Influence of ocean primary production on the activity pattern of wintering Common Terns

Abstract Birds migrating through different flyways and wintering in different areas face spatial differences in food availability, climate, and threats that may affect their breeding performance and survival during both the breeding and the non-breeding seasons. Using light-level immersion geolocators, we studied activity patterns of Common Terns Sterna hirundo during the non-breeding period, whilst migrating along two different routes (western and eastern) and wintering in three distinct areas along the western, southern and eastern African coast. Ocean primary production data were used as a proxy for food availability amongst the three wintering areas, and indicated an increase in foraging activity in those birds wintering in east Africa, where production is the lowest. Along both routes, terns spent more time foraging at stopover sites than during travelling. Stopovers lasted longer along the eastern than along the western route. The ratio of days spent at stopovers did not significantly differ between terns using west and south wintering areas, showing no effect on the length of migration. Higher food abundance may compensate for the longer travelling distance of birds on the East Atlantic flyway that winter in southern Africa compared to those wintering in western Africa. Knowledge of the use of wintering and stopover areas by different tern populations may be used to understand potential threats and improve conservation activities.