 1989). Klopfer (1959) suggested thatwhere seasonal environmental fluctuationsare minimal, the type of cover, nesting sites,and food which are available remain fairlyconstant.

METHODS

STUDY AREASpecies included in this study (Table 1)

were found within a 35 km radius of Estan-cia Fortin Toledo proper (hereafter, referredas Toledo) (22°33'S,60°30'W), DepartmentBoqueron, 35 km W of the Mennonite Col-ony, Filadclfia. This area, like much of thecentral Paraguayan Chaco, has been exten-sively cleared for cattle production (Benir-schke et al. 1989). The second-growth hab-

itat in the vicinity of Toledo is a mosaic of"quebracho" woodland and grassland (Short1975), characterized by thorny bushes,shrubs, and cacti, with scattered trees up to13 m high. Prosopis niscifolia, a thorny le-gume, and Opuntia sp. cactus are the domi-nant species (Lopez et al. 1987). Isolatedtracts of thick, impenetrable, thorny forestare sometimes left when land is beingcleared for agrarian purposes. The under-story in such forest consists of thorny Bro-melia serra and Cleistocactus baumanii(Stabler 1985). Tajamares (man-made, sea-sonal ponds) and filled gulleys from mas-sive rains are present throughout the studyarea.

SAMPLING METHODSThis study took place from August 1989

to August 1990. Narosky & Yzurieta (1987),Meyer de Schauensee (1982), and Dunning(1987) were used to identify unknown spe-cies. Abundance data were obtained fromobservations of live birds and were rankednumerically using a standardized scale forall taxa. During some months certain spe-cies were absent from Toledo proper butpresent within the 35 km radius of the cen-terpoint, so a special rank (1) was used toindicate such situations of local movements.Additionally, ranks of 2 and 3 were indica-tive of singletons being present rather thanmultiple individuals, reflecting transitorymovements or a low point during migration.The following monthly scale was used: 0 =absent: not observed during a given month;1 = local movements: observed within 35km of the study area's centerpoint but not atToledo proper; 2 = monthly transient indi-vidual: observation of one individual permonth persisting in study area no more than24 hrs.; 3 = monthly resident individual:observation of one individual per monthpersisting in study area more than 24 hrs; 4= uncommon: two to five individuals ob-served per survey day; 5 = common: six tonine individuals observed per survey day; 6- abundant: ten or more i n d i v i d u a l s ob-served per survey day.

Data were collected by walking an av-erage of 1.75 km of transect daily through a

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DANIEL M. BROOKS

Table L Abundant Species at Toledo+

GUILD

Species

INSECTIVORESDark-billed Cuckoo Coccyzus melacoryphusSmooth-billed Ani Crotophaga aniGuira Cuckoo Guira guiraWhite Woodpecker Leuconerpes CandidasNarrow-billed Woodcrccpcr Lepidocolaptes angustirostrisRufous Horncro Furnarius rufusCrested Horncro Furnarius cristutusChotoy Spinctail Schoeniophylax phryganophilaLittle Thornbird Phacellodomus sibilatrixLark-like Brushrunncr Coryphislera alaudinaSmall-billed Elaenia Elaenia parvirostrisWhite Monjita Xolmis iruperoBlack-backed Water-tyrant Fluvicola albiventerCattle Tyrant Machetarnis rixosusTropical Kingbird Tyrannus melancholicusFork-tailed Flycatcher Tyrannus savanaCrowned-slaty Flycatcher Griseotyrannus aurantioatrocristatusGreat Kiskadec/^'iarcgwi1 sulphuratusWhite-banded Mockingbird Mimus triurusMasked Gnatcatcher Polioptila dumicolaEpaulet Oriole Icterus cayanensis

(n=21)

