Hatching Asynchrony, Egg Size, Growth, and Fledging in Tree Swallows

AssTr•cT.--Marked asynchronous hatching was observed in Tree Swallows (Iridoprocne bicolor), apparently because incubation started before the full clutch was laid. Asynchronous hatching resulted in pronounced size differences of nestlings. Immediately after hatching, the first-hatched nestling was, on average, 94% heavier than the last-hatched. Clearly, a weight hierarchy was established, with small late-hatched young at a disadvantage. Egg weight increased with laying order, and last-laid eggs tended to hatch last. Thus, egg-size differences did not enhance the weight hierarchy due to hatching asynchrony. Early hatchlings had longer nestling times than late hatchlings. The former also had a higher observed asymptotic weight and longer primaries at fledging than the latter. Late-hatched young were less likely to fledge than early hatchlings. The number of young fledged increased with hatching asynchrony but so did brood size. Tree Swallows appear to use brood reduction to maximize reproductive output, but alternative hypotheses cannot be ruled out. Received 21 September 1981, accepted 8 March 1982.