Estacionalidad en las aves del Chaco Paraguayo central Brooks, D. M. 1997

Cita: Brooks, D. M. (1997) Estacionalidad en las aves del Chaco Paraguayo central. 014 (04) : 193-203

www.digital.bl.fcen.uba.ar Puesto en linea por la Biblioteca Digital de la Facultad de Ciencias Exactas y Naturales Universidad de Buenos Aires Hornero 14: 193-203, 1997

AVIAN SEASONALITY AT A LOCALITY IN THE CENTRAL PARAGUAYAN CHACO

DANIEL M. BROOKS Texas A&M University; Department ol Wildlije and Fisheries Sciences; Texas Cooperative Wildlije Col/ections; Col/ege Station, Texas 77843. USA. E-Mail: [email protected]

ABSTRACT. Abundances of different species of were recorded in the central Paraguayan 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 or water dependence. iThe number of species (abundant : rare) in each guild is as fol- lows: insectivores (21 :35), granivores and foliovores (20:6), faunivores (14: 13), hy- drophilic species (4:28), detrivores (3: 1), nectarivores (1: 1), and frugivores (0:5). Insectivores show the strongest seasonality (SD = 1.63) folIowed by hydrophilic spe- cies (SD = 1.43), nectarivore (SD = 1.41), faunivores (SD = 1.33), granivores and foliovores (SD = 1.20), and detrivores (SD = 0.50). Chi-square tests indicated that differences between numbers of abundant versus rare insectivores (P < 0.0 1), granivores (P < 0.005), and hydrophilic species (P « 0.005) were highly significant. Results are intrepreted in light of ecological and evolutionary processes. Key words: seasonality, resources, avian community, Chaco,

Estacionalidad en las aves del Chaco Paraguayo central

RESUMEN. Registré la abundancia de especies de aves en el Chaco Paraguayo cen- tral, Agosto de 1989 hasta Agosto de 1990, para investigar la variación estacional al nivel de grupo funcional o gremio (guild). Las características usadas para definir grupos funcionales fueron la dependicia del agua o la dieta primaria. El numero de especies (abundancia: raro) por grupo funcional son: insectívoras (21 :35), granívo- ras and folívoras (20:6), faunívoras (14:13), especies asociadas al agua (4:28), detri- tívoras (3: 1), nectarívoras (1: 1), y frugívoras (0:5). El grupo con la variación estacio- nal mayor fue las insectívoras (SD = 1.63), siguiéndoles las especies asociado al agua, (SD = 1.43), las nectarívoras (SD = 1.41), faunívoras (SD = 1.33), granívoras and folívoras (SD = 1.20), y detritívoras (SD = 0.50). Pruebas de chi cuadrado indican que la diferencia entre especies abundantes y raras son muy significativas para las insec- tívoras (P < 0.01), granívoras (P < 0.005), Yespecies asociadas al agua (p« 0.005). Los resultados se interpretan en términos de procesos, de ecología y evolución. Palabras clave: estacionalidad, recursos, comunidad 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 , western in the central portion of South America Paraguay, and northern , The area where several different neotropical biomes is characterized by low avian endemism bio-

