Male chicks are more costly to rear than females in a monogamous seabird, the Common Murre

Feeding rates and mass loss during chick rearing were compared for individually marked parents of male and female Common Murre (Uria aalge) chicks at Great Island, Newfoundland, Canada, from 1997–2001. Both parents in this socially monogamous seabird species share parental care duties until colony departure, after which the single chick is fed only by its father. Because murres provision their single chicks with one clearly visible fish per trip, it is possible to accurately determine whether parents differentially feed male and female chicks. Based on slightly greater mass of males in adulthood, possibly favored by sex differ-ences in breeding roles, we predicted that male nestlings would be fed more than females. Fathers ’ feeding rate to sons, but not daughters, increased with chick age, whereas maternal feeding rate increased with chick age for both sexes. When year-corrected feeding rates of pairs rearing both sexes were compared, both mothers and fathers fed their sons significantly more than their daughters in the later part of the chick-rearing period. Moreover, parents rearing male chicks lost mass at a significantly higher rate than those rearing females. There was no difference in fledging age for sons and daughters. These results indicate that differential parental allocation occurs and has measurable costs even in a species with only slight adult sexual dimorphism. Key words: Common Murre, feeding rate, parental care, sex differences, sexual size dimorphism, Uria aalge. [Behav Ecol] Raising offspring is costly, both in terms of parental energyexpenditure and risks to adult survival (Gabrielson 1996; Jönsson et al. 1998). To maximize lifetime reproductive suc-cess, parents must balance current investment in young agains