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 (Ն5 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 Ͼ10 yr and high breeding success took Ͼ60% 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.

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.

Abstract. There has been considerable debate in the study of hybrid zones as to whether hybrids may be superior to parental types within the area of contact (bounded hybrid superiority). In birds, naturally occurring hybridization is relatively common, and hybridization within this group always involves mate choice. If hybrids are superior, females choosing heterospecific mates should be expected to show higher fitness under the conditions prevalent in the hybrid zone. Hybrid superiority under these circumstances would reduce reinforcement and thereby help to maintain the hybrid zone. To examine this issue, we studied reproductive performances of hybrids and parental species of gulls (Larus occidentalis and Larus glaucescens) at two colonies within a linear hybrid zone along the west coast of the United States. This hybrid zone contains predominantly gulls of intermediate phenotype. Previous studies indicated that hybrids were superior to one or both parental types, but provided no data on possible mechanisms that underlie this hybrid superiority. Using a hybrid index designed specifically for these species, we identified to phenotype more than 300 individuals associated with nests, including both individual males and females within 73 pairs in the central portion of the hybrid zone and 74 pairs in the northern portion of the hybrid zone. There was little evidence of assortative mating, and what little there was resulted solely because of pairings within intergrades. In the central hybrid zone, females paired with hybrid males produced larger clutches and hatched and fledged more chicks compared with females paired to western gull males. This was a result of heavy predation on eggs in sand habitat, where male western gulls established territories. In contrast, many hybrid males established territories in vegetated cover that was less vulnerable to predation. In the northern part of the hybrid zone, clutch size did not differ among pair categories, however, there were differences in hatching and fledging success, with females paired to hybrid males showing better success compared to females paired to glaucous-winged gull males. Hybrids showed better hatching and fledging success in the north because hybrids are more like western gulls than glaucous-winged gulls in foraging behavior, taking a higher percentage of fish in their diet, which enhances chick growth and survival. This is believed to be the first documentation of bounded hybrid superiority that delineates the mechanisms that underlie hybrid superiority.

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