Recibido el 20/02/96. Aceptado el 29/01/97

- 193 - AVIAN SEASONALITY IN THE PARAGUAYAN CHACO ••• geographically (Short 1975) and is an ef- itat in the vicinity of To1edo is a mosaic of fective barrier to forest birds, but not wood- "quebracho" wood1and and grassland (Short land or grassland birds (Nores 1992, Hayes 1975), characterized by thorny bushes, 1995). shrubs, and cacti, with scattered trees up to The Chacoan avifauna was documented 13 m high. Prosopis ruscifolia, a thorny le- by Short (1975), who concentrated on bio- gume, and Opuntia sp. cactus are the domi- geographic aspects. Since then several stud- nant species (Lópezet al. 1987). Isolated ies have been undertaken on birds in the tracts of thick, impenetrable, thorny forest Paraguayan Chaco (e.g., Short 1976; Short are sometimes left when land is being 1980; Contreras & Mandelburger 1985; cleared for agrarian purposes. The under- Contreras & Gonzalez-Romero 1989; story in such forest consists of thorny Bro- Gonzalez-Romero & Contreras 1989; Hayes melia serra and Cleistocactus baumanii et al. 1990; Peris 1990; Peris et al. 1987; (Stab1er 1985). Tajamares (man-made, sea- Neris & Colman 1991; Hayes et al. 1991; sonal ponds) and filled gulleys from mas- Brooks 1991, 1995, 1997; Hayes 1995). Al- sive rains are present throughout the study though some of these studies have investi- area. gated seasonal variation for specific assem- blages such as shorebirds and waterbirds, SAMPLING METHODS none have attempted to investigate season- This study took place from August 1989 al variation for an entire avian community to August 1990. Narosky & Yzurieta (1987), at the guild level (Hayes pers. comm.). The Meyer de Schauensee (1982), and Dunning objective of this paper is to determine, the (1987) were used to identify unknown spe- role oflimiting resources in influencing sea- cies. Abundance data were obtained from sonality of birds from a site in the semi-xe- observations of live birds and were ranked ric Paraguayan Chaco. numerically using a standardized scale for Seasonal variation can be defined as all taxa. During some months certain spe- variation in annual abundance. Many tropi- cies were absent from Toledo proper but cal environments are sharply seasonal, and present within the 35 km radius of the cen- associated with rainfall rather than temper- terpoint, so a special rank (1) was used to ature variations. These changes affect habi- indicate such situations oflocal movements. tat structure and food supplies, and one Additionally, ranks of 2 and 3 were indica- would expect the species to respond tive of sing1etons being present rather than (Wiens 1989). Klopfer (1959) suggested that multiple individuals, reflecting transitory where seasona1 environmenta1 fluctuations movements or a low point during migration. are minimal, the type of cover, nesting sites, The following monthly scale was used: O= and food which are available remain fairly absent: not observed during a given month; constant. 1 = local movements: observed within 35 km ofthe study area's centerpoint but not at Toledo proper; 2 = month1y transient indi- METHODS vidual: observation of one individual per month persisting in study area no more than STUDYAREA 24 hrs.; 3 = monthly resident individual: Species included in this study (Table 1) observation of one individual per month were found within a 35 km radius of Estan- persisting in study area more than 24 hrs; 4 cia Fortín Toledo proper (hereafter, referred = uncommon: two to five individual s ob- as Toledo) (22°33'S,60030'W), Department served per survey day; 5 = common: .six to Boquerón, 35 km W of the Mennonite Col- nine individuals observed per survey day; 6 ony, Filadelfia. This area, like much of the = abundant: ten or more individuals ob- central Paraguayan Chaco, has been exten- served per survey day. sivelycleared for cattle production (Benir- Data were collected by walking an av- schke et al. 1989). The second-growth hab- erage of 1.75 km oftransect daily through a

- 194 - DANIEL M. BROOKS

TaMe 1. Abundan! Species a! Toledo+ GUlLO Months SO0.990.672.050.930.852.152.232.670.750.792.092.000.511.891.240.620.730.450.662.492.190.280.960.901.051.411.521.861.831.941.911.651.081.631.601.161.731.881.971.441.261.531.621.201.67score 444421204245444000055544445654544454045544440241654444444455014454444444045444446544044545444444222444424454666445556655000004445544444424445544000222444444465545400000454444444444004455444044002244544240234455444444024440144242Mean644555656544566545665444044440440040666654444444454500444444044544440000000004445454066554400000054024044204045444444200666444544444020004454440000225244444444544545444141444454444455645444444654454444554065654444444414444444144044444421042066445444044444445455444544444204566444344444244666655566655144445444454666646655444000444445544064245444454000554544444655442344455104425456654 SD= Species SONOJFMAMJJA 000445444242 INSECTIVORES 454444424454545420142444 Dark-billed Cuckoo Coccyzus melacoryphus Smooth-billed Ani Crotophaga ani Guira Cuckoo Guira guira White Woodpecker Leuco"erpes candidus Narrow-billed Woodcreeper Lepidocolaptes angustirostris Furnarius rufus Crested Hornero Furnarius cristatus Chotoy Spinetail Schoeniophylax phryganophila Little Thornbird Phacellodomus sibilatrix Lark-like Brushrunner Coryphistera alaudina Small-billed Elaenia Elaenia parvirostris White Monjita Xolmis irupero Black-backed Water-tyrant Fluvicola albiventer Cattle Tyrant Machetornis rixosus Tropical Kingbird Tyrannus melancholicus Fork-tailed Flycatcher Tyrannus savana Crowned-slaty Flycatcher Griseotyrannus aurantioatrocristatus Great Kiskadee Pitangus sulph uratus White-banded Mockingbird Mimus triurus Masked Gnatcatcher Polioptila dumicola Epaulet Oriole lcterus cayanensis (n=21)

