Residency, recruitment, and stopover duration of hatch-year Roseate Terns (Sterna dougallii) during the pre-migratory staging period

Abstract

Avian migratory stopover and staging sites represent important energetic bottlenecks and may influence population dynamics as much as breeding or wintering periods. Roseate terns ( Sterna dougallii ) are an ideal species to examine staging demography because >70% of the entire endangered northwest Atlantic population stages at accessible locations around Cape Cod, MA before southward migration. We quantified hatch-year tern weekly residency, weekly recruitment rate into the staging population, and derived weekly staging population growth rate during two post-breeding, pre-migratory staging seasons (2014 and 2015) at Cape Cod National Seashore. We also estimated hatch-year tern staging duration at Cape Cod staging grounds. Tern residency probability at Cape Cod National Seashore during 2014 and 2015 was nearly 1 during the first weeks of the season and decreased steadily over the last four weeks to ~0.5 in the final week of the study. Recruitment rates into the staging population, representing the weekly per capita increase in hatch-year terns during the staging season, indicated that most terns arrived on the staging grounds during the first weeks of the staging season (16 July–19 August). We also identified differences in staging duration between birds from the two breeding regions. Hatch-year terns from the southernmost region spent less time staging at Cape Cod National Seashore than their northern counterparts in both 2014 and 2015. These differences may indicate alternative staging strategies for individuals originating in different regions and possibly reveal differences in conditions between these areas; for example, in the availability of ephemeral prey fish.