Higher probability of tick infestation reveals a hidden cost of army ant‐following in Amazonian birds

The foraging specialization of army ant‐following birds has long intrigued ecologists and provided numerous questions such as why, how and when did this foraging guild specialization arise and evolve. Many of the answers to these questions have focused on ecological interactions such as predation and competition, whereas little has been done to study the potential effects of host–parasite interactions among members of this foraging guild. Using 1177 Amazonian birds from 187 species, we studied the probability of tick infestation in relation to attendance of birds at army ant swarms. We demonstrate that the probability of tick infestation was higher among professional and occasional ant‐followers than in bird species that do not follow army ants to prey upon leaf‐litter flushed arthropods. Moreover, occasional ant‐followers harbored a greater diversity of nymphal ticks. We argue that although the evolutionary transition toward specialized ant‐following behavior has a genetic basis, a trade‐off between the cost of evolution of this trait and its fitness benefits might exist. In this case, the cost is in the form of higher parasite pressure. Therefore, parasites may impose a selective pressure not only during foraging but also through the allocation of resources for an immunological response. Our results of an analysis of tick parasitism on Amazonian birds emphasizes the importance of parasitic organisms on ecology and evolution of behavioural traits of their hosts. Considering the effects imposed by parasite pressure is essential for understanding the evolution and maintenance of behavioral and life‐history traits.