Leapfrog migration systems, whereby more-northern breeding populations overwinter the farthest south, provide unique opportunities to further our understanding of how environmental variation shapes migratory behavior and the seasonal distributions of birds. Leapfrog migration in a western Fox Sparrow subspecies complex (Passerella iliaca unalaschcensis, Sooty Fox Sparrow) was described as early as 1920, and has served as an exemplar of leapfrog systems in subsequent theoretical work. However, migration behavior within P. i. unalaschcensis has never been studied directly, nor has the proposed leapfrog pattern been confirmed through the tracking of individuals. Using light-level geolocators and GPS tags, we tested the long-standing pattern of leapfrog migration in Sooty Fox Sparrows by determining spatiotemporal movement patterns for individuals originating from a northern (Vancouver Island, British Columbia) and a more southern (Point Reyes, California) overwintering region, where migratory timing, routes, and breeding locations were predicted to differ. Our results did not support the proposed leapfrog migration pattern in several ways. Individuals overwintering on Vancouver Island were predicted to be sedentary and/or breed locally, but we found they traveled more than 3,000 km to breeding sites in coastal northwestern British Columbia and southern Alaska. Birds overwintering in California had breeding locations that overlapped those of birds from British Columbia, as well as the predicted breeding regions of 4 other subspecies. Lastly, spring and fall migration routes were largely coastal for both groups, and we found no evidence of a proposed transoceanic fall migration route between Alaskan breeding sites and Californian overwintering sites. Thus, our results do not support the long-held pattern of leapfrog migration in Sooty Fox Sparrows and further highlight that bio-logging tools can reveal important new insights into patterns of migratory behavior, even in relatively well-studied systems.
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