GRANIVORES AND FOLIOVORES Brushland Tinamou Nothoprocta cinerascens Spotted Tinamou Nothura maculosa Greater Rhea Rhea americana Chaco Chachalaca Ortalis canicollis Picazuro Pigeon Columba picazuro Eared Dove Zenaida auriculata Picui Ground-dove picui White-tipped Dove Leptotila verreauxi Blue-crowned Parakeet Aratinga acuticaudata Nanday Parakeet Nandayus nenday Monk Parakeet Myiopsitta monachus Blue-fronted Parrot Amazona aestiva House Sparrow Passer domesticus Red-crested CardinalParoaria coronata Many-colored Chaco-finch Saltatricula multicolor Red-crested Finch Coryphospingus cucullatus Saffron Yellow-Finch Sicalisjlaveola Golden-billed Saltator Saltator aurantiirostris Bay-winged Cowbird Molothurus badius Shiny Cowbird Molothurus bonariensis (n=20)

FAUNIVORES Plumbeous Ibis Theristicus caerulescens Buff-necked Ibis Theristicus caudatus White-tailed Kite Elanus leucurus Snail Kite Rostrhamus sociabilis Great Black-Hawk Buteogallus urubitinga Savannah Hawk Buteogallus meridionalis Roadside Hawk Buteo magnirostris White-tailed Hawk Buteo albicaudatus American Kestrel Falco sparverius Aplomado Falcon Falco femoralis Red-Iegged Seriema Cariama cristata Black-Iegged Seriema Chunga burmeisteri Southern Lapwing Vanellus chilensis

195 - AVIANSEASONALITYINTHEPARAGUAYANCHACO...

Table 1 (continuation)

Rufous-Iegged Owl Strix rulipes 404440454442 1.65 (n=14) Mean SO= 1.33

HYOROPHILIC SPECIES Ringed Teal Calonetta leucophrys 654446666544 0.95 Whistling Heron Syrigma sibilatrix 444444444454 0.28 Great Egret Casmerodius albus 400050445544 2.19 Blaek-erowned Night-heron Nycticorax nycticorax 004455544000 2.31 (n=4) Mean SO= 1.43

OETRIVORES Blaek Vulture Coragyps atratus 444444445544 0.38 Turkey Vulture Cathartes aura 444445544444 0.38 Crested Caracara Polyborus plancus 654555654444 0.75 (n=3) Mean SO = 0.50

NECTARIVORES Glittering-bellied Ernerald Chlorostilbon aureoventris 455640444444 1.41 (n=l) Mean SD= 1.41

+Taxonorny follows Hayes 1995.

