The Condor 102:7X4-794 0 The Cooper Ornithological Society 2000 ANTBIRD GUILDS IN THE LOWLAND CARIBBEAN RAINFOREST OF SOUTHEAST NICARAGUA1 MARTIN L. CODY Department of OrganismicBiology, Ecology and Evolution, Universityof California, Los Angeles, CA 90095-1606, e-mail: [email protected] Abstract. Some 20 speciesof antbirdsoccur in lowland Caribbeanrainforest in southeast Nicaragua where they form five distinct guilds on the basis of habitat preferences,foraging ecology, and foraging behavior. Three guilds are habitat-based,in Edge, Forest, and Gaps within forest; two are behaviorally distinct, with species of army ant followers and those foraging within mixed-species flocks. The guilds each contain 3-6 antbird species. Within guilds, species are segregatedby body size differences between member species, and in several guilds are evenly spaced on a logarithmic scale of body mass. Among guilds, the factors by which adjacent body sizes differ vary between 1.25 and 1.75. Body size differ- ences may be related to differences in preferred prey sizes, but are influenced also by the density of the vegetation in which each speciescustomarily forages. Resumen. Unas 20 especies de aves hormiguerasviven en el bosque tropical perenni- folio, surestede Nicaragua, donde se forman cinquo gremios distinctos estribando en pre- ferencias de habitat, ecologia y comportamiento de las costumbresde alimentacion. Las diferenciasentre las varias especiesson cuantificadaspor caractaristicasde1 ambiente vegetal y por la ecologia y comportamientode la alimentaci6n, y usadospara definir cinco grupos o gremios (“guilds”). Tres gremios se designanpor las relacionesde habitat: edge (margen), forest (selva), y gaps (aberturasadentro la selva); dos mas por comportamiento,partidarios de army ants (hormigasarmadas) y mixed-speciesflocks (forrejando en bandadasde especies mexcladas). Estos gremios contenien 3-6 especies hormiguera. Dentro de grupos hay di- ferencias regularesen peso de cuerpo entre las especies;diferencias que igualmenteexisten entre 10s gremios diferentes, con raz6nes medios de 1.25 a 1.75 en peso de cuerpo. Las diferencias en el tamaiio de especiesindividuales probablementeestan relacionadascon las diferencias entre 10stamaiios de las presaspreferidas, pero quizas son influenciadaspor la densidadde la vegetation en donde se alimentan. Key words: antbirds, body size, foraging behavior,foraging height, guild, size segre gation. INTRODUCTION resident species [Stiles 19831; Barro Colorado Antbirds currently comprise two Neotropical Island, Panama: 15 species, some now rare or families (Formicariidae and Thamnophilidae; extinct [Willis and Eisenmann 19791). AOU 1997) of ca. 250 species. Most are gen- Most studies of community composition, eco- eralized insectivores of forest, forest edge, or logical segregation, or interspecific relationships gaps within the forest; some habitually consort in antbirds have been conducted on either ant- in multispecies flocks that forage in the forest following or mixed-flock species. Pearson understory or subcanopy, and a minority (10% (1977) studied the ecological relationships of according to Willis and On&i, 1978) follow small antbirds typical of mixed-species subcan- army ant swarms. Local assemblages of antbirds opy flocks in Peru, where Munn (1980) recorded number up to several dozen species in lowland some 17 antbird species in understory and can- Amazonia (Rio Napo, eastern Ecuador: 31 spe- opy flocks. Jones (1977) investigated territorial cies [Bochan, unpubl. data]; Rio Manu area, and foraging attributes of the antwrens of Peru: 53 species [Terborgh et al. 19841; Belem, mixed-species flocks in Panama; Gradwohl and Bra&: 17 common species among mistnet sam- Greenberg (1980) studied flock formation and ples [Lovejoy 19741). In Central America, spe- behavioral interactions at the same site. Similar cies counts are lower (La Selva, Costa Rica: 18 work has been conducted on antwren flocks in Ecuador (Whitney 1994). Detailed life histories have been published on several ant-following I Received 19 November 1999. Accepted 18 July species (Willis 1967, 1968, 1973) and their be- 2000. havior (On&i 1971, 1972, Oniki and Willis [7841 ANTBIRD GUILDS IN NICARAGUA 785 1972, Willis and On&i 1978). However, rela- gaps that are either natural or the result of a few, tively little is known of the more usual territorial select tree removals for lumber needs. Within species of non-antfollowers of forest, edge, and the study site are several areas of second growth, second-growth habitats (Howell 1957, Willis resulting from clearing for small fincas (aban- 1983). doned 5-15 years ago) and around the Refugio I present observations on antbirds from a low- buildings and Gran Reserva guard station. land tropical forest site in southeastern Nicara- This site supports a high diversity of the plant gua. The distributions, habitat preferences, and and animal species typical of undisturbed low- foraging ecology of 19 of the 20 species ex- land rainforest. Over the two visits, 256 bird pected at this site are reported here, with