To eat or to feed? Prey utilization of Common Terns Sterna hirundo in the Wadden Sea

Abstract

Prey availability to seabirds has a profound influence on individual decisions about allocating somatic and reproductive investment. These decisions can be expressed in foraging behavior and prey utilization and have consequences for establishing relationships between changes in the fish populations and responses in seabird breeding performance. We report here results of an unusual opportunity to investigate the relationships between fish abundance and at-sea foraging behavior, prey utilization and food provisioning of partners and chicks of Common Terns Sterna hirundo breeding in the German Wadden Sea. High quality prey was carried out of the foraging area disproportionately often, while almost all low quality prey items were ingested by the foraging adult bird itself. Proportions of prey being used for provisioning were more similar to prey being carried out of the foraging area than to prey caught. The preferential utilization of high quality food for provisioning suggests that large proportions of low quality food being delivered to the colony may indicate a shortage of high quality food and, consequently, poor prospects of good breeding performance. Moreover, seabirds feeding whole, undigested prey items may indicate a higher abundance of high quality fish in the sea, due to selecting high quality prey for provisioning. This may result in overestimating the abundance of high quality prey fish when calculated from colony-based diet studies of single-loading seabird species such as terns alone.