Food-provisioning behaviour of male and female Manx shearwaters, Puffinus puffinus

Recent attention has focused on the foraging and food-provisioning strategies that allow procellariiform seabirds (e.g. albatrosses and shearwaters) to exploit pelagic food resources whilst providing food for dependent offspring at the nest. Several studies have found sexual differences in foraging and provisioning strategies in sexually dimorphic procellariiforms. However, there are few such data for species such as Manx shearwaters, that have negligible sexual size dimorphism. We used radiotracking coupled with periodic weighing of chicks to examine foraging trip durations and overall contributions to food provisioning by individual male and female Manx shearwaters at Skomer Island, U.K. Both sexes made trips of 1-4 days but females also made longer trips of up to 7 days. As a result, mean trip duration was significantly longer in females (2.1 days) than in males (1.5 days). Males fed their chick on a greater proportion of nights, delivered food to chicks at a higher rate, and made a greater contribution to overall food provisioning than females. Meal size increased with foraging trip duration but the yield to the chick in g/day was greater after short trips. Trips of 5-7 days would give females access to the highly productive nursery grounds for sardines, Sardina pilchardus, in the southeast Bay of Biscay, which were previously thought to be exploited by birds from Skomer but were more recently thought to be beyond the foraging range of chick-rearing adults, based on a mean trip duration of 2 days. (C) 2001 The Association for the Study of Animal Behaviour.