Contrasting foraging strategies of gannets Morus bassanus at two North Atlantic colonies: foraging trip duration and foraging area fidelity

Seabirds may be able to increase their foraging efficiency by learning the whereabouts of predictable sources of prey and returning repeatedly to these locations. The occurrence of such foraging area fidelity has been little studied, particularly for pelagic species. We used satellite telemetry to study foraging behaviour and foraging area fidelity of individual chick-rearing gannets in the North Sea (at the Bass Rock, SE Scotland) and in the Celtic Sea (at Great Saltee, SE Ireland), 2 areas that differ in the distribution of prey. Foraging ranges of adults covered a wide area of ocean up to a maximum range of 540 km at the Bass Rock and 240 km at Great Saltee. At the Bass Rock, individual birds foraged in a single direction or at most in 2 distinct directions, with very similar bearings on successive trips in each direction, significant differences in bearings between individuals and significant repeatability in distances travelled. These results strongly suggest that individuals learned and remembered the locations of feeding sites and used that knowledge on subsequent foraging trips. By contrast at Great Saltee, bearings of successive trips by individuals were much less similar, with highly variable distances to destinations, no differences in bearings among individuals and no significant repeatability in distance travelled. These results indicate a much lower degree of foraging area fidelity at Great Saltee, probably due to a more uniform or less predictable distribution of prey in the Celtic Sea than in the North Sea. Despite marked differences between colonies in distances to trip destinations, durations of foraging trips and foraging area fidelity, the behaviour of birds during foraging trips was very similar at the 2 sites: the average speed of travel during foraging trips was almost identical and birds at both colonies spent about half their time at sea in flight.