Resource allocation in breeding seabirds: responses to fluctuations in their food supply

In the vicinity of the Pribilof Islands in the Bering Sea, abundance of foo d available to surface-foraging seabirds was greater during the chick-reari ng period in 1988 than in 1987, whereas abundance of food available to purs uit-diving seabirds was greater in 1987. Here we examine how breeding succe ss and resource allocation of surface-foraging black-legged kittiwakes Riss a tridactyla (BLKI) and pursuit-diving thick-billed murres Uria lomvia (TBM U) Varied with the fluctuations in their food supply. We also examine a dif ference in resource allocation among parents raising chicks at the large co lony on St. George Island and those at the nearby small colony on St. Paul Island. We studied breeding success (BS), field metabolic rates (FMR, asses sed by using doubly labeled water), foraging distribution, and nest attenda nce of parents and growth rate (GR) of chicks. The BS of BLKIs was lower in 1987 (a season of less abundant food for kittiwakes) than in 1988 (a seaso n of more abundant food), and parents had higher FMRs in 1987 than in 1988. At-sea distributions and nest attendance suggested that in 1987 BLKIs fora ged farther from the colonies, which could have resulted in the higher FMR of the parents. GR of BLKI chicks did not vary between 1987 and 1988. The B S of TBMUs was not significantly different between 1987 (a season of more a bundant food for TBMUs) and 1988 (a season of less abundant food). Parent T BMUs had similar FMRs between the seasons. Densities of foraging TBMUs were higher within 20 km around colonies in 1987 than in 1988. Although the tot al time parent TBMUs spent foraging did not vary inter-seasonally, they per formed more foraging trips of a shorter duration in 1987 than in 1988, and the GR of TBMU chicks was higher in 1987 than in 1988. Inter-colony compari sons do not suggest that parents reproducing at the large colony work harde r to raise their young compared to parents breeding at the small colony. In 1987 parent BLKIs failed in raising young at the large colony, whereas one -third of BLKIs fledged their chicks at the small colony. In 1988, however, RS and FMRs of parent BLKIs were not significantly different between the c olonies. Also, TBMUs at the large colony had higher BS than those at the sm all colony in both 1987 and 1988. Furthermore, in both years parent TBMUs f eeding young at the small colony foraged farther from the colony and had si gnificantly higher FMRs than at the large colony. These results suggest tha t fluctuations in food supply affect resource allocation in seabirds. Howev er, a decrease in food abundance is Likely to cause an increase in energy e xpenditures of parent BLKIs, whereas growth rates of their chicks are less affected. For the TBMUs, food shortages are likely to cause a decrease in g rowth of the chicks, but not an increase in energy expenditures of the pare nts.