mosaic of habitat types, including two adelfia and back. RT2 was sampled month- tajamars. This was complemented by an av- ly and involved surveys extending 9.3 km erage of 225 min of observation from one of through western Toledo. Approximately one three blinds daily. Two of the blinds were stop per survey was averaged to identify spe- located in quebracho woodland at feeding cies that were not immediately recognizable. sites baited primarily with succulent cactus, Birds would occasionally retreat to cover squash and corno The third blind was ele- (e.g., deeper into the brush) before it was vated approximately 9 m offthe ground and possible to identify the species. These indi- located next to a mulberry tree where many vidualswere excluded from the data. species foraged. Weather elements often trigger in- Although these methods accounted for creased or decreased reproductive or forag- most of the species present in the study, ing activity that could alter detectability of longer road transects through all habitats samples resulting in overcounted or missed were employed to increase the sampling individual s (Robbins 1981). To test wheth- area. Road transects were easily performed er such biases in detectability occurred, abi- in the relatively open central Chaco, in con- otic variable data were collected to corre- trast to more closed forest where many spe- late with abundance of species that were cies would go undetected. The predominant present at Toledo year-round, without ranks habitats along road transect 1 (RTl) includ- of O or 1 for any given month. Temperature ed quebracho woodland, agrarian pasture, was recorded using a standard high-Iow cel- and grassland, although forest edge and cius thermometer, rainfall was recorded in some tajamares were also present. In addi- millimeters using a standard rain gauge, tion to the habitats represented along RT 1, cloud cover (clear = 1, partly cloudy = 3, road transect 2 (RT2) contained one of the cloudy = 5, overcast = 7, or rainy = 9) and largest, most contiguous tracts of forest in relative wind velocity (stagnant = 1, occa- the study area. This forest was sampled by sionallight breeze = 3, consistent light wind direct scanning to insure that forest species = 5, or windy = 7) were recorded an aver- were adequately accounted for. RTl was age of five times per day. Monthly means sampled weekly and involved 70 km surveys were obtained for temperature, cloud cover, conducted through eastern Toledo to Fil- and relative wind velocity; a monthly total

- 196 - DANIEL M. BROOKS was obtained for rainfall. An intercorre1at- principie component which accounted for ed suite of these four abiotic factors was 55% of the total variation among the four computed with principal component analy- variables. Spearman rank correlations were sis (PCA) using Pearson product-moment used to measure the effects of these abiotic correlations with the computer program factors upon detectability. Each abiotic vari- SYSTAT (Wilkinson 1986). PCA scores for able was paired with abundance of each spe- each month were calculated using the first cies that was present year-round (Table 2) rabie 2. Rarer Species al roledo with Insufficienl Dalafor Analyses+ GUILD Months

Species SONDJFMAMJJA INSECTIVORES Little Nightjar Caprimulgus parvulus 000000040001 Scissor-tailed Nightjar Hydropsalis brasiliana 102000000000 Ashy-tailed Swift Chaetura andrei 000000640000 White-fronted Woodpeckcr Melanerpes cactorum 000020200100 Checkered Woodpecker Picoides mixtus 022240202442 Lineated Woodpecker Dryocopus lineatus 020000022000 B1ack-bodied Woodpccker Dryocopus schulzi 040020022420 Cream-backcd Woodpecker Campephilus leucopogon 020100444052 Scimitar-billed Woodcreeper Drymornis bridgesii 044200002522 Yellow-throated Spinetail Certhiaxis cinnamomea 000000002000 Firewood-Gatherer Anumbius annumbi 000000004000 Rufous Cacho lote Pseudoseisura cristata 000000000100 Great Antshrike Taraba major 000000004241 Barred Antshrike Thamnophilus doliatus 002000000442 Variable Antshrikc Thamnophilus caerulescens 000000004402 Stripe-backcd Antbird Myrmorchilus strigilatus 000040000020 Olive-crowned Crescent-chest Melanopareia maximílliani 000000004000 Pearly-vented Tody-tyrant Hemitriccus margaritaceiventer 000200002454 Greater Wagtail-tyrant Stigmatura budytoides 000400000400 Vermillion Flyeatcher Pyrocephalus rubinus 440000044224 Black-crowned Monjita Xolmis coronata 000000000002 Brown-crested Flycatcher Myiarchus tyrannulus 444000000000 Variegated Flyeatcher Empidonomus varius 040400000000 Streaked Flycatcher Myiodynastes maculatus 004444400000 Piratic Flyeatcher Legatus leucophaius 000000002000 Crested Becard Pachyramphus validus 000200200000 Rufous-browcd Peppershrike Cyclarhis gujanensis 020000020242 Creamy-bellied Thrush Turdus amaurochalinus 020000004444 House Wren Troglodytes aeodon 046444000220 Creamy-bellied Gnatcatcher Polioptila lactea 044400000000 Southern Martin Progne modesta 400000000000 Gray-breasted Martin Progne chalybea 000100000000 Barn Swallow Hirundo rustica 000020000000 Tropical Parula Parula pitiayumi 000000200202 Troupial lcterus icterus 400000002000 (n=35)

