Hatching date influences age at first reproduction in the black-headed gull

In long-lived colonial birds, age at recruitment is an important life-histo ry character. Variation in this parameter may reflect differences in severa l factors, including competitive ability and breeding strategies. Further, these differences may be due to timing of hatching (for instance through di fferences in competitive ability). We investigated the age of first-time br eeders in relation to hatching date in a black-headed gull Larus ridibundus colony situated in central France, from 1979 to 1993. Age at first breedin g was estimated for four groups of individuals (total n=550) according to t heir hatching date, using a recent capture-recapture methodology which allo wed us to estimate recruitment rate without the limiting assumptions of met hods relying on simple return rates. The age at first breeding was negative ly correlated with the hatching date of individuals: individuals hatched ea rlier in the season started breeding at a younger age than individuals born later. Proportionally more 2-year-old late-hatched individuals were seen b reeding on small peripheral colonies than young early-hatched individuals. This difference disappeared after age 3 years. These results strongly sugge st that individuals hatched late in the season start to breed on peripheral colonies before recruiting to their natal colony. A difference of few week s in hatching date has consequences which can last for several years.