Growing evidence indicates that producing eggs may constitute a considerable cost of reproduction. If female parental care, which in many species exceeds male performance, is added to these initial costs, it may be concluded that females contribute more to reproduction than males. However, this additional burden on females should reduce their survival and skew the usually equal sex ratio, but this is generally not the case. A resolution of this apparent paradox requires studies estimating the parental investments of both sexes at different stages of breeding, with particular focus on the initial reproductive stage. In the present study, egg composition and its energetic value was estimated in the Dovekie (or Little Auk, Alle alle), a seabird exhibiting bi-parental care except for the end of the chick-rearing period, when the female deserts the brood while the male continues the feeding and escorts the chick during fledging. Condition estimates (size-adjusted body mass and several hematological and biochemical parameters, including concentrations of hemoglobin, glucose, cholesterol, and triglycerides, as well as the leukocyte profile) were also examined in both sexes during the prelaying, laying/relaying, and chick-rearing periods. The egg composition and its energy content indicate that the energetic demands of egg production are not as high as previously assumed, and the non-resource–based costs in females seem to be similar to those experienced by males during the mating period (nest site and/or paternity guarding). However, it was also found that females had lower body mass than males throughout the whole breeding season, suggesting overall female-biased costs of reproduction despite very similar parental performance. This suggests that females are more susceptible to the negative effects of reproduction. If so, their earlier brood desertion may be a response to the additive costs of parental investments.
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Vol. 131 • No. 4