ORNITOWGIA NEOTROPICAL 8: 185-193, 1997 @ The Neotropical Ornithological Society

VARIATION IN CAPTURE RATES OF UNDERSTORY BIRDS IN EL REY NATIONAL PARK, NORTHWESTERN

John G. Blake1, 2 & Mercedes Rouges1, 2

1 Department of Biology, University of Missouri-St. Louis, St. Louis, MO, 63121, U.S.A. 2 Laboratorio Investgaciones Ecológicas de las Yungas, Tucumán, Argentina.

Abstract. Montane forests located in northwestern Argentina are important areas for conservation but have received little study. We used mist nets in a preliminary survey of understory birds at El Rey National Park, Salta; the park encompasses 44,162 ha over an altitudinal range from 700 to 2300 m. Study sites were at two elevations representing distinct forest types: Selva Basal (SB; 900-1200 m) and Selva de Mirtaceas (SM; 1200-1600 m). Birds were sampled during the dry and wet seasons 1994 and during the dry season 1996. We captured 660 birds (359 in SB; 301 in SM) representing 42 species (38 in SB; 31 in SM). Number of species per sample ranged from 17 to 27 but differences were largely due to difference in number of captures. Considerable variation in capture rates was noted between seasonsand elevations. Importance of fruit in the diet was substantially greater during the wet season. Accepted 22 August 1997. Key words: Argentina, diet, elevational gradient,. fruit, migrant, mist net, montane forest.

INTRODUCTION bird species along an elevational gradient of Most studies on structure of bird communities montane forests in one of these isolated reserves have been conducted in northern temperate areas (El Rey National Park). More specifically we and, to a much lesser extent, in tropical regions address the question of how capture rates of of Central and northern . Rela- understory birds differ between forests at two tively little is known about bird community elevations and between dry (winter, nonbreed- structure in subtropical habitats of the southern ing) and wet (summer, breeding, period of fruit hemisphere. Similarly, most research on migrants abundance) seasonsand how diets differ among has been conducted in northern hemisphere species and between seasons. Results presented habitats with less attention given to "austral" here are based on samples from two dry seasons migrants (Marantz & Remsen 1991, Chesser but only one wet season; thus, interpretation of observed patterns must be considered tentative 1994, Stotz et al. 1996). Comparative studies on the importance of latitudinal and altitudinal and require further substantiation. Nonetheless, movements (and similar scale movements) and the data do provide a preliminary assessment diet shifts in structuring bird populations are of variation in bird communities in a poorly generally lacking from subtropical sites in the studied region. southern hemisphere. Given that many species in subtropical montane forests may undergo STUDY SITE altitudinal movements (Vides 1992), it is clear Argentinean montane forests are located on the that additional studies on these systems are eastern slopes of a series of discontinuous moun- needed. tain ranges separatedby more arid intermountain Montane forests, or "yungas", reach their valleys (Brown & Ramadori 1989). These forests southernmost extent in a series of isolated re- typically receive from 1000 to 3000 mm annual serves in northwestern Argentina. These forests precipitation (Bianchi 1981) although long-te,rm are important areas for conservation but have records are lacking. Precipitation is concentrated received little ecological research (e.g., Brown during November to April (summer and fall) 1986). We initiated this study to obtain a prelimi- when about 80 % of the total annual precipita- nary description of variation in distribution, tion occurs. The climate is subtropical with a composition, abundance, and diets of understory mean annual temperature of 20°C (Chalukian