HYDROPHILlC SPECIES White-tufted Grebe Rollandia rolland 000000032000 Least Grebe Tachybaptus dominicus 200000000540 Pied-billed Grebc Podilymbus podiceps 000004444024 Neotropic Cormorant Phalacrocorax brasilianus 330440100000 Southern Screamer Chauna torquata 000240000200 White-faced Whistling-duck Dendrocygna viduata 000004000000 Masked Duck Oxyura dominica 300000030000 Muscovy Duek Cairina moschata 330442422000 Comb Duck Sarkidiornis melanotos 000044000000 Brazilian Teal Amazonetta brasiliensis 000000440000 Snowy Egret Egretta thula 025000100000

197 - AVIAN SEASONALlTY IN THE PARAGUAYAN CHACO ...

Table 2 (continuation)

White-necked Heron Ardea cocoi 000024100010 Striated Heron Butorides striatus 003440400000 Bare-faced Ibis Phimosus injuscatus 000000000010 Roseate Spoonbill Ajaja ajaja 002000000000 Wood Stork Mycteria americana 003214420000 Maguari Stork Ciconia maguari 000012000000 Jabiru Stork Jabiru mycteria 000000000004 Giant Wood-Rail Aramides ypecaha 000020000000 Purple Gallinule Porphyrio martinica 000043400000 Spot-flanked Gallinule Gallinula melanops 000032000300 White-winged Coot Fulica leucoptera 010000000000 Limpkin Aramus guarauna 000022320000 Wattled Jacana Jacana jacana 000014444300 Upland Sandpiper Bartramia longicauda 001000000000 Solitary Sandpiper Tringa solitaria 000002000000 Pectoral Sandpiper Calidris melanotos 554440109902 Ringed Kingfisher Ceryle torquata 000020300000 (n=28)

FAUNIVORES Cattle Egret Bubulcus ibis 000000000014 Pearl Kite Gampsonyx swainsonii 000020020010 Mississippi Kite lctinia mississippiensis 000000002000 Rufous-thighed Hawk Accipiter erythronemius 000000000033 Crane Hawk Geranospiza caerulescens 434404202142 Harris' Hawk Parabuteo uncinctus 000002000004 Black-collared Hawk Busarellus nigricollis 000000002200 Black-chested Buzzard-eagle Geranoaetus melanoleucus 000000000100 Laughing Falcon Herpetotheres cachinnans 002000242454 Greater Ani Crotophaga major 000000020000 Barn Owl Tyto alba 114000000000 Great-horned Owl Bubo virginianus 000000040202 Burrowing Owl Athene cunicularia 200000040202 Plush-crowned Jay Cyanocorax chrysops 000000110050 (n=13)

GRANIVORES Ruddy Ground-dove Columbina talpacoti 205541440000 Hooded Siskin Carduelis magellanica 420000000001 Black-capped Warbling-finch Poospiza melanoleuca 000040004454 Blue-black Grasquit Volatinajacarina 002004400000 Lined Seedeater Sporophila lineola 000244500000 Grayish Saltator Saltator coerulescens 000000400000 (n=6)

FRUGIVORES Red-eyed Vireo Vireo olivaceus 001002000000 White-lined Tanager Tachyphonus rujus 000000002000 Sayaca Tanager Thraupis sayaca 054422202202 Blue-and-yellow Tanager Thraupis bonariensis 044220000342 Purple-throated Euphonia Euphonia chlorotica 000020000000 (n=5)

DETRIVORES King Vulture Sarcoramphus papa 000000002141 (n=l)

NECTARIVORES Blue-tufted Starthroat Heliomasterfurcifer 000444420000 (n=l)

+ follows Hayes 1995. Progne modesta is not included in his list. Asturina nitida and Saltator maximus were tentatively not included in the above list until species designation is further verified.

