Diets of Four Sympatric Amazonian Woodcreepers (Dendrocolaptidae)’
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The Condor 93:904-915 0 The CooperOrnithological Society I99 I DIETS OF FOUR SYMPATRIC AMAZONIAN WOODCREEPERS (DENDROCOLAPTIDAE)’ ANGELACHAPMAN~ AND KENNETH V. ROSENBERG Museum of Natural Scienceand Department of Zoology and Physiology,Louisiana State University, Baton Rouge, LA 70803-3216 Abstract. Contents of 78 stomachsfrom four widespread and sympatric speciesof Am- azonian woodcreepers(Dendrocolaptidae) were examined to assessthe role of diet in resource partitioning. Orthopterans (2%35%) and beetles (lO-32%) dominated the diets of all four species,despite large differencesin foraging behavior. A higher proportion of spiders in the diet of Dendrocinclamerula was associatedwith specialized ground-foragingat army ant swarms; however, specialization on dead leaves by Xiphorhynchusguttatus resulted in no significant dietary differences from the substrategeneralist X spixii. Overlap in diet was much greater than overlap in behavior for all speciespairs, and the degree of diet special- ization was unrelated to behavioral specialization. Taxonomic representation of prey in stomach contents differed significantly from field sampling of available prey in the three speciestested, with orthopterans apparently selectedby all speciesand beetles selectedby D. fuliginosa. We suggestthat behavioral differences may have evolved to reduce overt aggression(interference competition) but may not serve to reduce diffuse competition for food among syntopic species.Segregation in substrateuse, however, may allow high diet overlap, suggestinga case of niche complementarity among these species. Key words: Dendrocolaptidae;diet;foraging ecology; resource partitioning;specialization; woodcreepers;tropical forest birds. INTRODUCTION Munn 1985). Others follow swarms ofarmy ants How large numbers of closely related speciesco- (E&on spp.) to feed on escaping arthropods exist in diverse tropical communities has in- (Willis and Oniki 1978). In addition, many spe- trigued ecologistsfor decades.Although studies cies do not feed directly on trunk or branch sur- of resource partitioning among potential avian faces,but investigateclusters of dead leaves,vine- competitors are numerous, studies that comple- tangles, or palm fronds, or sally after flying prey ment data on habitat selection and foraging be- (e.g., Pierpont 1986). The extent to which these havior with data on diet are few (e.g., Rotenberry behavioral differences may result in differences 1980, Robinson and Holmes 1982, Rosenberg in diet is unknown. et al. 1982, Dahlsten et al. 1985). This is es- Becauseup to 17 speciesof woodcreepersmay pecially true for species-richtropical forestguilds; coexist in parts of the Amazon Basin (e.g., Ter- only Sherry (1984) has investigated the dietary borgh et al. 1984) their foraging relationships relationships among sympatric neotropical in- are a significant component of the overall com- sectivores,using 16 flycatcher speciesfrom Costa munity dynamics. In this paper, we describe the Rica. diets of four widespread speciesat two sites in The woodcreepers (Dendrocolaptidae) are a southwesternAmazonia. We choseDendrocincla large and characteristic family of neotropical fuliginosa (Plain-brown Woodcreeper), D. mer- forest birds. Although superficially similar in ex- ula (White-chinned Woodcreeper), Xiphorhyn- ternal morphology and in their tendency to climb thus guttatus (Buff-throated Woodcreeper), and vertical trunks and branches, woodcreeper spe- X. spixii (Spix’s Woodcreeper)because these were cies exhibit a wide range of foraging behaviors. the most common large (> 30 g) woodcreepers Many species typically join mixed-species for- at each of these two well-sampled sites. Both aging flocks in the understory or canopy (e.g., species of Dendrocincla are “professional” fol- lowers of army-ant swarms throughout much of Amazonia (Willis 1972, 1979), although D. fu- ’ ’ Received 12 February 1991. Final acceptance 7 liginosa Xi- June 1991. also forages away from ants. Both z Presentaddress: Department of Biology, Universiti phorhynchus spp. rarely follow army ants but Brunei Darussalam, Bandar Seri Begawan, 3 186 Bru- routinely join mixed-species flocks in the un- nei, Darussalam. derstory and subcanopy (Munn 1985, Pierpont [9041 DIETS OF AMAZONIAN WOODCREEPERS 905 1986; Rosenberg, unpubl. data). To address such as beetles. We believe, therefore, that with whether diet was important in niche partitioning knowledge of the particular fragments repre- by these species, we asked the following ques- senting each food-type, we could detect hard- tions: (1) do differencesin foraging behavior cor- and soft-bodied prey equally well. Further ex- respond to differencesin diet? (2) do thesespecies perimentation would be needed to verify our specialize on particular prey? and (3) how do impression. species’ diets relate to food availability? The proportion of each prey category was de- termined separatelyfor each stomach; diets were METHODS then determined as the average of the propor- Diets were determined by direct observation of tions in the individual stomachs of each species stomach contents from 78 individual birds: 32 (i.e., samples were not pooled). Diet overlap be- X. guttatus,18 X. spixii, 11 D. jiuliginosa,and tween pairs of woodcreeperswas calculated as: 17 D. merula(Table 1). Specimenswere collected 0, = ~(P,,P,,)/\/(~Pz,.)