GRANIVORES AND FOL10VORESBrushland Tinamou Nothoprocta cinerascensSpotted Tinamou Nothura maculosaGreater Rhca Rhea americanaChaco Chachalaca Ortalis canicollisPicazoro Pigeon Columba picazuroEared Dove Zenuida auriculataPicui Ground-dove Columbina picuiWhite-tipped Dove Leptotila verreauxiBlue-crowned Parakeet Aratinga acuttcaudataNanday Parakeet Nandayus neiidayMonk Parakeet Myiopsitta tnonachusBlue-fronted Parrot Amazona aeslivaHouse Sparrow Passer domeslicusRed-crested Cardinal Paroaria coronataMany-colored Chaco-finch Saltatricula multicolorRed-crested Finch Coryphospingus cucullatusSaffron Yellow-Finch Sicalis jlaveolaGolden-billed Saltator Saltator auranliirostrtsBay-winged Cowbird Molothurua badiitsShiny Cowbird Moloihurus bonariensis(n-20)

FAUNIVORESPlumbeous Ibis Theristicus caerulescensBuff-necked Ibis Theristicus caudatusWhite-tailed Kite Elanus leucurusSnail Kite Rostrhamus sociabilisGreat Black-Hawk Buteogallus urubitingaSavannah Hawk Buteogallus meridionalisRoadside Hawk Buteo magniroslriaWhite-tailed Hawk Buteo albicaudatusAmerican Kestrel Falco sparveriusApiomado Falcon Falco femoralisRed-legged Scricma Cariama cristaiaBlack-legged Scricma Chunga hurmeixteriSouthern Lapwiny Vunellus chilensis

Months

SONDJFMAMJJA

000445444242045544440241644555656544020004454440444344444244044544440000064245444454000222444444000004445544654454444554000444445544144445444454045444444200444424445544066445444044465545400000066554400000065654444444444000055544444445455444054024044204Mean SD -

545420142444222444424454445654544454414444444144566545665444000554544444666655566655655442344455045444446544044440440040666646655444455645444444044444421042666445556655000004445454544444204566044545444444654444444455666444544444666654444444Mean SD =

45444442445444454454544400022524444400224454424044442120424523445544444410442545665400445544404401445444444402444014424214 1444454444454444444444454500444444

SD

score

1.861.830.792.090.622.151.521.732.230.671.940.991.880.731.912.492-671.522.190.451.971.63

1.601.080.671.160.852.000.511.051.412.050.930.661.650.752.231.651.260.660.900.961.20

0.730.451.831.831.530.791.891.941.441.621.240.281.67

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AVIAN SEASONALLY IN THE PARAGUAYAN CHACO...

Table 1 (continuation)

Rufous-legged Owl Strix rujipes(n=14)

HYDROPHIL1C SPECIESRinged Teal Calonetta leucophrysWhist l ing Heron Syrigma sibilalrixGreat Egret Casmerodius albusBlack-crowned Night-heron Nycticorax nyciii-orax(n=4)

DETRIVORESBlack Vulture Caragyps atratusTurkey Vulture Calhartes auraCrested Caracara Polyborusplancus(n=3)

NECTARIVORESGlittering-bellied Emerald Chlorostilbon aureoventris( n = l )

+Taxonomy follows Hayes 1995.

404440454442Mean SD =

1.651.33

654446666544444444444454400050445544004455544000Mean SD =

444444445544444445544444

654555654444Mean SD =

455640444444Mean SD =

0.950.282.192.311.43

0.380.380.75

0-50

1.411.41

mosaic of hab i ta t types, including twotajamars. This was complemented by an av-erage of 225 min of observation from one ofthree blinds daily. Two of the blinds werelocated in quebracho woodland at feedingsites baited primarily with succulent cactus,squash and corn. The third blind was ele-vated approximately 9 m off the ground andlocated next to a mulberry tree where manypasserine species foraged.

