Stopover ecology and habitat use of migratory Wilson's warblers

The conservation of long-distance migratory songbirds is complicated by their life-history characteristics and the spatial scales that they traverse. Events during migratory stopovers may have significant consequences in determining the population status of migratory songbirds. Using Wilson's Warbler (Wilsonia pusilla) as a focal species, we investigated effects of age and sex on stopover parameters during spring and fall migration. Variation in stopover patterns was predominantly sex related in spring and age related in fall. In spring, males appeared at study sites earlier than females, and they carried higher fat stores and had lower recapture rates and higher rates of mass gain. In fall, migratory passage overlapped temporally among age and sex classes, but young birds had lower fat stores, higher recapture rates, and longer stopovers than adults. Habitat use differed between spring and fall migration and was affected by the combination of age and sex. Wilson's Warblers appeared to prefer native willow habitat during spring migration. Agriculture field/edge habitats may have represented "sink habitats" because birds in these habitats tended to be immatures and had a lower rate of fat deposition and longer stopovers. The data suggest that immatures were particularly vulnerable to habitat disturbances at stopover sites during fall migration, probably because of their lower social status and inexperience with long-distance migration. We conclude that knowledge of intraspecific variation in migration strategies is important for effective management of stopover habitats for migratory songbirds.