- 198 - DANIEL M. BROOKS over time (n=12 months) using STAT- P=.006) and the abiotic suite of variables GRAPHICS (STSC 1986). The a1pha level (r=.798, P=.008) (Table 3). Because only two was set at 0.02 to control for bias due to Type species were significantly correlated with II error. three factors, detectability was not strongly biased due to behavioral cues triggered by SEASONALITY weather elements. Monthly abundance ranks were obtained The avian community at Toledo is com- for all species. Species were divided into two prised of 152 species in 47 families. Num- groups: abundant species (defined as those ber of species belonging to each guild is as species having an abundance rank of 4-6 for follows: 21 insectivores, 20 granivores and at least 6 months - Table 1), and rarer spe- foliovores, 14 faunivores, 4 hydrophilic spe- cies (all other species - Table 2). Ranks for cies, 3 detrivores and 1 nectarivore in the abundant species in Table 1 (Le., species abundant species group (Table 1); 35 insec- with sufficient data for analysis) were sub- tivores, 28 hydrophilic species, 13 fauni- jected to standard deviation (SD) computa- vores, 6 granivores, 5 frugivores, and 1 each tion using a TI-35X statistical calculator for detrivores and nectarivores in the rare (Texas Instruments 1992) to measure seaso- species group (Table 2). nality. Insectivores show the strongest season- All species were grouped into guilds ality (SD = 1.63) followed by hydrophilic based upon primary diet (insectivores, fauni- species (SD = 1.43), nectarivore (SD = 1.41), vores, detrivores, granivores/foliovores, fru- faunivores (SD = 1.33), granivores and fo- givores, and nectarivores) or water depen- liovores (SD= 1.20), and detrivores (SD = dence (hydrophilic species) from direct field 0.50) (Table 1). observations supplemented with information Results ofChi-square tests indicated that from Hilty & Brown 1986, Ffrench 1980, differences between numbers of abundant and Terborgh el al. 1990. Means of SDs versus rare insectivores (X2 = 3.25, P < 0.01), were obtained for each guild in Table 1 to gran ivores (X2 = 8.25, P < 0.005), and hy- assess how seasonality is constrained by lim- drophilic species (X2 = 20.15, P « 0.005) iting resources. The higher the mean SD were highly significant. In contrast, analy- value, the more seasonal variation exhibit- ses were not performed for faunivores, de- ed by a guild for a particular resource. trivores, and nectarivores due to similar Chi-square tests (Sokal & Rohlf 1969) numbers or low sample size. were used to test for significant differences between numbers of guild members in abun- dant (Table 1) versus rare (Table 2) species. DlSCUSSION Significant differences would reflect ecolog- ical and evolutionary processes (e.g., re- The most diverse families in this study source distribution, competition, etc.) that were also the most diverse families at an influence species packing mechanisms with- Argentine Chaco locality. The number of in guilds, to be entertained in the discus- species follow each family name parenthet- sion to follow. ically for Paraguay (this study) and Argen- tina (Capurro & Bucher 1988), respective- ly, as follows: Tyrannidae (17, 24), Ember- RESULTS izidae (13, 18), Accipitridae (13, 12), and Furnariidae (8, 11). Of the 24 species (16% of the commu- nity) represented at Toledo year-round, the SPECIES CONSUMING INSECTS only species significantly correlating with Insectivores are the most speciose guild abiotic factors were the Guira Cuckoo with in both abundant (21) and rare (33) species cloud cover (r=.737, P=.015), and Golden- groups, although number of abundant insec- billed Saltator with rainfall (r=-.825, tivores versus number of abundant grani-

- 199 - AVIAN SEASONALITY IN THE PARAGUAYAN CHACO ••.

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

Spotted Tinamou Savannah Hawk Narrow-billed Woodcreeper Greater Rhea Crested Caracara Lark-like Brushrunner RingTeal Black-Iegged Seriema Cattle Tyrant Whistling Heron Picazuro Pigeon Masked Gnatcatcher Plumbeous Ibis Picui Ground-dove Red-crested Cardinal Buff-necked Ibis Monk Parakeet Golden-billed Saltator* Black Vulture Blue-fronted Parrot Bay-winged Cowbird Turkey Vulture Guira Cuckoo* Shiny Cowbird

* Significant correlations with P~.02 (to control for type 11error) were found only with Guira Cuckoo and cloud cover (r=.737, P=.OI5), and Goldco-billed Saltator with rainfall (r= -.825, P=.O(6) and the abiotic suite ofvariablcs (r=.798, p=.008).