Although these methods accounted formost of the species present in the study,longer road transects through all habitatswere employed to increase the samplingarea. Road transects were easily performedin the relatively open central Chaco, in con-trast to more closed forest where many spe-cies would go undetected. The predominanthabitats along road transect 1 (RTl) includ-ed quebracho woodland, agrarian pasture,and grassland, a l though forest edge andsome tajamares were also present. In addi-tion to the habitats represented along RTl,road transect 2 (RT2) contained one of thelargest, most contiguous tracts of forest inthe study area. This forest was sampled bydirect scanning to insure that forest specieswere adequately accounted for. RTl wassampled weekly and involved 70 km surveysconducted through eastern Toledo to Fil-

adelfia and back. RT2 was sampled month-ly and involved surveys extending 9.3 kmthrough western Toledo. Approximately onestop per survey was averaged to identify spe-cies that were not immediately recognizable.Birds would occasionally retreat to cover(e.g., deeper into the brush) before it waspossible to identify the species. These indi-viduals were excluded from the data.

Weather elements often trigger in-creased or decreased reproductive or forag-ing activity that could alter detectability ofsamples resulting in overcounted or missedindividuals (Robbins 1981). To test wheth-er such biases in detectability occurred, abi-otic variable data were collected to corre-late with abundance of species that werepresent at Toledo year-round, without ranksof 0 or 1 for any given month. Temperaturewas recorded using a standard high-low cel-cius thermometer, rainfall was recorded inmillimeters using a standard rain gauge,cloud cover (clear = 1, partly cloudy = 3,cloudy = 5, overcast = 7, or rainy = 9) andrelative wind velocity (stagnant = 1, occa-sional light breeze = 3, consistent light wind= 5, or windy = 7) were recorded an aver-age of five times per day. Monthly meanswere obtained for temperature, cloud cover,and relative wind velocity; a monthly total

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DANIEL M. BROOKS

was obtained for rainfall. An intercorrelat-ed suite of these four abiotic factors wascomputed with principal component analy-sis (PCA) using Pearson product-momentcorrelations with the computer programSYSTAT (Wilkinson 1986). PCA scores foreach month were calculated using the first

pr inciple component which accounted for55% of the total variation among the fourvariables. Spearman rank correlations wereused to measure the effects of these abioticfactors upon detectability. Each abiotic vari-able was paired with abundance of each spe-cies that was present year-round (Table 2)

Table 2. Rarer Species at Toledo with Insufficiem Data for Analyses*

GUILD

Species

Months

SONDJFMAMJJA

INSECTIVORESLittle Nightjar Caprimulgus parvulusScissor-tailcd Nightjar Hydropsalis brasilianaAshy-lailcd Swift Chaelura andreiWhite-fronted Woodpecker Melanerpes cactorumCheckered Woodpecker Picoidex mixtusLincalcd Woodpecker Dryocopux linealuxBlack-bodied Woodpecker Dryocopus schulziCream-backed Woodpecker Campephihis IcucopogonScimitar-billed Woodcrccper Drymornts bridgesiiYe I low-throated Spinctail Certhiaxis cinnamomeaFirewood-Gatherer Anumblus annumbiRufous Cacholotc Pseudaselsura cristalaGreat Antshrikc Tarubu majorBarred Antshrikc Thamnophilus dolialusVariable Antshrikc Thamnophilus caemlescensStripe-backed Antbird Myrmorchilus strigilalusOlive-crowned Crescent-chest Mclanoptireia maximillianiPearly-vented Tody-tyrant Hemitritcus margaritaceiventerGreater Wagtail-tyrant Stigmaturu budyioidexVcrmillion Flycatcher Pyracephalus rubinusBlack-crowned Monjita Xolmis coronalaBrown-crested Flycatcher Myiarchus lyrannulusVariegated Flycatcher Empidonomns variusStreaked Flycatcher Myiodynasles maculalusPiratic Flycatcher Legatus leucophaiusCrested Bccard Pachyramphus validusRufous-browed Pcppcrshrikc Cyclarhis gujanensisCreamy-bellied Thrush Turdus amaurochallnusHouse Wren Troglodytes aeodanCreamy-bellied Gnatcatchcr Polioptila lacteaSouthern Martin Prague modes faGray-breasted Martin Progne chalybeaBarn Swallow Htrundo rusticaTropical Parula Parula pitiayumiTroupial Icterus icterus(n-35)

