Sampling on the migration route: genetic structure and population assignment of Roseate Terns on the East Atlantic Flyway

The Roseate Tern Sterna dougallii is a locally threatened seabird that breeds in four discrete metapopulations across the Atlantic Ocean Basin: Northwest Atlantic, Caribbean, Azores and Northwest Europe. While this species is well‐monitored and the focus of long‐term management efforts across the breeding distribution, significant knowledge gaps remain for the Northwest Europe and Azores metapopulations with regard to genetic diversity, population structure and connectivity outside the breeding season. Here, we address this gap by leveraging data collected as part of a long‐term mist‐netting programme in Senegal, West Africa. We genotyped 240 individuals sampled during the northward migration in 2021–2023, including 112 ringed or resighted in breeding colonies. We used a panel of 15 microsatellite loci to investigate genetic diversity and population structure for birds of known origin (determined by location of ringing or resighting), and compared them to individuals sampled from a site in the Northwest Atlantic. We found relatively low genetic diversity indices within the Northwest Europe and Azores metapopulations, consistent with previous reports from the Caribbean, Azores and Northwest Atlantic metapopulations. We found no significant genetic differentiation among sites within Northwest Europe, consistent with metapopulation modelling studies. Significant differentiation was detected among the Northwest Europe, Azores and Northwest Atlantic metapopulations. We used discriminant analysis of principal components to assign 79% of individuals of unknown origin to the Azores or Northwest Europe metapopulations with ≥ 80% probability, while no birds were assigned to the Northwest Atlantic metapopulation. These results contribute the first comprehensive documentation of the genetic diversity of Roseate Terns utilizing the East Atlantic Flyway and the Northwest Europe metapopulation, document gene flow within Northwest Europe and demonstrate the value of sampling birds during migration for population genetic studies.