Fragmentation and elevation effects on bird–army ant interactions in neotropical montane forest of Costa Rica

Army ants (Formicidae: Ecitoninae) are top predators in neotropical forests. Army ant raids support a community of diverse organisms, including birds that attend the raids to collect prey. While elevation and forest fragmentation influence army ant and insectivorous bird communities, their effects on the interaction between army ants and bird species is unknown. We studied the size and species composition of bird flocks attending army ant swarms in forest fragments and continuous forest across an elevational gradient (1100–1680 m asl) in a neotropical montane region (Monteverde, Costa Rica). We observed a total of 41 bird species attending army ant swarms. Neither the number of birds, nor the total body mass of birds, nor the number of bird species in attending bird flocks was related to elevation. However, we found a higher bird species richness, larger flock size and greater total body mass of birds attending army ant swarms in continuous forest. Continuous and fragmented forest shared many attending bird species in common, but there was elevational segregation of attending bird species. Some montane endemic birds, and neotropical migrants, attend swarms regularly and use army ant raids as a food source.