Classic pattern of leapfrog migration in Sooty Fox Sparrow (Passerella iliaca unalaschcensis) is not supported by direct migration tracking of individual birds

Abstract

ABSTRACT 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...