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 tridactyla (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. KEY WORDS: Doubly labeled water · Field metabolic rates · Food abundance · Kittiwakes · MurresResale or republication not permitted without written consent of the publisher

Does location really matter? An inter-colony comparison of seabirds breeding at varying distances from productive oceanographic features in the Bering Sea

“…However, this might not always be the case, for example if foraging conditions on the shelf deteriorate further, the additional distance to the productive continental shelf-break may be cost-prohibitive for murres to reproduce successfully on St. Paul Island. Conditions on St. Paul have certainly been shown to be poorer than St. George at least in some years, with higher CORT levels (Benowitz-Fredericks et al, 2008) and lower chick-feeding rates (Kitaysky et al, 2000).…”

“…Certainly, wild murre chicks have been shown to consume under their maximum limit (Harris and Wanless, 1995). Alternatively, murre parents may have limited ability to increase parental effort (Erikstad et al, 1997;Kitaysky et al, 2000; but see Paredes et al, 2005). Or, chick feeding rates may be constrained by the need for parents to spend time at the colony, with colony attendance by both parents more important on Bogoslof because of the presence of predatory glaucous-winged gulls Larus glaucescens (Martindale, 1982), whereas there are almost no avian predators on the Pribilof Islands.…”

“…The closely related common murre has been shown to buffer chick-feeding rates and fledging success across a wide range in food availability by reallocating discretionary time spent at the colony to foraging effort Piatt et al, 2007). Reproductive performance is therefore not a good indicator of foraging conditions (Kitaysky et al, 2000), whereas the behavior of adults (colony nest-attendance; Harding et al, 2007), and blood levels of stress hormones have been shown to be directly related to changes in the availability of food resources to chick-rearing murres.…”

“…Although both parents share chick-provisioning duties, trip duration and distance is also constrained by the need for one member of the pair to be present with the chick at all times to prevent predation and exposure to poor weather conditions (Birkhead and Nettleship, 1981). Flexible time budgets allow murres to maintain chick-feeding rates and breeding success (Kitaysky et al, 2000) at fairly constant rates over a range of foraging conditions, by allocating more time to foraging when conditions are poor . However, very little is known about how foraging distances vary under different conditions of food availability (e.g., Wilson et al, 2005).…”

Foraging distance affects reproductive success in Magellanic penguins

“…Oceanographic conditions vary from year to year, affecting the foraging locations and breeding success of marine central-place foragers, such as seabirds and pinnipeds, that feed at sea but breed on land (Boyd et al 1994, Kitaysky et al 2000, Sydeman et al 2001, Davoren & Montevecchi 2003. Foraging locations at sea, particularly distance to food, influence offspring feeding frequency (Pinaud et al 2005), reproductive success (Inchausti et al 2003), and adult energy balance (Shaffer et al 2003).…”