Abstract
BACKGROUND: Interspecies variation in avian egg shape and size is understandable in terms of adaptation, allometry and phylogeny. Within-species variation in egg properties influences offspring fitness and can be explained by differences in allocation of resources into reproductive components of life history in mulidimensionally variable environments. Egg size is inherently traded-off with clutch size, which may also be true of egg shape in some cases. We investigated long-term variation in egg shape and size between two geographically close populations of blue tits Cyanistes caeruleus in relation to clutch size and habitat differences. RESULTS: The main finding is that there exists a persistent long-lasting pattern of spatial variation of egg size and shape between the two study populations of blue tits, 10 km apart, controlling for clutch size. Eggs in the urban park site were on average larger in volume and less spherical in shape than eggs in the forest site over 12 years of this study. Egg sizes were negatively associated with clutch sizes. Egg shape was not correlated with clutch size. CONCLUSIONS: Our findings suggest that the pattern of variation in egg size and shape results from different trophic richness of the breeding habitats of the study populations, demanding different allocation of resources and, especially, from the contrasting difference in the availability of calcium.