185 BLAKE & ROUGE

1991)j frost and even snow are not uncommon in 20/day) were 30 to 50 m apart. The number of some areas, particularly at higher elevations. nets operated varied because of logistical con- We conducted this research at El Rey Nátio- straints and the desire to prevent birds from re- nal Park (24 o 45' S 64 o 40' W), Salta, Argentina. maining in nets for more than 1 h. Nets general- El Rey was created in 1948 and encompasses ly were opened close to dawn and kept open for 44,162 ha over an altitudinal range from 700 to 6 to 7 hs/day, depending on weather (rain, wind). 2300 m. Study sites were located at two eleva- Captured birds were identifiéd, weighed, and tions that represent two distinct forest types: banded with a numbered aluminum leg band. Selva Basal (900-1200 m) and Selva de Mirtaceas Each bird was held in a plastic container for ap- (1200-1600 m) (Brown 1986). Selva Basal re- proximately 5 min to collect fecal samples. Birds places lower elevation transition (between dry were processed and released at the capture site. cliaco and humid forest) forestj it is typically Birds were assignedto migratory guilds based on taller with a more developed understory than literature accounts (Hilty & Brown 1986, Isler & the lower forest. The canopy is dominated by Isler 1997, Narosky & Yzurieta 1993, Curson et Cinnamomum (Phoebe)porphyria, Blepharocalix al. 1994, Stotz et al. 1996,) and personal observa- gigantea, Cedrella lilloi, and C. angustifolia. A tions. second stratum is composed of species typically Mist nets are a valuable tool for sampling less than 20 m, such asAllophyllus edulis, Zlntho- birds (Stotz et al. 1996) but their use is somewhat xylum (Fagara) coco,and Prunus tucumanensis. In controversial (see Remsen & Good [1996] for a the understory (2-5 m), Chusquea lorentziana, recent review). As in previous papers (e.g., Blake Urera baccifera, Miconia ioneura, and several & l.oiselle 1991, l.oiselle & Blake 1991), we use shrubs in the Solanaceae family are common. capture rates (i. e., numbers of birds captured per Epiphytes are abundant, particularly bromeliads. 100 mist net hours [1 mist net open 1 hour = At higher elevations (above 1200 m) Selva 1 mist net hour]) as an index of the activity of Basal is replaced by Selva de Mirtaceas, a forest birds in the understory without making claims dominated by trees in the Mirtaceae family about abundance per se. We do use changes in (Myrcianthes mato, M. pseudomato, Blepharocalix capture rates between seasonsas an indication of gigantea, Eugenia unifiora, E. pungens, and changes in activity and suggest that in many Myrrhinium lorathoides). In the understory, Piper cases(e.g., for migrants) these changes in capture tucumanum, Celtis sp., Pteris defiexa, Polystichum rates do reflect changes in abundance; in other planthyphyllum, Aphelandra hieronimy, and Justi- cases, changes in capture rates likely reflect cia sp. are most common. Epiphytes are also an changes in behavior (i. e., birds shifting foraging important element in this forest. Selva de Mirta- height from understory to canopy in concert ceas is replaced at higher elevations by forest with changes in season). typically dominated by Podocarpusand/or Alnus Fecal samples from 1994 were analyzed by and grasslands. separating fruits from arthropods (1996 samples Fruit production reaches a peak (number of are not yet analyzed). Fruit species were identi- speciesin fruit) in both Selva Basal and Selva de fied (when possible) using a reference collection Mirtaceas forests during November-January, of seedsfrom the area and insects were identified generally coinciding with the peak in rainfall (when possible) to order. All samples were re- (Brown 1986). The main breeding period for tained so that unidentified fruit seeds can be birds is between October-December (pers. obs.) identified later as the reference collection in- so that many young are f1edging as fruit is creases.For each bird species, we recorded the becoming particularly abundant. total number of times each item (e.g., Coleopte- ra, Diptera, seeds, etc.) appeared in METHODS fecal samples as well as the number of fecal Birds were sampled during the dry aune, July) samples that contained only insects, only fri1it and wet (December, January) seasons 1994 and seedsor pulp, or a combination of insects and during the dry season auly, August) 1996. We fruit. used mist nets (12x2.6 m, 36 mm mesh) set Sampling effort (number of nets) varied along narrow trails to sample birds; nets (10 to among sites and seasonsso we expressed results

