Understanding why the shapes of avian eggs vary between species has long puzzled researchers, but a new study sheds light on the issue. Mary Caswell Stoddard of Princeton University and co-workers studied egg-shape diversity in terms of asymmetry and ellipticity using a sample of 49,175 eggs from some 1400 species of bird in 35 extant orders and two extinct ones. The team was able to explain the distribution in terms of a simple model based on geometric and mechanical properties of the egg membrane. The researchers also found that the shape of eggs correlates with the ability of birds to fly, and suggest that adaptations for flight may have been critical factors driving egg-shape variations.