Mixed-species bird flocks from Brazilian Atlantic forest: the effects of forest fragmentation and seasonality on their size, richness and stability

The effects of forest fragmentation on species richness, size and stability of bird mixed-species flocks were studied during rainy and dry seasons at nine Atlantic Forest fragments in Minas Gerais state, southeastern Brazil. Two distinct kinds of mixed-species flocks were identified at the study area: heterogeneous flock and understory flocks. The first type of association was observed in all forest fragments and was composed mainly of canopy and midstory species, but may include a few understory species. Understory flocks were composed mainly of understory species and were detected in only three forest fragments. Forest fragment area and season significantly affected species richness, size and stability of heterogeneous flocks. Moreover, species richness in heterogeneous flocks was correlated with total species pool in forest fragments. Species richness and size of understory flocks were different among the three forest fragments, but season influenced only flock size. Understory flock stability differed neither among forest fragments nor between seasons. The red-crowned ant-tanager Habia rubica was the nuclear and leader species of understory flocks, and determined the occurrence of this association in forest fragments. Our results show that forest fragmentation can affect species’ interactions in foraging associations, and that conservation plans for the region should preserve nuclear species of mixed-species flocks and their preferred habitats.