186 UNDERSTORY BIRDS IN EL REY NATIONAL PARK, NW ARGENTINA in terms of capture rates. We compared capture Number of species per sample ranged from rates between seasons within forest, between 17 to 27 but differences were largely due to dif- forests within season, and between years for dry ference in number of captures. When all samples season samples. Because samples were collected were compared based on 59 captures (the lowest from single sites at each elevation, results of total, from dry season 1996 in SM), mean species statistical tests must be considered primarily richness (based on 1000 si1Jlulations for each descriptive and valid only for the sites being sample) ranged from 17.5 to 20.4. Species rich- compared. We used Fisher's Exact test for species ness (based on rarefaction to 128 captures) for SB represented by at least 6 captures (combined total wet (mean of 1000 simulations = 23.7 [21.8- for both seasons or sites being compared) and 25.6]) was, however, slightly less than during the Chi2 tests for total captures. Comparisons were dry season 1994 (27 species; see Table 1). based on actual numbers (not rates) with ex- Capture rates. Considerable variation in capture pected values based on number of net hours. rates was noted between wet and dry seasonsin Because samples differed in total numbers of SB, between dry seasonsin SM, and between SB captures, we could not directly compare number and SM during dry seasons (Table 2). Capture of species between samples (i. e., elevations, sea- rates (birds captured/100 mist net hours) were sons). Instead, we compared species richness lowest during dry season 1996 in SM but highest (number of species) between samples using rare- during dry season 1994 in the same site. Capture faction techniques based on a null model simula- rates at lower elevations did not change between tion (Ecosim V.1.0, Gotelli & Entsminger 1997). years. Rarefaction is a useful technique when sample Marked changes in capture rates between sea- sizes differ (i. e., numbers of individuals; see sons also were noted for several individual species Gotelli & Graves 1996). (Table 2), particularly latitudinal and altitudinal migrants. In SB, several species rarely or never RESULTS captured in dry season were captured more Community composition. We captured a total of frequently in wet season (e.g., Elaenia strepera, 660 birds (359 in Selva Basal-SB hereafter; 301 in Lathrotriccus euleri, Turdus nigriceps, Thlypopsis Selva de Mirtaceas-SM hereafter) representing sordida). Comparable changes also were noted in 42 species (38 in SB; 31 in SM) and 14 families SM, especially for Elaenia strepera, Turdus nigri- (counting subfamilies of Emberizidae separately) ceps,and Thraupis sayaca. Other species, in con- (Table 1); 8 specieswere captured only in SB and trast, were captured less often during the wet sea- 3 only in SM. Recaptures accounted for only son (SB: Basileuterus bivittatus, Chlorospingus 3.6% of the totals (12 in each site) and are not opthalmicus; SM: Eriocnemis glaucopoides,Meco- considered further. Most samples tended to be cerculus leucophrys, Phylloscartes ventralis, Basi- dominated by relatively few species;three to five leuterussignatus, Chlorospingusopthalmicus); many speciesin each sample accounted for over 50 % of of these species are not latitudinal migrants and changes in capture rates likely reflect changes captures (Table 1). Species represented by only 1 or 2 captures accounted for at least 50 % of all in behavior, habitat shifts within an elevation, speciescaptured except during dry season 1994 in or altitudinal movements. Reciprocal changes Selva de Mirtaceas when such species only between seasons at the two elevations (i. e., accounted for 25 % of the total. Particularly decreasing during the wet season at SM while common species in the dry season included Syn. increasing at SB) may indicate altitudinal shifts dactyla rufosuperciliata, Phylloscartes ventralis, in distribution (e.g., Atlapetes torquatus). Basileuterus bivitattus, and Atlapetes torquatus in Several species showed marked differences in SB and Basileuterus signatus, Atlapetes torquatus, capture rates between elevations. Particularly and Chlorospingus opthalmicus (1994 only) in notable were Eriocnemis glaucopoides, Ela~nia SM. During the wet season,Syndactyla rufosuper- strepera, Mecocerculus leucophrys, Lathrotriccus ciliata, Lathrotriccus euleri, Turdus nigriceps, Atla. euleri, Turdus nigriceps, Basileuterus bivittatus, petes torquatus, and 7hlypopsis sordida were com- B. signatus, and Thlypopsis sordida. Differences mon in SB whereas Elaenia strepera and Turdus between elevations were most pronounced during nigriceps dominated captures in SM. wet season 1994 and dry season 1996.

187 BLAKE & ROUGES

Diets. We obtained 226 fecal samples during 1994 rent items, 19% three items, and about 11% four (from 43 % of captures), representing 29 of the or five items. 42 speciescaptured (Tables 3, 4). Number of dif- There were substantial differences between ferent items recorded in each fecal sample varied; wet and dry seasonsin number of fecal samples approximately 70 % contained one or two diffe- containing fruit seedsor pulp. Fecal samples were

TABLE 1. Number of captures in Selva Basa! and Selva de Mirtaceas during dry (1994, 1996) and wet (1994) seasons in El Rey Nationa! Park, Argentina. Scientific names follow Stotz et al. (1996).

a Migrant codes: A -altitudinal migrant, A? -possible altitudinal migrant, L- latitudinal migrant (at least partially).

