DIFFERENTIAL TIMING OF WILSON'S WARBLER MIGRATION IN ALASKA

We examined age- and sex-related differences in the timing of Wilson's Warbler (Wilsonia pusilla pileolata) migration at four locations in Alaska: Fairbanks, Tok, Mother Goose Lake, and Yakutat. We captured Wilson's Warblers with mist nets for >= 5 years during spring (northbound) and autumn (southbound) migration. In spring, males passed through our two northernmost sites-Tok and Fairbanks-earlier than females. During autumn, timing of adult migration did not differ by sex, but immatures passed through earlier than adults at all four sites. During previous studies of autumn passage sampled at lower latitudes, the lack of age-related differences in migration timing could be attributed to adults migrating faster than immatures (i.e., if immatures from higher latitudes began migration earlier than the adults, then the adults may have caught up to them at lower latitudes) or to the mixing of breeding populations from different locales. Autumn migration of adults and immatures netted at our two southernmost sites, both coastal locations, preceded migration at our two interior sites. These site-specific differences in the timing of autumn migration are likely the result of our coastal stations sampling birds that breed farther south and arrive earlier than birds breeding in more northerly regions of Alaska (and sampled at our interior stations). Early-arriving populations are likely able to complete their breeding season activities earlier and, subsequently, initiate their autumn migration earlier.