Foraging trip time-activity budgets and reproductive success in the black-legged kittiwake

Seabirdsare central place foragersduring the breeding season and, as marine food resources are often patchily distributed, flexibility in foragingbehaviour may be important in maintaining prey delivery rates to chicks. We developed a methodological approach using a com- bination of GPS data loggersand temperature-depth recorders that allowed us to describe the behaviour of surface-feeding seabirds. Specifically, we tested whether differences in foragingbehaviour of black-legged kittiwakes Rissa tridactylacould be linked with reproductive success by comparing 2 consecutive years at 2 sites. At Rathlin Island (Northern Ireland) during 2010, for- aging differed markedly from that during 2009 and from that at Lambay Island (Republic of Ire- land) during both years. Birds exhibited foragingtrips of greater duration, travelled a greater total distance, spent more time in transit and spent longer recuperating on the surface of the water. This notable shift was associated with a decline in breeding success, with greater loss of eggs to preda- tion and lower prey delivery rates, resulting in the starvation of 15% of chicks. We suggest that food resources were reduced or geographically less accessible during 2010, with suitable foragingareas located further from the colony. Birds did not invest greater amounts of time attempting to catch prey. Thus, our results indicate that kittiwakesat Rathlin modulated their foragingbehav- iour not by increasing foragingeffort through feeding more intensively within prey patches but by extending their range to increase the probability of encountering more profitable prey patches.