Is asynchronous hatching adaptive in herring gulls (Larus argentatus)?

Hatching asynchrony commonly induces a size hierarchy among siblings and th e resultant competition for food between siblings can often lead to starvat ion of the smallest chicks within a brood. We created herring gull (Larus a rgentatus) broods with varying degrees of hatching synchrony by manipulatin g the timing of incubation while maintaining the originally laid eggs. The degree of hatching asynchrony affected sibling size hierarchy at the time o f hatching of the last-hatched "c-chick." In unmanipulated broods, there wa s no disadvantage of being a c-chick. However, when asynchrony was experime ntally increased, we found reduced survival of the c-chick only in the exag gerated asynchronous experimental group. The effects were observable only d uring the first 10 days of chick life. We recorded no cases of the chicks d ying of starvation. Furthermore, behavioral observations indicated that the re was no sibling competition, and no selective feeding of larger sibs in t he study colony. We propose that the observed lower survival rates of c-chi cks in exaggerated asynchronous broods resulted from their lesser motor abi lities, affecting their chances of escaping predators. Fledging success for the whole colony was generally high and almost half of all pairs fledged a ll three chicks, which is indicative of a good feeding environment. We argu e that normal hatching asynchrony is a favorable solution in a good feeding environment, but that increased asynchrony reduces breeding success. We do not view asynchrony in the herring gull as an adaptation for brood reducti on and propose instead that it may come about because there has been select ion for incubation to start before clutch completion.