Migrant–habitat relationships during stopover along an ecological barrier: extrinsic constraints and conservation implications

For migrating land birds, the influence of extrinsic factors (e.g., weather and energetic condition) on stopover site selection relative to factors intrinsic to habitat (e.g., food) may be especially acute after crossing a large ecological barrier. We quantified bird distributions during spring and autumn migration along the coast of Mississippi after birds had crossed the Gulf of Mexico using weather surveillance radar data to test our prediction that bird distributions would be less closely associated with a proximate cue to intrinsic habitat quality (the amount of forest cover in the landscape) as proximity to the coastline increased during spring. Relative bird density was positively associated with the amount of hardwood forest in the landscape during both seasons, and broadly with proximity to the coastline during spring. In spring, bird density was less closely associated with the amount of hardwood forest with greater proximity to the coast. Due to extrinsic constraints associated with crossing the Gulf of Mexico during spring, most migrants appear to select sites for resting within 18 km from the coastline during their initial landfall and seek resource-rich habitats for refueling further inland during subsequent stopovers. During autumn, most birds seek resource-rich stopover sites inland from the coast in which to refuel before crossing the Gulf of Mexico. Because stopover habitat may be limiting to some populations of migratory land birds, the disproportionately high rates of human development and deforestation along with the reduced availability and amount of protection of forests in near-coastal areas create urgency for the conservation of forested stopover sites along the Gulf of Mexico.