Use of Amazonian Forest Fragments by Understory Insectivorous Birds

We sampled understory insectivorous birds in Amazonian forest fragments from before isolation through 9 yr after isolation. We accumulated 3658 mist net captures of 84 insectivorous species in five 1—ha fragments and four 10—ha fragments. Abundance and species richness declined dramatically after isolation, even though fragments were separated from continuous forest by only 70—650 m. Three species of obligate army ant followers disappeared within the first 2 yr after isolation. Mixed—species flocks containing 13 commonly netted species disintegrated within 2—3 yr after isolation, although three species that dropped out of flocks persisted in fragments. Among insectivores not associated with flocks or army ants, only two species of edge specialists were unaffected by fragmentation. Overall, loss of forest insectivores was not compensated for by an increase in nonforest or previously uncommon species. Secondary vegetation surrounding fragments strongly affected use of fragments after isolation. Fragments surrounded by Vismia, the dominant regrowth where felled forest was burned and temporarily used as cattle pasture, remained depauperate. In contrast, many species returned to fragments by moving through regenerating forest dominated by Cecropia, which occurred in areas where the felled forest was not burned. Both 1— and 10—ha fragments surrounded by Cecropia were used by ant followers by 5 yr after isolation. Mixed—species flocks reassembled in 10—ha fragments surrounded by Cecropia by 7—9 yr after isolation, and augmented their group territories by foraging in secondary forest outside fragments. Solitary species were more variable in their responses, although several species returned to 10—ha fragments surrounded by Cecropia. Terrestrial insectivores, such as Sclerurus leafscrapers and various antbirds, did not return to any fragments, and appear to be the group most vulnerable to fragmentation. Ordination of the insectivore community showed that 1—ha fragments diverged from their pre—isolation communities more than did 10—ha fragments. Communities in 10—ha fragments surrounded by Cecropia were more closely associated with pre—isolation communities than those in fragments surrounded by Vismia. Over time, communities in 10—ha fragments surrounded by Cecropia became more like pre—isolation communities, although communities in other fragments generally continued to diverge.