Egg size variation in birds with asynchronous hatching: is bigger really better?

In many animals large size at birth enhances offspring survival, but comparative evidence remains equivocal for birds. Failure to consider asynchronous hatching (ASH) may have confounded previous analyses. We assessed effects of egg size and ASH on growth and survival of common grackle (Quiscalus quiscula) nestlings to test the hypothesis that females adjust the size of last-laid eggs to modify effects of ASH. Although positive, the effect of egg size on nestling growth and survival was overwhelmed by the effect of ASH, with late-hatched nestlings being most likely to starve. Egg size did significantly affect growth late in the nestling period, but only because starvation had greatly reduced hatching asynchrony among surviving nestlings. Similarly, in experimentally synchronized nests, egg size and hatching asynchrony both affected offspring growth early in the nestling phase. Our results suggest that there is unlikely to be an adaptive advantage to females from varying the size of last-laid eggs in species with substantial ASH and that studies to assess the effect of a given maternal effect (e.g., varying egg size) should be done in the context of other maternal effects that may be operating simultaneously (e.g., ASH).