188 JNDERSTORY BIRDS IN EL REY NATIONAL PARK, NW ARGENTINA

TABLE 2. Results of Fisher's exact tests for differences in captures between seasons(D-W -dry vs. wet seasons 1994; D-D -dry 1994 vs. dry 1996) within a forest and within seasons (D94 -dry 1994; D96 -dry 1996; W94 -wet 1994) between forests. Species were tested if there was a total of a least 6 captures in the two sites (seasons) being compared (see Table 1). Total captures were compared using Chi2 tests. ns=not significant; *=P

Selva Basal Taxa D-W I D-D

Eriocnemis glaucopoides Synallaxis azarae Syndactyla rufosuperciliata ns *** Elaenia strepera Mecocerculus leucophrys Phylloscartes ventralis Lathrotriccus euleri Troglodytes solstitialis Catharus dryas Turdus nigriceps ** ns Turdus rufiventris ns ns Poospiza erythrophrys ns Atlapetes citrinellus * Atlapetes torquatus Passerina brissoni ,',' " Chlorospingus opthalmicus 7hlypopsis sordida **** 7hraupis sayaca Pipraeidea melanota ns Myioborus brunniceps ns Basileuterus bivittatus *** ns Basileuterus signatus ns

Total species tested 14 10

*** ns Total captures

dominated by insect remains during the dry sea- with fruit differed in both SB (Chi2 = 26.0) and son: 51 of 55 samples in SB and 29 of 36 samples SM (Chi2 = 26.0, P<0.001, both cases). Overall from SM contained only insect parts (Chi2 com- patterns did not differ between forests during the parisons for equal distribution between insects dry season (Chi2 = 3.0, P<0.10) but did in the only and samples with fruit = 40.2 and 13.4 for wet season (Chi2 = 7.7, P<0.01); differences in SB and SM, respectively; P<0.001 in both cases). the wet season were not pronounced between In contrast, fruit samples were much more com- forests when Turdus nigriceps was omitted from .mon during the wet season, particularly in SM: the comparison (Chi2 = 2.7, P<0.10). 39 of 78 samples from SB and 42 of 57 samples Species differed considerably in use of fruit. from SM contained fruit or fruit and insects Among the most frugivorous were Elaenia strepe. (Chi2 = 0 for SB; Chi2 = 12.8, P<0.001 for SM). ra (100% of samples with fruit), 7hraupis sayaca Samples from Turdus nigriceps accounted for 20 (100 %), Turdus nigriceps (65 %), and 7hljpopsis of the samples with fruit in SM; with this species sordida (60 %). A total of 15 species consumed excluded there was no difference in number of fruit whereas almost all species were recorded samples with insects only (15) or with fruit (22) with insect parts in fecal samples (except Passeri. (Chi2 = 1.3). Seasonalpatterns (dry vs wet season) na brissoni -only one fecal sample, containing in relative proportions of samples with insects or only pulp). Severa1species present in the area

189 BLAKE & ROUGES

TABLE 3. Number of fecal samples containing only insect parts (I), only fruit seedsor pulp (F), or a combination of insects and fruit (IF) collected during dry and wet seasons, 1994, from birds captured at two elevations in El Rey National Park, Argentina.

during both dry and wet seasonsincreased their general agreement with other studies (Vides use of fruit during the wet season(based on pro- 1992), it is important to emphasize that the ob- portional occurrence of fecal sample with fruit) served patterns and their interpretation are based (e.g., Turdus nigriceps, Atlapetes torquatus). In- on a limited data set. Consequently, several diffe- creased use of fruit corresponded with greater rent explanations are possible for the observed availability and diversity of fruits (pers. obs.). variation. Variation in capture rates and species composition may, for example, be related to site DlSCUSSION specific effects or sampling error. We sampled Bird communities in montane forests of north- only one area at each elevation so it is possible western Argentina exhibit considerable variation that the results obtained apply only to those sites in species composition and relative abundance and are not representative of forests at those ele- (capture rates) both between and within eleva- vations in general. However, given that vegeta- tions. Although the observed patterns are in tion present in the sampled areaswas similar to

