Severe declines of ant-following birds in African rainforest fragments are facilitated by a subtle change in army ant communities

Tropical rainforests are disappearing at a rapid rate. Although several studies have revealed the detrimental effects of forest fragmentation on tropical birds, the ecological mechanisms facilitating the decline of populations have been poorly documented. In the tropical rainforests of Africa and America, ant-following birds track the massive swarm raids of army ants and prey on animals which are flushed by the ants. We analyzed the persistence of five species of ant-following birds along a habitat fragmentation gradient in western Kenya and tested if bird populations are limited by the abundance of army ant colonies in forests. Abundances of four of five ant-followers declined by 52–100% in forest fragments 113 ha. Multi-model Bayesian inference suggests that the decline of the three most specialized ant-followers is facilitated by a decrease in the abundance of the army ant Dorylus wilverthi in small forest fragments. Our data suggests that a second, fragmentation-tolerant army ant species, Dorylus molestus, does not functionally compensate for the decline of D. wilverthi because, first, of a higher affinity of birds to D. wilverthi raids (found for one species). Second, because the daytime activity of D. molestus is dependent on high humidity conditions, a pattern which was not found for D. wilverthi. Consequently, specialized ant-followers in small fragments, where D. wilverthi is missing, probably suffer from food scarcity due to a cease of army ant foraging in the dry season. Our results suggest that a subtle alteration of army ant communities caused by habitat fragmentation may have large ecological consequences.