LONG-TERM REPRODUCTIVE OUTPUT IN WESTERN GULLS: CONSEQUENCES OF ALTERNATE TACTICS IN DIET CHOICE

Abstract

Abstract. Numerous studies reveal strong, positive skews in long-term breeding performance among free-living animals, yet few studies explore the mechanisms underlying such variation. We examine the results of a 12-yr study of a population of Western Gulls, Larus occidentalis. Of 112 pairs for which we have either long-term ($5 yr) or lifetime reproductive output, 44% bred for only a single year, and an additional 25% bred for only 2–3 yr. A few pairs bred successfully for 6–12 yr and showed higher average clutch size, hatching success, and fledging success within any single season than did less successful breeders. The principal trait influencing both survival and reproduction was individual diet, which consisted of a mix of human refuse and fish. A strong, positive relationship existed among the amount of fish taken, breeding life-span, and reproductive performance. Birds with short life-spans took little or no fish on an annual basis. Birds with breeding lifespans .10 yr and high breeding success took .60% fish. Diet choice was also important for successful recruitment; 90% of banded offspring returning to breed on the colony had parents that had taken predominantly fish. Diets of male, but not female, recruits were correlated with diets of their parents; 90% of male recruits banded as chicks in the colony were successful breeders, in contrast to 10% of other recruits. Despite apparent selective advantages, few recruits take a diet consisting predominantly of fish, which suggests the existence of at least two alternate tactics, i.e., highly risk-prone foraging for fish, or riskaverse foraging for refuse. This suggests that diet choice is passed between generations by means such as learning or cultural transmission from parents to offspring.