1qn UNDERSTORY BIRDS IN EL REY NATIONAL PARK, NW ARGENTINA that present at other areasat the same elevation nated by members of that family; trees produce in the park (Brown 1986; pers. obs.), it is likely abundant crops of fruits that are consumed by that the results are representative of the forest as birds. In other cases(e. g., Lathrotriccus), differen- a whole. Sampling error is more likley to be a ces between elevations are not likely due to diffe- problem for individual species (e.g., Chlorospin. rences in fruit resources as no evidence of fruit gus opthalmicus) than for overall community consumption was found. Birds may be respond- patterns. Variation in capture rates of individual ing to differences in insect numbers (often higher speciesmay reflect changes in behavior (flocking in wet seasons)but we have no information on VS. territorial; canopy VS. understory foraging) insect abundance with which to evaluate this .that are not revealed by mist net captures. possibility. Capture rates also are influenced by arrival Seasonalshifts along the elevational gradient arid departure of latitudinal migrants and sea- likely are an important component of the dy- sonal shifts along the elevational gradient by resi- namics of communities in this area. Vides (1992) dent species(altitudinal migration), patterns also suggestedthat as much as 50 % of the species in observed in bird communities in montane forests montane forests in northwestern Argentina (at a of Central America (Stiles 1985, Stiles & Clark site near Tucumán) may exhibit regular altitudi- 1989, Loiselle & Blake 1991). Variation in abun- nal shifts in distribution. Although our data are dance of resources, including insects, fruit, and preliminary and need further confirmation, it nectar, influences dynamics of bird communities appears that this type of movement occurs in a in many temperate and tropical regions (Blake & number of species in El Rey and helps account Hoppes 1986, Wiens 1989, Blake & Loiselle for differences in bird communities between ele- 1991, Loiselle & Blake ¡991, Levey & Stiles 1992, vations. Capture rates of Atlapetes torquatus, for Poulin et al. 1992, Lefebvre & Poulin 1996) and example, declined in Selva Mirtaceas coincident likely is an important influence on bird com- with higher capture rates in Selva Basal. Al- munities in El Rey National Park as well. Nec- though the distances traveled are not great, the tarivores (e.g., Sappho sparganura, Eriocnemis shift in elevation (from 1300 to 900 m) may glaucopoides) were more frequently encountered nonetheless be important. In Costa Rica, shifts during the dry season, when flower abundance of 200 m between seasons are exhibited by a and, presumably, nectar availability is higher number of species, particularly members of the (pers. obs.) whereas several species of frugivores Pipridae (Loiselle & Blake 1991, unpubl. data). were captured much more frequently during the Similarly, 7blypopsis sordida is known to shift wet season, during the period of peak fruit pro- along elevational gradients (Isler & Isler 1987) duction. Similarly, arrival of latitudinal migrants and the higher capture rates during the wet sea- for breeding is coincident with production of son in Selva Basal may be at least partially in fruit, which many speciesuse as a major part of their diet. Overall, there was a substantial in- response to increases in fruit resources. creaseboth in the number of specieseating fruit Altitudinal shifts may be in response to and the proportion of individuals within a changes in resources, as has been demonstrated species eating fruit during the summer (wet sea- elsewhere but also may be in response to sea- son), the period when fruit is most abundant. sonal changes in environmental conditions (see Among the more common latitudinal mi- Ramos 1983). Higher elevations in the park are grants were Elaenia strepera, Lathrotriccus euleri, more likely to experience colder conditions and Turdus nigriceps. Captures rates of speciesdif- during winter, with frost and snow a greater fered between elevations, suggesting differences possibility. The winter of 1996 was especially in habitat preferences. In some cases(e. g., Elae. severe,with considerable snow recorded in Selva nia, Turdus), differences between seasonsand ele- de Mirtaceas forest (H. Grau, p~rs. comm.). This vations may be related to differences in availabi- severe weather may be -one explanation for the lity of fruit resources. Production of fruit by much lower capture rates in that forest compared members of the Mirtaceae peak during the wet to winter 1994. Clearly, samples from additional seasonwhen breeding is occurring and migrants years and seasons are needed to evaluate the are present. Similarly, Selva de Mirtaceas is domi- effects of such weather conditions.

