Influence of Summer Biogeography on Wood Warbler Stopover Abundance
We evaluated the effect of summer biogeography of migrant wood warblers (Parulidae) on their stopover abundance. To characterize abundance patterns, we used mist-net capture data from spring and fall migration in the Middle Rio Grande Valley, New Mexico, spring migration on the Gulf Coast Of Louisiana, and fall migration on the Gulf Coast of Alabama. To describe the summer biogeography of 47 species of wood warblers, we used indices of their summer range size, their summer density, and distance between their summer ranges and our netting sites. Multiple linear regressions indicated that biogeographic indices explained 55% and 49% of variation in captures in the Middle Rio Grande Valley during spring and fall, respectively. On the Gulf Coast these regressions explained 25% of the variation during spring at the Louisiana site and 51% during fall at the Alabama site. Both summer range size and distance between the summer range and study sites explained significant portions of the variation in three of the four analyses. Interestingly, the importance of biogeographic factors was least evident among spring migrants along the Gulf Coast of Louisiana. The difference between this site and other sites may reflect differences between migrants arriving after a Gulf crossing and those migrating across continental land masses or possibly an increased importance of winter biogeography for migrants crossing the Gulf of Mexico in the spring. In general, these results indicate that abundance of migrant warblers at our netting sites in both the eastern and western United States during spring and fall migration were influenced by summer biogeography. Consequently, we suggest including biogeographic analyses in assessments of conservation priorities for local stopover sites.