Food supply and the consequences of egg size in the Thick-billed Murre

We examined the effect of egg size on post-hatching development in the Thic k-billed Murre (Uria lomvia), an Arctic seabird that lays a one-egg clutch, at a colony where food availability during chick-rearing was low (Digges I sland, Nunavut, Canada). We compared our results to those of a previous stu dy conducted at a colony where food availability was higher (Coats Island, Nunavut). To control for underlying phenotypic correlations between egg siz e and parental quality, we switched eggs at random among pairs. Egg size po sitively affected the rate of early wing-feather growth, bur contrary to pr ediction, the advantage enjoyed by chicks from large eggs over those from s mall eggs at Digges Island (1.6 days' feather growth on average) was no gre ater than at Coats Island (2.0 days). Egg size had no effect on the rate at which chicks gained mass at Digges Island, but young from large eggs tende d to remain heavier than those from small eggs. At Coats Island, this occur red only in a year in which chicks grew relatively slowly, offering some su pport for the hypothesis that a large egg confers greater advantage when fe eding conditions are unfavorable. Adults at Digges Island invested heavily in provisioning their chicks, but then: was no evidence of a trade-off betw een egg size and provisioning. As costs associated with large eggs have not been detected in Thick-billed Murres, the existence of considerable variat ion in egg size, beyond that associated with female age or experience, rema ins unexplained.