191 BLAKE & ROUGES

" . "1j ., "..~ " ~NNN~00~~N~~~N~~~N~~~~~0~~~~~N ~ o. " 8 ~ N ~ ~~ ~ N~N~~~N o.c :E ~ §° .-~ 8 o u ~,E N--NO~~N-O-~N~O~-~~~-~~~~O~N- -.r> ~ .. --~~ N -.'o N- N ~ "' \-(0 ~ II ~ '" >-u 'ii ~ ~ '"" N """-"'-~"' N o ¡-. .5 Q :::1""'('1 " " .. i ~u ~.. " " " ==o. .~ o" ., N 0-- 00 00... N o :1= ~-: "' II ° -.. '$ o" d '- ~~ '" N ~ ." A. ~ ~ ~ o: ~ t: II ... " .$ a o II ~ ~"' --N -o ..¡-00 -N N Or\N = :. -. .~ 7 ~ ~ Z ,,"Q. >, ..a ~ " ~ o.~ E 0"" o: u "¡iJ ,... .5 E~ "1j o f~ 11:!, ~ ~ a a >-" ~ :1: ~ ... N N N N " ~ " ~ .:z c--1 ..~ ~."r :E .~o. " a ~ 8 s ..., ("I ' ~ Ni'-"'-""",r¡ 0- " .. > ..., :1: ° ...§ :r: II.E;- a " ~ " o. "E. s ..,..., :1: .~ 8 " ~ ~ ",E ~." ... .u Jj ~ .. 0- N U"\ ""N """' N -o-oNNNoo-o 00 .§ .;; o ~ rL ..o: .5 ~o~ .~ "1j ., II 6 "1j . ..° '00' ° o: bJ) ~ II ~ ~~ M~ '~ 0 ""Z " . ..., II ~ a. ~ N- N :!:1 """ ~ ~ ~ u "' 0 " 6 .o: ~~ ~ " >"" ~,,- ° - N N,,",N O ...,t-.t-.N o o."' o - -.t"-.t"---~ ...'~Or\ o .-II .,... 0- .3 ~ u II °

o."' ~"1j .~ ."r '- " 0 " 0 '" "' " " ~ ~ .." '¡¡ t:' " " ","1j ~ " " o.U .-., ...A ~ 2 t " ... " "' ",NN N -C"'"N""""""" ~ ~ ->- ~ CO < 8~ II II ~.5 -.. .-3 "1j"1j "' 8~ ~"1j ~ "' ~ ~ 'ii 8 2J '! "' "' "' .N ~ ." " .-u .;; o. , ~~ "' ~~ ~ o. ~ .¡¡ ."' k' "' "' o "' -~ "' "' ~ tj 00 -" '- u t].N ~~~~ "'~ ~."' ~ °~¡:"'"' ~ ~ ~ "' ~ § 1 8' E ~ ~ ] ~ .~ .:t: ~ ~ J 5. :~ .;s¡ ~ 9 .~ .~ ¡: .~ II .5 JS~ -." ~",N~~~~=~~~... "'...0~""!~C-.~ '! N~" '15 ~ " 8~ '" .'"' ~~~:x ~~ ..~.~ ".~-'¡5 ~ ~~.t.~.E ~~ "'~ ~--.¡ ~j¡.~.~ ~ " .- "o. " ." ~ Z 9 i!"-~"'~~~"'"'~~"'k'.~..~'!...~.~~~"'~-I; "'~~ .~ o: "' ."'~ ~.~ ~~ ~.~.~.(j ~~~.~~~~.~ s~.~."' ~ '! ~ ~ ~ o:o~ E ..§~u .~ ~ 0 Nntn"",~n,,¡:~n"'~" ~"""'!~~""'¡:~~.~.N ;; " , ~ 1j O ~ .~ t ~ ~ § 'C ~ ~ ~ "' "' .~ O ~ ~ .~ :s~ ~.~~ ~ ~ ~ ~ C " "' ..o: 1 ~ ~ ;:g ~ 8::¡, ~ ~ ~ .~ ~~ i i ~ 5 ii ~ s ~ ~ ~ .2 ~ ~ ~ ~ < 9 ~~ II II o ...~ ~ ~ J';-J;,~:¡;-¡¡j~~~"i:~~~<3~~~~~~~6~~~~~~c~ ~

