Fitness consequences of egg shape variation: a study on two passerines and comments on the optimal egg shape model

Abstrakti

Amodel has been proposed that predicts a relationship between egg shape and fitness, arguing that eggs will be shaped to fit within the brood patch in an arrangement that is optimal for incubation . Deviations from the "optimal egg shape" would therefore entail a decrease of fitness of the parents. We explored the fitness consequences of egg shape variation, using data on egg characteristics and breeding performance of 354 Great Tit Parus major and 150 Blackbird Turdus merula clutches . "Optimal egg shape" for a particular clutch size was defined as the mean of the shape index for each clutch size . For each clutch, mean egg shape, an index of shape deviation from the optimal egg shape, and the coefficient of variation of the shape deviation index, were computed ("egg shape variables"). We found that (1) mean egg shape did not vary with clutch size ; (2) hatching and breeding success, and weight of nestlings at 15 days of age, were not related to egg shape variables, even after controlling for the effects of clutch size and egg size ; and (3) within a clutch, the relative shape of an egg did not affect its hatchability . Therefore, within the range found in natural populations, the shape of eggs does not seem to have fitness consequences, a conclusion that does not support the optimal egg shape model.