JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. SUMMARY(1) We studied age-specific breeding success of western gulls (Larus occidentalis) on south-east Farallon Island, California in relation to annual variation in food abundance, 1983-89. We assessed yearly reproductive performance on the basis of clutch initiation date, clutch size, hatching and fledging success, and the number of chicks raised to independence. We categorized age of parent into three groups: young, middle-aged and old.(2) Mature birds initiated breeding earlier and laid larger clutches; they fledged more chicks than younger birds as a result of hatching a greater proportion of their eggs and fledging a greater proportion of their hatched chicks. The oldest birds generally did not differ from middle-aged in breeding performance. The difference in reproductive performance between young and mature birds was most apparent during years when food availability was reduced.(3) Timing of breeding influenced reproductive performance independently of parental age; reproductive performance decreased throughout the breeding season. Within-season reductions in reproductive performance were most apparent for young breeders and individuals with small clutches or broods.(4) Breeding performance varied significantly among years independently of age of the study animals. Food availability accounted for part of the yearly variation in breeding success.(5) We suggest that clutch/brood size reflects individual quality. Age-specific breeding success in western gulls is best explained by previous breeding experience and individual quality. Age and reproduction in gulls & Parker 1975). The trade-off between current and future reproductive success has been the focus of much detailed research. However, few studies have provided unequivocal support for increasing reproductive effort with age because other variables intercorrelated with parental age may directly influence reproductive success without invoking an agespecific life-history strategy (Nur 1984; Reid 1988). Age-specific breeding performance may improve with previous breeding experience and foraging efficiency, or be a statistical artefact of individual variation if only high quality animals survive to the oldest ageclasses (see Coulson & Porter 1985; Coulson & Thomas 1985a).Much of the literature concerning age-specific reproductive performance is based upon colonially nesting seabirds relative effects of parental age are not well understood due to confounding by timing of breeding and yearly resource abundance.We studied age-specific reproductive performance in relation to food availability from 1983 t...
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