192

~ UNDERSmRY BIRDS IN EL REY NATIONAL PARK, NW ARGENTINA

Results of this study indicate that bird com- Gotelli, N.J., & G. L. Entsminger. 1997. EcoSim. Null munities in montane forests of northwestern modelssoftware for ecology. Version 1.0. Kesey-Be- Argentina display considerable spatial and tem- ar, Inc. pora1 variation in both composition and abun- Gotelli, N. J., & G. R. Graves. 1996. Null models in dance. There are a number of possible causesfor ecology. Smithsonian Institution Press, Washing- this variation, with different factors likely to be ton, D. C. important for different species.A more complete Hilty, S. L., & W. L. Brown. 1986. A guide to the birds of . Princeton. understanding of these patterns requires additio- Isler, M. L., & P. R. Isler. 1987. The tanagers: natural nal study. history, distribution, and identification. Washing- ton D. C. ACKNOWLEDGMENTS Lefebvre, G., & B. Poulin, B. 1996. Seasonal abundance We are grateful to Administracion de Parques of migrant birds and food resources in Panamanian Nacionales-Direccion Tecnica NOA and to mangrove forests. Wilson Bull. 108: 748-759. Levey, D. J., & F. G. Stiles. 1992. Evolutionary precur- Parque Nacional El Rey for permission to work sors of long-distance migration: resource avail- in the park and for logistical help. Support for ability and movement patterns in Neotropical this research has been provided by Consejo landbirds. Amer. Nat. 140: 447-476. Nacional de Investigaciones Cientificas y Tecni- Loiselle, B. A., & J. G. Blake. 1991. Temporal variation cas-Beca de Iniciacion, Sigma Xi, International in birds and fruits along an elevational gradient in Center for Tropical Ecology, National Science Costa Rica. Ecology 7:2: 180-193. Foundation Travel Grant, and U niversity of Mis- Marantz, C. A., & J. V. Remsen. 1991. Seasonal distri- souri -St. Louis Research Award. Additional bution of the Slaty Elaenia, a little-known austral support for continuing studies is being provided migrant of South America. J. Field Ornithol. 62: by National Geographic Society. Identification 162-172. of insects was done by Grace Servat. Narosky, T., & D. Yzurieta. 1993. Birds of Argentina and : A field guide. Buenos Aires. Poulin, B., Lefebvre, G., & R. McNeil. 1992. Tropical REFERENCES avian phenology in relation to abundance and Bianchi, A. 1981. Las precipitaciones en el noroeste exploitation of food resources. Ecology 73: 2295- argentino. INTA. 2309. Blake, J. G., & W. G. Hoppes. 1986. Resource abun- Ramos, M. A. 1983. Seasonalmovements of bird pOpU- dance and microhabitat use by birds in an isolated lations at a Neotropical study site in sourthern east-central lllinois woodlot. Auk 103: 328-340. Veracruz. Ph. D. diss., University Minnesota, St. Blake, J. G., & B. A. Loiselle. 1991. Variation in re- Paul. source abundance affects capture rates in birds of Remsen, J. V., Jr., & D. A. Good. 1996. Misuse of data three lowland habitats in Costa Rica. Auk 108: from mist-net captures to assessrelative abundance 328-340. in bird populations. Auk 113: 381-398. Brown, A. D. 1986. Autoecology de Bromeliaceas epifi- Stiles, F. G. 1985. On the role of birds in the dynamics tas y su vinculacion con Cebus apella (Primates) en of Neotropical forests. Pp. 49-59 in Diamond, el noroeste Argentino. Tesis Doctoral, Facultad de A. w:, & T. Lovejoy (eds.). Conservation of tropi- Ciencias Naturales y Museo, Universidad Nacional cal forest birds. Cambridge, U. K. de La Plata. Stiles, F. G., & D. A. Clark. 1989. Conservation of tro- Brown, A. D., & E. D. Ramadori. 1989. Patron de dis- pical rain forest birds: a casestudy from Costa Rica. tribucion, diversidad y caracteristicas ecologicas de American Birds 43: 420-428. especiesarboreas de las selvas y bosques montanos Stotz, D. F., Fitzpatrick, J. W., Parker, T. A., III, & del noroeste de Argentina. Anales VI Congreso D. K. Moskovitz. 1996. Neotropical birds: ecology Forestal Argentino: 177-181. and conservation. Chicago. Chalukian, S. C. 1991. Regeneracion, sucesion y plan- Wiens, J. A. 1989. The ecology of bird communities. tas invasoras en un bosque de Yungas, Salta, Argen- Cambridge. . tina. Ms. Thesis. Programa Regional en Manejo de Vida Silvestre para Mesoamerica y el Caribe. Vides, R. 1992. Estudio comparativo de las taxocenosis Chesser, R. T. 1994. Migration in South America: an de aves de los bosques montanos de la Sierra de San overview of the austral system. Bird Conserv. Intl. Javier, Tucumán: Basespara su manejo y conserva- 4: 91-107. cion. Tesis Doctoral. Fac. de Ciencias Naturales e Curson, J., Quinn, D., & D. Beadle. 1994. Warblers of Instituto Miguel Lillo, Universidad Nacional de the Americas: an identification guide. Boston. Tucumán, Tucumán, Argentina.

193