Bird egg shape related to flight efficiency

A bird’s egg shape may be related to the way it flies, a new study 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,” wrote Claire Spottiswoode of the University of Cambridge in an article related to the study.

There are various theories about why the shape of bird eggs varies so widely across species. One theory, for example, holds that cliff-nesting birds lay more cone-shaped eggs – which would roll in a tight circle so as to not tumble off the cliff.

However, the new research yields evidence that variable egg shape is driven by unique flight adaptations.

The study’s lead author, Mary Caswell Stoddard of Princeton University, and her research team analyzed 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.

The scientists also analyzed a wealth of biometric, life history, and environmental environmental parameters for all species.

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 is not related to clutch size, environmental factors, or nest characteristics. However, 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 said.

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By: David Beasley, Earth.com Staff Writer

Source: American Association for the Advancement of Science

Image Credit: M.C. Stoddard