Diet Choice in the Herring Gull: Constraints Imposed by Reproductive and Ecological Factors

Abstract

Studies of diet choice in carnivorous animals typically concentrate on maximizing rates of energy intake, and rarely emphasize either constraints on choice or alternate currencies. We examined diet choice in a generalist carnivore, the Herring Gull, Larus argentatus, in relation to both nesting habitat and reproductive performance over a 2—yr period. During prelaying and incubation periods 75—80% of Herring Gulls specialized on either intertidal organisms, human refuse, or other seabirds, while only 20—25% had generalized diets. Specific types of specialists tended to nest in particular habitats. Foraging tactics associated with each of three diets were related to time budgets and ecological constraints, i.e., levels of predation or intraspecific competition in specific habitats. Variation in diet choice was also strongly related to individual breeding performance. Intertidal specialists laid eggs earlier, produced larger and heavier clutches, and had higher rates of hatching than generalists and other specialists. This pattern held true over both years of the study even though weather conditions, and food accessibility, differed between years. Despite differences in performance related to laying and hatching, there were no significant differences in fledging rates, either between years or among diets. These results suggest that (a) choice of nesting habitat may constrain diet choice, (b) diet choice may have major effects on breeding performance, and (c) energy may not be the proper currency for examining diet choice in carnivores subject to a variety of constraints.