(~Pz,~), where Pi, and P,. from June through August, mainly with mist nets, are the proportions of prey category “a” in the near Cobija, Dpto. Pando, extreme northwestern diets of species“i” and “j” respectively (Pianka Bolivia, in 1986 (see Parker and Remsen 1987) 1974, May 1975). Niche breadth, or prey-type and near Abujao, Dpto. Ucayali, eastern Peru, diversity, was calculated for every stomach using in 1987 by field parties from Louisiana State B = (Zp,Z))‘, where p, is the proportion of taxon University Museum of Natural Science “i” in the stomach (Levins 1968). An average (LSUMNS). The two sites are in continuous, niche breadth was then calculated for each spe- lowland rainforest, separatedby roughly 200 km. cies. It is unclear whether pooled frequency dis- Additional foraging and insect availability data tributions of prey items from multiple stomach were gathered at the Tambopata Reserve, Dpto. samples can be compared statistically, because Madre de Dios, southeastern Peru, from 1987- of potential pseudoreplication (Hurlbert 1984). 1989. Although in most casesindividual arthropod prey Stomachs were preserved in 70% ethanol and captured by woodcreepersprobably representin- housed permanently in the LSUMNS collection, dependent events (except for rare attacks on ant along with referenceskins or skeletons.Contents nests),we have conservatively chosen not to ap- of each stomach were examined using a stereo- ply goodness-of-fit statistics to compare diets microscope(6-25 x) with a micrometer. Because among species. We believe, however, that the only a small number of items could be identified patterns in dietary overlap are clear enough to to family, most prey were identified to order or indicate biologically significant trends, and our suborder, using Borror and White (1970), Borror overall conclusions would not be altered. et al. (1981), and Ralph et al. (1985). Prey items For comparisons with the foraging behavior were sorted, measured, and counted, and sketch- of these species,we drew upon data from several es of identified material were made to facilitate sources.First, woodcreeperswere observed for- later identification. Prey sizewas determined from aging as part of a general community study at measurable fragments using regressionequations the Bolivia study site and at the Tambopata Re- in Calver and Wooller (1982) or computed from serve in southeastern Peru (J. V. Remsen and arthropods collected in the study areas (Rosen- Rosenberg, unpubl. data). These observations berg, unpubl. data). Reference series of identi- consisted of opportunistic sightings along forest fied, mounted fragments, drawings, and photo- trails, for each of which we recorded foraging graphs (Fig. 1) were prepared for future com- height (estimated to the nearest 1 m), foraging parisons. substrate, perch site, prey-capture method, and The question of differential digestion of hard- an estimate of foliage density around the bird, and soft-bodied prey is pertinent to any analysis as well as associatedspecies (e.g., mixed-species of stomach contents (Rosenberg and Cooper flocks) and general habitat features. Data nota- 1990, and referencestherein). Hard-bodied items tion followed Remsen and Robinson (1990). take longer to digest and may persist longer in Usually, fewer than five consecutive observa- the stomach. However, it seemed that the hard tions were made on each bird, although repeated fragments of soft-bodied prey, such as orthop- sightings of (probably) the same individuals in teran mandibles and spider fangs, were as prev- the same mixed-speciesflocks were included. Be- alent in our samplesas thoseof hard-bodied prey, cause these efforts focused on mixed-species- 1. FIGURE 1. Fragmented prey items found in the stomachsof four woodcreeperspecies. A. Homoptera(head); B. Lepidopteralarva (mandible);C. Coleoptera(elytron); D. lizard (jaw); E. Curculionidae (head); F. ant (head with mandibles); G. roach (head with mandibles); H. spider (fang); I. Coleoptera (head with mandibles); .I. Orthoptera (mandible part); K. spider (chelicerae with fangs); L. Orthoptera (mandible). Black bar indicates I mm. flocking species,our samples are adequate only then computed using the