Weak genetic structure, shared nonbreeding areas, and extensive movement in a declining waterbird

Understanding population mixing, movements, and connectivity of populations is an important first step towards effective conservation, particularly for long distance migrants that are suffering the greatest population declines, as this allows researchers to recognize how populations may face different risks throughout the annual cycle. We combined population genetic and individual tracking data to quantify the genetic structure and full-cycle movements of the declining North American Black Tern (Chlidonias niger surinamensis). A total of 147 genetic samples were collected from 9 breeding colonies across the range (Maine, Ontario, Michigan, Iowa, Wisconsin, Nebraska, Saskatchewan, and Oregon), and 19 light-level geolocators were recovered from 3 colonies (Ontario, Michigan, and Saskatchewan). Our results demonstrated weak genetic structure, and tracking data demonstrated the use of shared nonbreeding areas between central (Saskatchewan) and eastern (Ontario and Michigan) breeding populations. Our tracking data also provide novel evidence of long-distance breeding dispersal (~1,400 km between breeding locations across years) based on an individual tracked across multiple years, as well as short distance dispersal (~2.5-57 km) based on new recovery locations of 6 tracked individuals. Our results are consistent with the hypothesis that the shared use of nonbreeding areas influences physical condition, timing of departure, and subsequent reproductive timing in such a way as to facilitate dispersal across the breeding range and contribute to weak genetic structure among breeding populations. This study is the first to explore population genetics and migration of North American Black Terns. Extensive movement of individuals may pose a challenge from a conservation perspective as important areas and habitats throughout the annual cycle may be difficult to predict, and future studies should build on our work via extensive mark-resight effort using color bands, tracking individuals from more breeding sites, and examining carry-over effects to further investigate when in the annual cycle populations are most limited.