The relationship between egg size and posthatching development in the thick-billed Murre

Many nonexperimental studies have reported positive relationships between e gg size and posthatching survival or growth in birds. However, these result s might be confounded by underlying correlations between egg size and paren tal attributes. At Coats Island (Northwest Territories, Canada), in 1994 an d 1995, we examined the effects of egg size and parental quality on posthat ching growth in mass and wing length in the Thick-billed Murre, a colonial, cliff-nesting, Arctic seabird in which the single chick leaves the nest at a young age and at a preliminary stage of development. The relationship be tween egg size and parental quality was randomized by switching eggs among pairs. The size of the egg originally laid by the experimental females was used as a putative measure of their quality. The size of the egg from which the fostered chicks hatched had little effec t on the rate at which they gained mass. Conversely, the rate of wing growt h increased with egg size, the main difference occurring at 6-10 d of age, the period at which the primary coverts (the longest feathers on the wings of nestling murres) burst from the sheaths. It appears that the main differ ence in growth rate was created by the effect of egg size on the age at whi ch the sheaths burst. The difference in feather length created by this effe ct was maintained through the nestling period. The size of the experimental pairs' original egg was a weak predictor of the growth of the chicks they fostered. In one year, chicks that had their wings grow quickly departed th e nest at younger ages than those that had their wings grow slowly. This pa ttern is observed frequently in nonexperimental Thick-billed Murre studies and was found among unmanipulated chicks in one year of this study. The len gth of the wing feathers probably has an effect on the chick's ability to d epart successfully from the colony. We conclude that Thick-billed Murre chi cks that hatch from large eggs have a developmental advantage that has pote ntially important consequences for their survival.