vores (20) is virtually indistinguishable. low number of rare species may be a conse- Insectivore abundance may result from quence of plant parts not being a spatially insects being an evenly distributed resource predictable resource, permitting higher lev- at Toledo (pers. obs.). Despite even distri- els of coexistent with minimal competition. bution, insects are often a thinly distributed It is not beneficial for birds to defend terri- resource in Neotropical environments, re- tories ifthefood plants may not bloom with- sulting in increased territoriality and com- in that territory. Rather, it is beneficial for petition among insectivores (e.g. Snow species to share resource clumps, synchro- 1976). Such competitive forces can yield nously or asynchronously. more "supertramp" species (superior dis- The results herein are concordant with persers, inferior competitors; Diamond the findings of other studies in the Argen- 1975) within the community reflected by the tine Chaco (Capurro & Bucher 1982), Monte significantly higher number of rare species. (Marone 1992) and Venezuelan xeriscape However, it is possible that "rescue effect" (Poulin et al. 1993) where a positive rela- (Brown & Kodric-Brown 1977) occurs tem- tionship exists between density of granivores porally with incoming migrants replacing and seeds. Moreover, Capurro & Bucher conspecific migrants that are leaving (see (1982) found no correlation between diver- discussion below). sity of granivores and seeds in the Argen- These findings are concordant with tine Chaco, in contrast to the correlation Karr's (1976) hypothesis that insectivores between diversity ofinsectivores and insects, show the most seasonality of all guilds. suggesting considerable dietary overlap Additionally, Avery & Van Riper (1989) at- among sympatric granivores. tributed an insectivore-dominated commu- nity to a spatially broad array of insect dis- FRUGIVORES tribution within California woodlands, The unpredictable blooming strategy of where insects occupy a variety of niches. fruit attributes to the low number of frugiv- orous species (5) included in the rare spe- SPECIES CONSUMING PLANT PARTS cies group (none present in the abundant The number of granivores decreases dra- species group). Similarly, Poulin et al. matically from the abundant (20) to rare (6) (1993) speculated that a temporally patchy species groups. Species such as Ortalis con- presence offruit attributes to most frugivores sume more foliage than seed parts (e.g., being transients. Caziani & Protomastro 1994), but such fo- liovores comprise a small subset ofthe abun- NECTARIVORES dant species group. A similar situation is revealed by the low The high number of abundant granivo- numbers ofnectarivores (1 species abundant, rous species that co-occur compared to the 1 rare). Hummingbirds specialize on repro-

- 200 - DANIEL M. BROOKS ductive plant parts that are not temporal1y FINAL COMMENTS predictable, attributing to the low diversity Resource tracking plays a vital role in at Toledo (N=2 species). Hummingbirds in- shaping the community, through resource crease during the rainy season when necta- "explosions" (e.g., fruit) and seasonal ry sources increase at different sites in the changes that affect resource abundance (e.g., Neotropics (e.g., Toledo & Venezuelan Man- water) in one area, forcing the consumer to groves, Lefebvre el al. 1994),. move to another. Year-to-year variation in food availability may have a significant im- WATER-DEPENDENT SPECIES pact on species abundance (Karr 1976). Al- Hydrophilic species are significantly though data were collected for a continuous more speciose in the rare species group (28 year, it is possible that at least some of the species) than the abundant group (4 species), species in this study are typically more, or attributable to the stochastic and xeric na- less, abundant than during this particular ture ofthe Chaco. Permanent water is a spa- year of data collection. tio-temporally unpredictable resource at Birds exhibiting seasonality in one re- Toledo, with only one tajamar containing gion may occur year-round or during dif- water throughout the year. Nonetheless this ferent parts of the year in other regions. tajamar would reach a depth of < 1 m dur- Moreover, northern austral migrants can be ing the drier periods versus > 4 m during replaced by incoming conspecific southern extensive showers in the rainy season. To- austral migrants and vice-versa in certain ledo is located virtually in the center of the cases where South American species have Chaco, which is centered in the continent, extensive latitudinal ranges. In such instanc- and is surrounded by several major aquatic es it is more difficult to detect idiosyncra- systems: the Pilcomayo River to the south- sies of seasonality at the local leve!. None- west, the Paraguay River to the east, and the theless the importance of documenting sea- vast Pantanal wetland to the north. Numer- sonality at specific sites can not be over- ous aquatic species may stop-over briefly at emphasized because birds may occur year- a tajamar or other "staging area" (Myers round when inc1uding samples from sever- 1983) in transit from one region to the next, al different sites as a data set. reflecting the high number of rare species Habitat may play an important role in versus abundant species. determining which guild is the most diverse Hayes & Fox (1991) suggest that the in a community. For example, in a Costa evolution of migration for certain hydro- Rican tropical, mesic forest insectivores philic species (e.g., shorebirds) is influenced were the most speciose guild in the under- by seasonal precipitation cycles and the ef- story, whereas frugivores dominated the can- fects on habitat availability. opy (Loiselle 1988).

