Interspecific Competition and the Foraging Behavior of Plain‐Brown Woodcreepers

Abstract

Listen

Translate article

When Plain—brown Woodcreepers (Dendrocincla fuliginosa) follow swarms of army ants on Barro Colorado Island, Panama Canal Zone, they forage in the zone near the ground more frequently when Ocellated Antthrushes (Phaenostictus mcleannani) are absent. The ground—foraging ant thrushes regularly supplant the smaller woodcreepers, which then move to higher or to peripheral and less productive zones. This seems a clear example of competitive exclusion by dominance. On Trinidad, where competing low—foraging antbirds are nearly always absent, fuliginosa forages near the ground at swarms. In British Guiana, where several small antibirds saturate the lower levels over the ants, fuliginosa again forages high in the undergrowth. Instead of following Svardson's hypothesis that increasing interspecific competition restricts the habitat of a species, fuliginosa restricts the foraging—zone dimension of its habitat when interspecific competition is low. Both on Barro Colorado and on Trinidad the species gravitates to a narrow but more productive zone when interspecific competition is low; it seems to leave the upper zones unoccupied at such times. This failure to fill zones occupied under conditions of more stringent interspecific competition, as well as the inability of antthrushes to completely exclude fuliginosa from the lower levels of Barro Colorado, may be explained by the "irregularity principle": available food supplies are frequently left by biological or physical irregularities in time or space, since exploitation of food always lag behind its appearance. The principle suggests that superabundance of food may be frequent rather than exceptional. In the case of the woodcreeper and the antthrush, periodic low numbers of swarms may limit the numbers of antithrushes, leaving superabundant food supplies which the woodcreepers quickly exploit. The failure of fuliginosa to occupy the upper part of its foraging beat on Trinidad may, however, reflect absence of prey in that zone. Comparison of foraging zones of populations of a species are made difficult by differences in environment and heredity of organisms in different areas even thought this study shows that results of interspecific competition in one area are similar to results for different areas for Dendrocincla fuliginosa.