Offspring survival and subsequent fitness are assumed to increase with offspring size. Although the relationship between egg size and young size in birds has attracted considerable scientific attention, to our knowledge no study so far has investigated differences among nests with respect to this relationship. The aim of this study was to find out 1) whether some inter-nest variation in the egg mass — young mass relationship exists among nests of Common Pochards, and 2) whether such a variation could be attributed to the body mass of the female measured in the last 5 days of incubation. Egg mass explained a higher portion of the variability in young mass in nests of heavier females. Those females produced smaller young for eggs of a given size. We suggest a trade-off between current and future reproduction as being the evolutionary mechanism underlying the relationship between female body mass and hatchling body mass.
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