Experimental evaluation of bird movements in a fragmented Neotropical landscape

The potential negative effects of forest fragmentation on animal movement and dispersal, and its consequences for population persistence, require an understanding of how modified landscapes affect movement decisions of forest species. We used a dispersal challenge experiment, whereby we released individuals of six bird species in a cattle pasture at different distances from forest edge (0, 50, 100 and 150 m), in a fragmented tropical landscape in Mexico to investigate the gap-crossing abilities and movement behavior of six species of tropical forest birds. Gaps as narrow as 50 m affected movement behavior of tropical forest birds. A sharp change in movement behavior in gaps 100 m and larger suggested the presence of a threshold distance beyond which birds are less likely to attempt and successfully navigate during trans-gap flights. Bird responses varied with degree of forest dependence: three forest-restricted species showed greater latency to cross gaps, independent of gap width, as compared with forest-unrestricted species. Gap width had a stronger effect on the orientation and destination of forest-restricted species than that of forest-unrestricted species. The concordance of our results with those found in species-distribution and radio-tracking studies indicates that dispersal challenge experiments provide reliable predictive information about response of forest birds to gaps between isolated forest fragments. In the landscape we studied, reducing gaps to no more than 50 m and providing corridors or shade trees between fragments should facilitate movements of forest birds.