Migration patterns of San Francisco Bay Area Hermit Thrushes differ across a fine spatial scale

Abstract Effective conservation of short-distance migrants requires an understanding of intraspecific variation in migratory patterns across small spatial scales. Until the advent of ultra-light geolocation devices, our knowledge of the migratory connectivity of songbirds was limited. For the Hermit Thrush (Catharus guttatus), subspecies delineations and connectivity patterns have been unclear in the portion of their breeding range in western North America from southeastern Alaska to northwestern Washington, where individuals wintering in the San Francisco Bay Area of California purportedly breed. To determine breeding locations and migratory timing of the Bay Area’s wintering Hermit Thrushes, we deployed geolocators at sites to the north and south of the San Francisco Bay. We compared results from these two regions to one another and to connectivity patterns suggested by subspecies definitions. We collected morphometrics to identify regional differences. Hermit Thrushes that wintered in the North Bay had a wider and more southerly breeding distribution from the British Columbia coast to northwestern Washington, whereas South Bay thrushes migrated to southeastern Alaska and the British Columbia coast. In general, North Bay thrushes departed wintering grounds and arrived on breeding grounds earlier than South Bay thrushes, but we cannot eliminate sex as a factor in these differences. Regional morphology differed only in bill length. Intraspecific isolation in glacial refugia during the Late Pleistocene may explain these fine-scale geographic variations in migration patterns and morphology.