SPEClES CONSUMING MEAT Faunivores represent a guild with rela- ACKNOWLEDGMENTS tively little variation between abundant (14) and rare (13) species. Hayes (1991) found The companionship o/ nUmerous individuals that raptor abundance is most likely influ- in Paraguay, including Eddie and Sonja Mueller, enced by availability of preferred prey and the Unger /ami/y, Chris Yahnke, Mami/a Gamma- foraging strategy. Raptors are important rra de Fox, Flavio Colmán, Chaco Solar caballe- keystone species as they have a strong in- ros (Eduardo and Carlo) , and some o/ the local fluence on prey populations (Robinson Mennonites is hardly /orgotten. Thanks to my /a- 1994). Detrivores showed less than 1/3 the mi/y /or mutually supporting my interest. 1 am gra- SD as that of the most seasonal group (in- te/ul to K. Kleypas/or computing SD values. Cons- sectivores). attributable to a constant sup- tructive comments on this manuscript, or previous ply of road-killed carcasses. versions o/ this manuscript were provided by T

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Chesser, F. Hayes, S. Mayes, and an anonymous HAYES,F. E. 1991. Raptor densities along the Paraguay reviewer. Financial support was provided by Dr. River: seasonal, geographical and time of day varia- tion. J. Raptor Res. 25: 101-108. Kurt Benirschke and the Foundation for Endan- HAYES,F. E. 1995. Status, Distribution, and Biogeogra- gered . Local logistics were provided by phy of the Birds ofParaguay. ABA Monogr. Field the Zoological Society of San Diego through su- Ornithol Ner.:I. Colorado Springs, USA. HAYES,F. E. & J. A. Fox. 1991. Seasonality, habitat use, pport of Proyecto Taguá. and flock sizes of shorebirds at the Bahia de Asun- cion, Paraguay. Wils. Bull. 103: 637-649. HAYES,F. E., S. M. GOODMAN,& N. E. LÓPEZ.1990. New LITERATURE CITED or noteworthy bird records from the Matogrosense region of Paraguay. Bull. Brit. Ornitho\. Club 110: 94-103. AVERY,M. L.ANDC. VANRIPER1Il. 1989. Seasonal chang- HAYES,F. E.; P. A. SCHARF& H. LOFTIN.1991. A Birder's es in bird communities of the Chaparral and Blue- Field Checklist to the Birds ofParaguay. Russ' Na- Oak Woodlands in central California. Condor 91: tural History Books, Lake Helen, Florida. 288-295. HILTY,S. L. & W. L. BROWN.1986. A Guide to the Birds BENIRSCHKE,K.; M. L. BYRDANDR. J. LOWE. 1989. The ofColombia. Princeton Univ. Press, NJ, USA. Chaco region of Paraguay: pecc¡lTies and mennoni- Karr, J. R. 1976. Seasonality, resource availability, and tes. Interdisc. Sci. Rev. 14: 144-147. community diversity in tropical bird communities. BROOKS,D. M. 1991. Some notes on the ciconiiformes Am. Nat. 110: 973-995. in the Paraguayan Chaco. IUCN/IWRB Ciconiifor- KLOPFER,P.H. 1959. Environmental determinants offau- mes Spec. Grp. 4: 4-5. nal diversity. Am. Na!. 93 :337-343. BROOKS,D. M. 1995. Ecological partitioning between LEFEBVRE,G.; G. POULlN& R. McNEIL. 1994. 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