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

Abstract

In the vicinity of the Pribilof Islands in the Bering Sea, abundance of food available to surface-foraging seabirds was greater during the chick-rearing period in 1988 than in 1987, whereas abundance of food available to pursuit-diving seabirds was greater in 1987. Here we examine how breeding success and resource allocation of surface-foraging black-legged kittiwakes Rissa tridacty1a (BLKI) and pursuit-diving thick-billed murres Uria lomvia (TBMU) varied with the fluctuations in their food supply. We also examine a difference in resource allocation among parents raising chicks at the large colony on St. George Island and those at the nearby small colony on St. Paul Island. We studied breeding success (BS), field metabolic rates (FMR, assessed by using doubly labeled water), foraging distribution, and nest attendance 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 season 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 foraged 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 BS of TBMUs was not significantly different between 1987 (a season of more abundant food for TBMUs) and 1988 (a season of less abundant food). Parent TBMUs had similar FMRs between the seasons. Densities of foraging TBMUs were higher within 20 km around colonies in 1987 than in 1988. Although the total time parent TBMUs spent foraging did not vary inter-seasonally, they performed 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 comparisons do not suggest that parents reproducing at the large colony work harder 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 colonies. Also, TBMUs at the large colony had higher BS than those at the small colony in both 1987 and 1988. Furthermore, in both years parent TBMUs feeding young at the small colony foraged farther from the colony and had significantly higher FMRs than at the large colony. These results suggest that fluctuations in food supply affect resource allocation in seabirds. However, a decrease in food abundance is likely to cause an increase in energy expenditures of parent BLKIs, whereas growth rates of their chicks are less affected. For the TBMUs, food shortages are likely to cause a decrease in growth of the chicks, but not an increase in energy expenditures of the parents.