COST OF REPRODUCTION AND THE EVOLUTION OF DEFERRED BREEDING IN THE WESTERN GULL

Survival patterns in a population of western gulls (Larus occidentalis ) of known age of first breeding, alpha, indicate a cost of reproducti on related to the age of initial breeding. Among both sexes, birds tha t commenced breeding at the earliest ages (3 years in males and 4 year s in females) had higher annual mortality than those that deferred bre eding one or more years. In addition, females (but not males) evidence d a cumulative cost of reproduction: holding age constant, females wit h more annual bleeding attempts demonstrated poorer survival. These pa tterns of alpha-specific survival were statistically significant after controlling for Interannual variation in food availability and are no t explained simply by variation in the intrinsic quality of individual s. To assess tie effects of these sex-specific costs on fitness, we co mbined the observed survival patterns with data on prebreeding survivo rship and alpha-specific reproductive success to estimate rates of pop ulation growth and lifetime reproductive success for different ages at first reproduction. Males showed a clearly defined fitness optimum at alpha = 4 years, which coincided with the modal cr for males in tile population. Females showed no clear optimum, except that breeding at a ge 4 was suboptimal, hence females benefited from deferring breeding t o ages 5-7 years. Observed age of first breeding also showed no clear made for females, with slight peaks at ages 5 and 7. As a result, in b oth sexes, the fitness surface for alpha corresponded well with observ ed frequencies of alpha. We suggest that stabilizing selection has act ed to shape thr phenotypic distribution of alpha in males but, due to trade-offs between survival and early reproduction. stabilizing select ion Is weak or absent in females.