We examined predictions derived from three hypotheses (hurry-up, peak-load reduction, and brood reduction) regarding the adaptive significance of hatching asynchrony. The study was conducted, in 1991 and 1992, in a colony of Ring-billed Gulls (Larus delawarensis) on l'île de la Couvée, Montréal, Québec, Canada. The hypotheses were examined by comparing the mass and size of 18- to 20-day-old chicks from broods that hatched asynchronously (unmanipulated) and synchronously (manipulated). We also compared feeding rates and fledging success between asynchronous and synchronous broods. Also, in 1992, a sample of asynchronous and synchronous broods was experimentally food stressed by providing them with an additional chick when the oldest resident chick was 4 or 5 days old. Consistent with the hurry-up hypothesis, hatch spreads were significantly shorter in 1991 when the mean clutch initiation date was significantly earlier compared with 1992. Also consistent with the hypothesis, hatch spreads increased significantly through the breeding season. In agreement with the peak-load-reduction hypothesis, feeding rates were significantly lower in asynchronous broods compared with synchronous broods. Contrary to the hypothesis, however, the feeding rate was similar for food-stressed asynchronous and food-stressed synchronous broods. As predicted by the brood-reduction hypothesis, survivorship was higher for first-hatched chicks in asynchronous broods compared with chicks in synchronous broods. Total brood loss as a result of starvation tended to occur less often in asynchronous broods compared with synchronous broods. Likewise, fledging success was higher in asynchronous broods compared with synchronous broods. However, contrary to the brood-reduction hypothesis, survival rate of first-hatched chicks, total brood loss, and fledging success were similar in food-stressed asynchronous and synchronous broods. The above results indicate that several benefits accure to Ring-billed Gulls from hatching asynchrony.
