Oviposition drives hatching order and developmental disparities with brood mates.

Birth or hatching order can affect fitness. It has long been assumed that the order in which a bird's egg is laid (its oviposition) and first exposed to incubation relative to other eggs in the nest determines the order in which it hatches and the subsequent effects on development and survival. To my knowledge, this cause of hatching order has not been tested while controlling for laying-order effects on egg composition. Like those of many species, the last-laid eggs in clutches of the Lincoln's sparrow ( Melospiza lincolnii) hatch last and produce nestlings with reduced growth and survival rates. Last-hatched Lincoln's sparrows also develop wider bills that are associated with reduced attractiveness of adult courtship songs. Using wild, free-ranging Lincoln's sparrows, I replaced the first or fourth freshly laid egg in four-egg host nests with a freshly laid first egg from a donor nest. I discovered that the experimental elevation of oviposition-which controlled for egg composition-delayed hatching, inhibited growth and widened bill gape, thus confirming the mechanism for hatching order and revealing a potential role for oviposition in reproductive success. Similar effects of postnatal family position could affect fitness in other taxa, as well.