Avian Breeding Adaptations: Hatching Asynchrony, Brood Reduction, and Nest Failure

Hatching asynchrony within a brood, produced by the parent(s) initiating incubation before all eggs are laid, varies in degree among and within bird species. Its occurrence has been explained primarily as a mechanism for brood reduction in habitats with unpredictable resources, and less often as a response to nest predation. Using a simply model of the effect of total nest failure on optimal asynchrony, we predict that, even in the absence of brood reduction, most bird species should commence incubation before the last egg is laid. Data from 87 species of altricial birds support the nest-failure model and not the brood-reduction hypothesis. Other features of bird reproductive biology (e.g., variation in egg weight), which are problematic for the brood-reduction hypothesis, are also explained by the nest-failure model.