Determinants of local recruitment in a growing colony of Audouin's gull

1. Local recruitment of Audouin's gull (Larus audouinii Payraudeau) was stu died between 1988 and 1997 at the Ebro Delta colony (north-western Mediterr anean). Since its establishment in 1981, the colony has dramatically grown to include, in 1997, 65% of the total world population. Several hypotheses were tested, involving the effects of a badger predatory event in 1994, and sex, age and cohort (year of birth) on recruitment. 2. Results supported the prediction that colony size influenced recruitment : the probability for any individual to have previously bred increased thro ughout the study, together with colony size. At the end of the study, 90% o f breeders aged 4 years had already been recruited at age 3, the age of fir st reproduction by Audouin's gulls. As expected by the dramatic increase of breeding numbers, most local recruitment occurred at very young ages, espe cially when compared with other Laridae. 3. Neither food availability nor reproductive success affected recruitment. Recruitment was not affected by high nest predation by the badger, althoug h after the event, the proportion of Ebro Delta birds nesting on the nearby Columbretes Islands tripled. 4. Probability of first reproduction depended on age: it was the highest at ages 3 and 4, and then decreased sharply with age to stabilize beyond age 6 to a value depending on the year and cohort but always very low (< 5%). C ohort and sex did not influence local recruitment. 5. Annual resighting rates ranged between 35% and 82%, and were higher for females. This may represent a sex-dependent suspension of breeding, probabl y as a trade-off between early recruitment and future survival.