HYDROPHILIC SPECIESWhite-tufted Grebe Rollandia rollandLeast Grebe Tachybaplux ilominicuxPied-billed Grebe PodilymbuspodicepsNcotropic Cormorant Pkalacrocorax brasilianusSouthern Screamer Cfiauna torqitataWhite-faced Whisl l ing-duck Dendrocygna viduataMasked Duck Oxyuru ilominictiMuscovy Duck Cairiiia. muscfiataComb Duck Sarkldiornis melanatosBrazi l ian Teal Amazaiietia brasilienxisSnowy Lyrel Kgn'ttci ilmla

000000040001102000000000000000640000000020200100022240202442020000022000040020022420020100444052044200002522000000002000000000004000000000000100000000004241002000000442000000004402000040000020000000004000000200002454000400000400440000044224000000000002444000000000040400000000004444400000000000002000000200200000020000020242020000004444046444000220044400000000400000000000000100000000000020000000000000200202400000002000

000000032000200000000540000004444024330440100000000240000200000004000000300000030000330442422000000044000000000000440000025000100000

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AVIAN SEASONALITY IN THE PARAGUAYAN CHACO...

Table 2 (continuation)

White-necked Heron Ardea cocoiStriated Heron Butorides striatusBare-faced Ibis Phimosus infuscatusRoseate Spoonbill Ajaja ajajaWood Stork Mycteria americanaMaguari Stork Cicania maguariJabiru Stork Jabiru mycteriaGiant Wood-Rail Aramides ypecahaPurple Gallinule Porphyrio martinicaSpot-flanked Gallinule Gallinula melanopsWhite-winged Coot Fu/ica leucopteraLimpkin Aramus guaraunaWattled JacanzJacanajacanaUpland Sandpiper Bartramia longicaudaSolitary Sandpiper Tringa solttariaPectoral Sandpiper Calidris melanotosRinged Kingfisher Ceryle torquata(n=28)

FAUNIVORESCattle Egret Bubutcus ibisPearl Kite Qatnpsonyx swainsoniiMississippi Kite Iclinia missiasippiensisRufous-thighed Hawk Accipiter erythronemiusCrane Hawk Geranospiza caerulescensHarris' Hawk Parabuteo uncinctusBlack-collared Hawk Busarellus nigricollisBlack-chested Buzzard-eagle Geranoaetus melanoleucusLaughing Falcon Herpetotheres cachmnamGreater Ani Crolophaga major-Barn Owl Tyto albaGreat-horned Owl Bubo virginianusBurrowing Owl Athene cuniculariaPlush-crowned Jay Cyanocorax chrysops(n=13)

GRANIVORESRuddy Ground-dove Columbina lalpacoliHooded Siskin Carduelis magellanlcaBlack-capped Warbling-finch Poospiza melanoleitcaBlue-black Grasquit VblalinajacarinaLined Scedcater Sporophila lincolaGrayish Saltator Saltalor coerulescens(n-6)

FRUGIVORESRed-eyed Virco Vireo olivaceusWhite-lined Tanagcr Tachyphonus ru/usSayaca Tanager Thraupis sayucaBlue-and-yellow Tanager Thraupis bonariensisPurple-throated Euphonia Euphonia chloroticu(n-5)

DETRIVORESKing Vulture Sarcoramphus papa(n-l)

NECTARIVORESBlue-tufted Starthroat HeUomaster furcifer(n-l)

000024100010003440400000000000000010002000000000003214420000000012000000000000000004000020000000000043400000000032000300010000000000000022320000000014444300001000000000000002000000554440100002000020300000

000000000014000020020010000000002000000000000033434404202142000002000004000000002200000000000100002000242454000000020000114000000000000000040202200000040202000000110050

205541440000420000000001000040004454002004400000000244500000000000400000

001002000000000000002000054422202202044220000342000020000000

000000002141

000444420000

+Taxonomy follows Hayes 1995. Progne modesta is not included in his list. Asturina nitida and Salta!or maximuswere tentatively not included in the above list until species designation is further verified.

