Rushing in the spring and relaxing in the fall: seasonal and sex-specific migration profiles of Dunlin

Developing effective species conservation strategies relies on our ability to understand the spatiotemporal distribution of birds across their annual cycle. Assessing the connectivity between breeding and non-breeding areas remains challenging in migratory species that may exhibit intraspecific variation in migration strategies. Here we use light-level geolocation to test for sex-specific differences in the migration strategies (i.e. migration routes, stopovers, and wintering sites) of a population of Dunlin (Calidris alpina hudsonia) that breeds in Churchill, Manitoba, Canada. Churchill Dunlin exhibited relatively weak connectivity, as birds spread out evenly across the Atlantic Coast south of New Jersey, U.S.A, and the Gulf Coast from Mexico to Florida, U.S.A., the entire known non-breeding range of the species. We did identify important concentrated stopovers in James Bay, ON, Canada and along the coast of Delaware, U.S.A. Overall migration and stopover durations were significantly shorter during spring (northward) compared to fall (southward) migration. During spring migration, males followed a more direct migration route than females. In fall, male Dunlin departed breeding grounds earlier and spent more time on migration than females, likely driven by a trend of longer stopover durations. These sex-specific differences suggest that in spring, males deploy an energy minimization strategy to reach breeding grounds quickly and select the highest quality mating territories, whereas in fall, males take a much more relaxed migration. This research sheds light on the least understood period of the avian life cycle and informs our ability to target conservation initiatives to locations most important for this species.