Many different theories exist as to why the shape of bird eggs varies so much across species, and now, new research yields evidence that variable egg shape is driven by unique flight adaptations. The results counter theories suggesting that life history or nesting habitat are egg shape's key drivers. Ideas about egg shape have been numerous and vast in scope; among them is the theory that cliff-nesting birds lay more cone-shaped eggs - which would roll in a tight circle, when moving, so as to not tumble off the cliff. Another theory suggests that different egg shapes exist to maximize incubation efficiency in a clutch. Here, Mary Caswell Stoddard aimed to resolve this debate by analyzing the shape of 49,175 eggs representing about 1,400 species in 37 orders, two of which are extinct. The eggs were categorized based on their asymmetry and/or ellipticity. As well, a wealth of biometric, life history and environmental parameters for all species were analyzed. The team used biometric measurements from museum specimens to calculate the hand-wing index (HWI), a standard proxy for flight efficiency and dispersal ability in birds. They found that egg shape (asymmetry and ellipticity) is not related to clutch size, developmental mode, environmental factors, or nest characteristics - yet there was a correlation between egg shape and HWI. Given that HWI is positively related to flight efficiency, these results raise the intriguing possibility that adaptations for flight may be key drivers of egg shape variation in birds, the authors say. In a related Perspective Claire Spottiswoode, discussing how birds are streamlined for flight, suggests "perhaps streamlined birds need narrower eggs to negotiate their narrower pelvis, and because the only way to fit a chick into a narrower egg is to make the egg longer, elliptical or asymmetric eggs result."