198

DANIEL M. BROOKS

over time (n=12 months) using STAT-GRAPHICS (STSC 1986). The alpha levelwas set at 0.02 to control for bias due to TypeII error.

SEASONALLYMonthly abundance ranks were obtained

for all species. Species were divided into twogroups: abundant species (defined as thosespecies having an abundance rank of 4-6 forat least 6 months - Table 1), and rarer spe-cies (all other species - Table 2). Ranks forabundant species in Table 1 (i.e., specieswith sufficient data for analysis) were sub-jected to standard deviation (SD) computa-tion using a TI-35X statistical calculator(Texas Instruments 1992) to measure seaso-nality.

All species were grouped into guildsbased upon primary diet (insectivores, fauni-vores, detrivores, granivores/foliovores, fru-givores, and nectarivores) or water depen-dence (hydrophilic species) from direct fieldobservations supplemented with informationfrom Hilty & Brown 1986, Ffrench 1980,and Terborgh et al. 1990. Means of SDswere obtained for each guild in Table 1 toassess how seasonality is constrained by lim-iting resources. The higher the mean SDvalue, the more seasonal variation exhibit-ed by a guild for a particular resource.

Chi-square tests (Sokal & Rohlf 1969)were used to test for significant differencesbetween numbers of guild members in abun-dant (Table 1) versus rare (Table 2) species.Significant differences would reflect ecolog-ical and evolutionary processes (e.g., re-source distribution, competition, etc.) thatinfluence species packing mechanisms with-in guilds, to be entertained in the discus-sion to follow.

RESULTS

Of the 24 species (16% of the commu-nity) represented at Toledo year-round, theonly species significantly correlating withabiotic factors were the Guira Cuckoo withcloud cover (r=.737, P=.015), and Golden-bil led Saltator wi th ra infal l ( r—.825,

P=.006) and the abiotic suite of variables(r=.798, P=.008) (Table 3). Because only twospecies were significantly correlated withthree factors, detectability was not stronglybiased due to behavioral cues triggered byweather elements.

The avian community at Toledo is com-prised of 152 species in 47 families. Num-ber of species belonging to each guild is asfollows: 21 insectivores, 20 granivores andfoliovores, 14 faunivores, 4 hydrophilic spe-cies, 3 detrivores and 1 nectarivore in theabundant species group (Table 1); 35 insec-tivores, 28 hydrophilic species, 13 fauni-vores, 6 granivores, 5 frugivores, and 1 eachfor detrivores and nectarivores in the rarespecies group (Table 2).

Insectivores show the strongest season-ality (SD = 1.63) followed by hydrophilicspecies (SD= 1.43), nectarivore (SD = 1.41),faunivores (SD = 1.33), granivores and fo-liovores (SD - 1.20), and detrivores (SD =0.50) (Table 1).

Results of Chi-square tests indicated thatdifferences between numbers of abundantversus rare insectivores (X2 = 3.25, P < 0.01),granivores (X2 - 8.25, P < 0.005), and hy-drophilic species (X2 - 20.15, P « 0.005)were highly significant. In contrast, analy-ses were not performed for faunivores, de-trivores, and nectarivores due to similarnumbers or low sample size.

DISCUSSION

The most diverse families in this studywere also the most diverse families at anArgentine Chaco locality. The number ofspecies follow each family name parenthet-ically for Paraguay (this study) and Argen-tina (Capurro & Bucher 1988), respective-ly, as follows: Tyrannidae (17, 24), Ember-izidae (13, 18), Accipitridae (13, 12), andFurnariidae (8, 11).

