BROOD SIZE, HATCHING ORDER AND HATCHING DATE - EFFECTS ON 4 LIFE-HISTORY STAGES FROM HATCHING TO RECRUITMENT IN WESTERN GULLS

1. We examined survival and recruitment (into the breeding population) in three cohorts of western gulls Larus occidentalis (Audubon) in relation to brood size at hatching, hatching order and hatching date. We divided the pre-breeding period into four stages: (i) the natal stage-'pre-fledging' from hatching to independence; (ii) the juvenile stage-the period from independence to age 1 year; (iii) the subadult stage- the second and third years of life; and (iv) the recruitment stage-the period when adults breed for the first time. 2. Hatching date and hatching order (in order of importance) each had a strong, negative effect on survival in the natal stage. Brood size had little effect on natal survival; however, after controlling for hatching date there was a significant negative effect. Juvenile survival covaried positively with brood size, negatively with hatching date, and negatively (but weakly) with hatching order. No variate correlated with survival in the subadult stage; however, recruitment probability of adults covaried negatively with hatching date. Effects of each variate did not reflect confounding by parental age. 3. Of the three variables, only the effect of hatching date varied between cohorts, and only in the natal stage. 4. The post-fledging survival advantage of hatching early in the season (expressed in the juvenile and recruitment stages) may reflect a direct effect; chicks fledging early in the season may establish lasting dominance over other chicks, and/or an indirect effect (better parental quality). We suggest that the post-fledging advantage of hatching into a large brood probably reflects a direct effect; competitive abilities may covary positively with brood size due to interaction between siblings, and/or an indirect effect (parental quality). 5. Relative recuitment rate of different cohorts was accurately predicted by natal survival. However, natal survival was a poor indicator of the probability that a given hatchling recruited because of selection occurring post-fledging on differences related to brood size and to a lesser extent on hatching order and hatching date. As a result, each parent's reproductive success was poorly measured by natal survival of their young. The data suggest that survival of young to age 1 year provides a more reliable estimate of recruitment probability and a parent's reproductive success.