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Five species of obligate ant-following birds – Phlegopsis nigromaculata, Myrmeciza fortis, Rhegmatorhina melanosticta, Gymnopithys salvini (Thamnophilidae), and Dendrocincla merula (Dendrocolaptidae) – and two species of army ants (Eciton burchelli and Labidus praedator) were studied in Amazonian Peru over five years. Here, I explore aspects of species coexistence in these five ecologically similar birds through analyses of their population ecology, resource use, behavior, and spatial movement patterns.
Mean home-range size of each antbird species was reduced through reliance on the unpredictable but abundant foraging opportunities at L. praedator swarms. This little-known ant species played a pivotal role in expanding the foraging resource available to the obligate ant-following birds, which allowed an increase in the birds' population densities well above what would be supported solely by the better-known E. burchelli army ants.
Two of the five bird species (D. merula and M. fortis) displayed resource selectivity among antswarms by foraging significantly more with one of the two ant species. The woodcreeper D. merula further segregated from the four antbirds in its utilization of white-lipped peccary (Tayassu peccari) herds as a foraging resource; the peccaries act as "beaters'' of arthropod prey in a manner similar to that of the army ants. The three antbirds that did not prefer one army ant species over the other (P. nigromaculata, R. melanosticta, and G. salvini) segregated by body mass, which may allow differential use of space along the width of an antswarm front. That size difference would permit a smaller, more subordinate species to "fit" along the front of a swarm that was already "full" to a different bird species.
Population dynamics of the birds were not stable over five years of data collection, and total population of obligate ant-followers declined by almost half over the course of the study. It is suggested that periodic population fluctuations are a normal occurrence in guilds of obligate ant-followers and may be exacerbated by the lack of territoriality exhibited by most of these species. Lower population density correlated with decreased interference competition among individuals. Population fluctuations may increase the ability of the subordinate species R. melanosticta to coexist with the larger, dominant P. nigromaculata in floodplain forest.
Nest-site selection may contribute to niche breadth among the obligate ant-followers. I provide descriptions of the nests, eggs, and nestlings of P. nigromaculata, R. melanosticta, and G. salvini, which were undescribed at the start of the present study.