SPECIES CONSUMING INSECTSInsectivores are the most speciose guild

in both abundant (21) and rare (33) speciesgroups, although number of abundant insec-tivores versus number of abundant grani-

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AVIAN SEASONALLY IN THE PARAGUAYAN CHACO...

Table 3. Avian Species Present Year-Round at Toledo.

Spotted TinarnouGreater RhcaRing TealWhistling HeronPlumbeous IbisBuff-necked IbisBlack VultureTurkey Vulture

Savannah HawkCrested CaracaraBlack-legged SericmaPicazuro PigeonPicui Ground-doveMonk ParakeetBlue-fronted ParrotGuira Cuckoo*

Narrow-billed WoodcrceperLark-like BrushrunncrCattle TyrantMasked GnatcatchcrRed-crested CardinalGolden-billed Saltator*Bay-winged CowbirdShiny Cowbird

* Significant correlations with P<.02 (to control for type II error) were found only with Guira Cuckoo andcloud cover (r=.737, P=.015), and Golden-billed Saltator with rainfall (r= -.825, P=.006) and the abioticsuite of variables (r=.798, p=.008).

vores (20) is virtually indistinguishable.Insectivore abundance may result from

insects being an evenly distributed resourceat Toledo (pers. obs.). Despite even distri-bution, insects are often a thinly distributedresource in Neotropical environments, re-sulting in increased territoriality and com-petition among insectivores (e.g. Snow1976). Such competitive forces can yieldmore "supertramp" species (superior dis-persers, inferior competitors; Diamond1975) within the community reflected by thesignificantly higher number of rare species.However, it is possible that "rescue effect"(Brown & Kodric-Brown 1977) occurs tem-porally with incoming migrants replacingconspecific migrants that are leaving (seediscussion below).

These findings are concordant withKarr's (1976) hypothesis that insectivoresshow the most seasonally of all guilds.Additionally, Avery & Van Riper (1989) at-tributed an insectivore-dominated commu-nity to a spatially broad array of insect dis-t r ibu t ion wi th in Ca l i fo rn ia woodlands,where insects occupy a variety of niches.

SPECIES CONSUMING PLANT PARTSThe number of granivores decreases dra-

matically from the abundant (20) to rare (6)species groups. Species such as Ortalis con-sume more foliage than seed parts (e.g.,Caziani & Protomastro 1994), but such fo-liovores comprise a small subset of the abun-dant species group.

The high number of abundant granivo-rous species that co-occur compared to the

low number of rare species may be a conse-quence of plant parts not being a spatiallypredictable resource, permitting higher lev-els of coexistent with minimal competition.It is not beneficial for birds to defend terri-tories if the food plants may not bloom with-in that territory. Rather, it is beneficial forspecies to share resource clumps, synchro-nously or asynchronously.

The results herein are concordant withthe findings of other studies in the Argen-tine Chaco (Capurro & Bucher 1982), Monte(Marone 1992) and Venezuelan xeriscape(Poulin et al. 1993) where a positive rela-tionship exists between density of granivoresand seeds. Moreover, Capurro & Bucher(1982) found no correlation between diver-sity of granivores and seeds in the Argen-tine Chaco, in contrast to the correlationbetween diversity of insectivores and insects,suggesting considerable dietary overlapamong sympatric granivores.

FRUGIVORESThe unpredictable blooming strategy of

fruit attributes to the low number of frugiv-orous species (5) included in the rare spe-cies group (none present in the abundantspecies group) . Similar ly , Poulin et al.(1993) speculated that a temporally patchypresence of fruit attributes to most frugi voresbeing transients.

