Mega-hydroelectric dams in lowland Amazonia create large archipelagos of upland forest, yet few studies have taken advantage of these fragmented landscapes to examine the effects of habitat insularization on vertebrate populations. We investigated the effects of insularization on bird species richness and composition at 27 variable-sized islands formed by the Balbina Hydroelectric Reservoir in the central Brazilian Amazon after 25 years of isolation, which were compared to three mainland sites in adjacent continuous forest. Islands were characterized by patch and landscape metrics, and the local avifauna was surveyed at all sites (including 172 sampling points) four times using point counts, and twice using playbacks. Island size was the strongest predictor of species richness and similarity, whereas other metrics showed weak or no effect. On the basis of 61 forest bird species distributed across six foraging guilds, terrestrial omnivores and insectivores, small understorey frugivores and large-bodied trunk frugivore/faunivore comprised the guilds most affected by habitat insularization. Our results demonstrate a striking decline in avifaunal species richness, particularly in islands smaller than 55 ha, which represent ~. 90% of the ~. 3500 islands in the Balbina reservoir. We conclude that other major hydropower reservoirs across the lowland Amazonian basin are likely to experience similarly high avian extinction rates and suggest that the environmental licencing of these impoundments should be reassessed or at least include protected area offsets. © 2016.
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