Army ants and obligate ant-following birds: A study of ecology, spatial movement patterns and behavior in Amazonian Peru

Five species of obligate ant-following birds (Phlegopsis nigromaculata, Myrmeciza fortis, Rhegmatorhina melanosticta, Gymnopithys salvini , Thamnophilidae; Dendrocincla merula, Dendrocolaptidae) and two species of army ants (Eciton burchelli and Labidus praedator) were studied in Amazonian Peru over five years. The question of species coexistence in these five ecologically similar bird species was explored through analysis of population ecology, behavior and spatial movement patterns of the birds. Two of the five bird species ( D. merula and M. fortis) utilized one or the other of the two army ant species as a preferred foraging resource. The three obligate ant-following antbirds that did not prefer one army ant species over the other (P. nigromaculata, R. melanosticta, and G. salvini ) are segregated by body mass, which may allow differential utilization of space along an antswarm front's width. This would permit a smaller, more subordinate species to “fit” along the front of an antswarm that was already “full” to a different bird species. Behaviorally, species implemented different strategies when choosing where to forage among available E. burchelli antswarms. Nest site selection contributed to niche breadth among the obligate ant-followers, and may act to constrain populations of the dominant species P. nigromaculata from expanding to utilize more of the available army ant resource. Constraints on the dominant species may provide the socially subordinate R. melanosticta and G. salvini with the niche space they need to coexist with P. nigromaculata. Population dynamics of the birds were not stable over five years of data collection, and the total population of obligate ant-followers dropped by almost half over the course of the study. I suggest that periodic population declines are a normal occurrence in obligate ant-following guilds. Population fluctuations decreased interference competition among individuals and may increase the ability of the subordinate species R. melanosticta to coexist with the larger, dominant P. nigromaculata at Cocha Cashu. Home range size of each antbird species was minimized in some years through reliance on the unpredictable but abundant foraging opportunities at L. praedator antswarms. This poorly studied ant species played a pivotal role in expanding the available foraging resource for obligate ant-following birds, which allowed an increase in population densities well above what would be supported solely by E. burchelli army ants. The army ant E. burchelli has been well studied on a micro-habitat scale, but measures of behavioral ecology and population density are largely unknown for the Amazon basin and are estimated for the first time in this study. Nests and reproductive biology of four of the five bird species were undescribed at the start of this study, and nests, eggs and nestlings are described for P. nigromaculata, R. melanosticta and G. salvini. The dissertation concludes with a comparative examination of obligate ant-following guilds at three additional Neotropical sites.