BREEDING PERFORMANCE IN THE WESTERN GULL - EFFECTS OF PARENTAL AGE, TIMING OF BREEDING AND YEAR IN RELATION TO FOOD AVAILABILITY

(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.