Common Terns Sterna hirundo and Roseate Terns Sterna dougallii frequently rest on the sea surface in winter quarters and during migration

ABSTRACT Capsule Common Terns Sterna hirundo and Roseate Terns S. dougallii rarely rest on the sea surface during the breeding season but frequently do so in winter and on migration. Aims To investigate the behaviour of Common and Roseate Terns throughout the annual cycle using immersion sensors. Methods We analyzed data from immersion sensors attached to 11 Common Terns and 6 Roseate Terns that bred in the northeastern USA and wintered either on the north or east coasts of South America, to investigate temporal patterns of resting on saltwater throughout the annual cycle. Results Both species of tern rarely contacted salt water during the breeding season, except when bathing or plunge-diving, but in winter they spent several hours each day resting on the sea surface. This resting was most frequent around midday and least frequent in early mornings and late afternoons. Night-time immersion varied widely among individuals but terns wintering in eastern Brazil were less likely to spend the night on the sea surface than those wintering on the north coast of South America. We suggest that this reflected the availability of dry-land roosting sites. Conclusion Our results are consistent with and extend studies from Europe, confirming that although Common and Roseate Terns spend very little time resting on the sea surface in the breeding season, this behaviour is frequent during migration and wintering periods.