NECTARIVORESA similar situation is revealed by the low

numbers of nectarivores ( I species abundant,1 rare). Hummingbirds specialize on repro-

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DANIEL M. BROOKS

ductive plant parts that are not temporallypredictable, attributing to the low diversityat Toledo (N=2 species). Hummingbirds in-crease during the rainy season when necta-ry sources increase at different sites in theNeotropics (e.g., Toledo & Venezuelan Man-groves, Lefebvre et al. 1994),.

WA TER-DEPENDENT SPECIESHydrophilic species are significantly

more speciose in the rare species group (28species) than the abundant group (4 species),attributable to the stochastic and xeric na-ture of the Chaco. Permanent water is a spa-tio-temporally unpredictable resource atToledo, with only one tajamar containingwater throughout the year. Nonetheless thistajamar would reach a depth of < 1 m dur-ing the drier periods versus > 4 m duringextensive showers in the rainy season. To-ledo is located virtually in the center of theChaco, which is centered in the continent,and is surrounded by several major aquaticsystems: the Pilcomayo River to the south-west, the Paraguay River to the east, and thevast Pantanal wetland to the north. Numer-ous aquatic species may stop-over briefly ata tajamar or other "staging area" (Myers1983) in transit from one region to the next,reflecting the high number of rare speciesversus abundant species.

Hayes & Fox (1991) suggest that theevolution of migration for certain hydro-philic species (e.g., shorebirds) is influencedby seasonal precipitation cycles and the ef-fects on habitat availability.

SPECIES CONSUMING MEATFaunivores represent a guild with rela-

tively l i t t le variation between abundant (14)and rare (13) species. Hayes (1991) foundthat raptor abundance is most likely influ-enced by availability of preferred prey andforaging strategy. Raptors are importantkeystone species as they have a strong in-f luence on prey popu la t ions (Robinson1994). Detrivores showed less than 1/3 theSD as that of the most seasonal group (in-scctivores), attributable to a constant sup-ply of road-killed carcasses.

FINAL COMMENTSResource tracking plays a vital role in

shaping the community, through resource"explosions" (e.g., f rui t ) and seasonalchanges that affect resource abundance (e.g.,water) in one area, forcing the consumer tomove to another. Year-to-year variation infood availability may have a significant im-pact on species abundance (Karr 1976). Al-though data were collected for a continuousyear, it is possible that at least some of thespecies in this study are typically more, orless, abundant than during this particularyear of data collection.

Birds exhibiting seasonality in one re-gion may occur year-round or during dif-ferent parts of the year in other regions.Moreover, northern austral migrants can bereplaced by incoming conspecific southernaustral migrants and vice-versa in certaincases where South American species haveextensive latitudinal ranges. In such instanc-es it is more difficult to detect idiosyncra-sies of seasonafity at the local level. None-theless the importance of documenting sea-sonality at specific sites can not be over-emphasized because birds may occur year-round when including samples from sever-al different sites as a data set.

Habitat may play an important role indetermining which guild is the most diversein a community. For example, in a CostaRican tropical, mesic forest insectivoreswere the most speciose guild in the under-story, whereas frugivores dominated the can-opy (Loiselle 1988).

ACKNOWLEDGMENTS

The companionship of numerous individualsin Paraguay, including Eddie and Sonja Mueller,the Unger family, Chris Yahnkc, Manilla Gamma-rra ile Fox, Flavio Co/man, Chaco Solar caballe-ros (Eduardo and Carlo), and some oj the localMcnnoniies is hardly forgot fen. Thanks to my ja-mify for mutually supporting my interest. I am gra-teful to K. Kleypasfor computing SD values. Cons-tructive comments on this manuscript, or previousversions of this manuscript were provided by T.

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AVIAN SEASONALITY IN THE PARAGUAYAN CHACO...

Chesser, F. Hayes, S. Mayes. and an anonymous

reviewer. Financial support was provided by Dr.

Kurt Benirschke and the Foundation for Endan-gered Animals. Local logistics were provided bythe Zoological Society of San Diego through su-pport of Proyecto Tagud.

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