Habitat Specialization and Foraging Behavior by Birds of Amazonian River Islands in Northeastern Peru

Roughly 15% of the rich Amazonian avifauna has been shown to be dependent on riverine habitats, particularly islands that form in the channels of the Amazon River and its largest tributaries. In northeastern Peru and southeastern Colombia, over 230 species have occurred on river islands, about 19 of which appear to be restricted to habitats found almost exclusively on islands, such as Tessaria-scrub and large tracts of Cecropia forest. Habitat specialization and foraging behavior were studied quantitatively for 31 bird species that used only terrestrial habitats on two study islands on the Napo River in northeastern Peru. Density estimates for residents on a small, Tessaria-dominated island, exceeded 160 birds/ha, a figure an order of magnitude greater than bird densities previously reported for forest habitats in the Neotropics. Obligate river island species showed a higher degree of microhabitat specialization than did nonobligate island species. Several of the most specialized species were found mainly in the Tessaria scrub or the understory of the Cecropia forest. Most of the species were primarily insectivorous, but several also took advantage of the abundant Cecropia fruit and were at least partially frugivorous. Little difference in the degree of foraging specialization was found between obligate and nonobligate island species, however, both groups were specialized with respect to foraging technique and substrate use. Water fluctuation of up to 20 m between seasons, completely flooding the young islands and the understory of the older islands, creates the need for species to be good colonizers, as does the natural, rapid, succession of vegetation on the islands from Tessaria, to Cecropia, to an older, more complex vegetation. Conservation plans for the Amazon basin should take into account the high dependency of species on riverine habitats, particularly islands.