partic- species were recorded in ca. 1 km2, with a jack- ular reference to ecological segregation by hab- knife estimate (based on species seen only in itat, foraging behavior and height, and body size, one or the other visit) of 298 bird species. and summarized by species allocation to five Among species recorded are a number which antbird guilds. typically are lost from disturbed sites, such as many mammals (primates, felids, edentates, and METHODS tapirs), three cracids, and two species of Ara ma- caws. STUDY SITE The observations reported here were made PROCEDURES April-May 1994 and 1999 at Refugio Bartola Antbirds were located by sight and sound within and the Gran Reserva Biologica “Rio Indio- 1.5 km of the Refugio both east and west of the Maiz” in southeastern Nicaragua. The site is lo- Rio Bartola. Antbird songs are distinctive (Stiles cated at the confluence of the Rio Bartola and and Skutch 1989), and are useful phenotypic the Rio San Juan, on the east and west banks of characters in studies of antbird systematics (Isler the former and the north bank of the latter et al. 1998, 1999) and in the recognition of new (10.97”N, 84.16W, elev. 30 m). The climate is species previously overlooked (Whitney and Al- wet tropical, with around 4 m of annual precip- varez 1998). The approximate locations of ter- itation, and a dry season from February-April ritories in edge habitats, primary forest, and in during which about 15% of the annual rainfall or around treefall gaps were mapped, with ref- is recorded. During the study periods, 100 mm erence to a surveyed trail system that effectively of precipitation fell in 1994, 160 mm in 1999. covered the local terrain. Antbird songs were re- Temperatures were equable in this period, gen- corded for species identification purposes, for erally around 23°C at dawn, occasionally reach- subsequent analysis and preparation of sono- ing 36°C in the open on cloudless days, and grams, and for playback experiments both to lo- 31°C within the forest. cate conspecifics and to test for the possibility The vegetation in this area is lowland tropical of interspecific responses to song playback. In rainforest; within the Gran Reserva, some half April-May, antbird singing was consistent in million hectares of pristine lowland rainforest several territorial species, but rather sporadic in are protected (although precariously), constitut- others, limited to one or two repetitions of the ing perhaps the largest extent of primary forest song at dawn. Although the taping and playback in Central America. Canopy trees reach >50 m of songs assisted in locating territories, it did not in height, and the composition of the forest ap- overcome problems of small sample sizes of ter- pears similar to that at La Selva, Costa Rica (100 ritories in some of the rarer antbird species at km to the southwest), although the dominant for- the site. est tree Pentaclethra macroloba is less common The vegetation characteristics of located ter- at the Bartola and Dipteryx panamensis more so ritories were measured. Vegetation density was (Hartshom 1983). recorded as the inverse of horizontal distance to Although most of the vegetation in the vicin- 50% cover of an imaginary board in the vertical ity of the site is primary forest, there are natural plane at a series of height (m) intervals: 0.15, edge habitats besides those around abandoned 0.3, 0.6, 1.2, 2, 3, 4.5, 6, 7.5, 9, 10.5, and 15 m, clearings. They include forest edges along the and the approximate canopy height at the terri- banks of the major rivers and within the forest tory center was noted. along the larger streams, and around tree-fall Antbird foraging behavior and ecology was 786 MARTIN L. CODY these differences was used as a null model to which the observed weight intervals were con- g2 trasted. The procedure again assigned a proba- bility to the observations, the one-tailed test as- : 1 u. sessing the likelihood that the observed spacing SO of species on the log(weight) axis could be the E-1 result of a random selection of species’ body E -‘ 2 weights. Third, the Barton-David test for uni- ;,- 6-3 form spacing of species along the body weight axis was used (Barton and David 1956). I com- -4 -6 -4 -2 0 2 4 6 puted the test statistic [l-Pr(G, < a)], the prob- Discriminant Factor (1) ability that the magnitude of the ratio of inter- I+‘lGURE 1. Discriminant function analysis of the vals r, s on the log(weight) scale (with ratios vegetation characteristicsin the territories of eight spe- size-ordered 1, 2, 3, . , r, . , s, . ) might cies of antbirds. Taller habitat is representedto right be observed as high as a by chance alone. For on the abscissa,vegetation more open at intermediate a given number of rz intervals measured after a heights ranks higher on the ordinate. Forest interior species are distributed to the right (two species; n = random selection of (n + 1) points, Pr(G,, < a) 18 territories), Edge antbirds are positioned to the left is computed as (four species;n = 18 territories), and gap species(two r-l s-r-l species;n = 10 territories) are intermediate.