The effects of roads and their edges on the movement patterns and community composition of understorey rainforest birds in central Amazonia, Brazil

Abstract

Understorey rainforest birds appear to be exceptionally sensitive to habitat fragmentation and disturbance. This study aims to examine the responses of understorey birds to roads and their edges in the lowland rainforests of central Amazonia. A two-year mist-net study captured 3681 birds at six study sites, along a small (30 - 40m wide) unpaved road. Bird movement across the road and bird composition within the forest was compared among three road-verge treatments: cleared, moderate regrowth and tall regrowth. Understorey birds varied considerably in their ability to cross a small road. Of the seven understorey guilds studied, the frugivorous and the edge/gap guilds appeared unaffected by the road at all sites. The five insectivorous guilds, however, demonstrated significantly fewer movements across a cleared road. While forest regrowth along road verges facilitated the movements of some insectivorous bird guilds, the solitary understorey birds still showed significant inhibition. Radio-tracking experiments of translocated birds were undertaken across a highway (45 - 65 m), a large farm clearing (250 m wide), and in continuous forest. Translocated birds returned to their home ranges across the highway and in continuous forest, but not across the large farm clearing. Edge effects, caused by the road clearing, were found to significantly alter the distribution of many understorey birds. Total bird captures declined significantly near forest edges irrespective of the type of habitat bordering the edge. Of all the bird guilds, the understorey insectivores showed the strongest pattern of edge avoidance. Captures of army-ant followers, solitary species, mixed-species flocks and terrestrial species declined dramatically near borders whereas edge/gap specialists increased. Frugivores, nectivores, and canopy and mid-storey insectivores did not vary as a function of edge distance. These thesis data were integrated with two other bird datasets from the BDFFP, to examine six species-traits which could predispose understorey birds to decline and possible extinction in forest fragments. Three significant traits were identified as predictors of post-fragmentation abundance: edge response, natural abundance, and foraging guild. The relative fragmentation response was also examined, and edge response was found to be a highly significant predictor explaining almost 50% of the variability in the data. The implications of this study for reserve design and bird conservation are discussed.