Influence of Forest Cover on the Movements of Forest Birds: A Homing Experiment

Habitat loss and fragmentation affect forest birds through direct loss of breeding habitats, detrimental edge effects such as increased nest predation and brood parasitism, and possibly by limiting movements among remaining forest patches. Despite indirect evidence suggesting that landscape-scale bird movements are constrained by open areas, skepticism remains because birds routinely cross inhospitable terrain during migration. Here, we report evidence from 201 independent homing trials showing that landscape composition and configuration influence the movements (1–4 km) of two neotropical migrant (Black-throated Blue Warbler, Dendroica caerulescens and the Ovenbird, Seiurus aurocapillus) and one resident (Black-capped Chickadee, Poecile atricapillus) forest bird species in Quebec, Canada. Trials consisted of translocating territorial, mated males and measuring the time they needed to return to their territories (homing time), as well as the probability with which they returned to their territories within 30 h (homing success). Birds took more time and were less likely to return to their territories as forest cover decreased in the landscape. Once the simple linear variation due to forest cover was removed from landscape configuration variables, their influence on homing time and success was nonexistent or subtle, suggesting that landscape composition has greater predictive value than landscape configuration to infer constraints on forest bird movements. Indeed, and contrary to our expectation, mean nearest-neighbor distance between forest patches had no impact on homing time or success, but its coefficient of variation was positively correlated with homing time and negatively correlated with homing success. On the other hand, homing time and success were not influenced by the number of forest patches or the amount of edge per unit of forest cover. These results were consistent for all three bird species that we studied. Our data support the hypothesis that movements are constrained when forest birds travel in deforested and fragmented landscapes outside migratory periods. Such an impediment is likely to disrupt habitat selection processes, reduce the colonization of isolated forest patches, and ultimately, alter population structure and dynamics.