Using accurate measures of size and shape, we studied the eggs of 955 extant species across the avian phylogeny, including 39 of the 40 orders, and 78% of the 249 families. We show that the elongation component of egg shape is largely the result of constraints imposed by the female’s anatomy during egg formation, whereas asymmetry is mainly an adaptation to conditions during the incubation period. Thus, egg elongation is related to the size of the egg in relation to the size of the female’s oviduct and also to either the shape of her pelvis or general body conformation associated with her mode of locomotion. Egg asymmetry is related mainly to clutch size and the structure of the incubation site, factors that influence thermal efficiency during incubation and the risk of breakage. Importantly, general patterns across the phylogeny do not always reflect the trends within families and orders. We argue that the analysis of avian egg shape is most profitably conducted within taxa where all species share similar life histories and ecologies, as there is no single factor that influences egg shape in the same way in all birds.

Data from field and museum studies, as well as from published images and datasets.