Why Do Bird Eggs Have Different Shapes? Look to the Wings

With Eggxtractor, the researchers plotted nearly 50,000 eggs, representing all major bird orders, from a database of digital images by the Museum of Vertebrate Zoology in Berkeley, Calif.


“We could see then that egg shapes varied from spherical, to elliptical, to very pointy, to almost everything in between,” said Mary Caswell Stoddard, an assistant professor of ecology and evolutionary biology at Princeton University and the lead author of the study.

Next, the researchers attempted to answer how eggs might acquire varying shapes. Rather than looking at the shell, as one might expect, they focused on the egg’s membrane (the film you see when peeling a hard-boiled egg), which is essential to the egg’s shape.

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Mary Caswell Stoddard of Princeton University, the lead author of the study, which examined egg shapes across 1,400 bird species.

Credit
Denise Applewhite

The scientists identified two parameters that could influence egg form: variations in the membrane’s composition and differences in pressure applied to the membrane before the egg hatches.

By adjusting these two parameters, “we were able to completely recover the entire range of observed avian egg shapes” — a good test of the model, said L. Mahadevan, a professor of applied math, biology and physics at Harvard University and an author of the study.

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Finally, the researchers looked into why egg shapes might be so spectacularly diverse. One popular hypothesis centered on nest location: Cliff-nesting birds, it was thought, lay pointy eggs so that if the eggs are bumped, they spin in a circle rather than rolling off the cliff. Another suggested that birds lay eggs in shapes that pack together best in different-size clutches.

But when the authors related egg shape to these and other variables, they were surprised to find that none of them fit on a global scale (though they may still play important roles on smaller scales). Instead, egg shape was strongly correlated with a measure of wing shape, called the hand-wing index, that reflects flight ability.

So what connects flight to egg shape? In general, birds want to pack as many nutrients as possible into their eggs. But, in order to fly, they must maintain sleek bodies — meaning their eggs can’t be too wide.

Common murres, for instance, are fast, powerful fliers and have asymmetric eggs, as do least sandpipers, which migrate long distances.

Photo

An egg of a common murre.

Credit
Harvard Museum of Comparative Zoology

Photo

An egg of a least sandpiper.

Credit
Harvard Museum of Comparative Zoology

Wandering albatrosses are one of the most far-ranging fliers — some have been known to circumnavigate the Antarctic Ocean three times in a year — and have elliptical eggs.

Photo

An egg from a wandering albatross.

Credit
Harvard Museum of Comparative Zoology

Eastern screech owls rarely move beyond their small territory, where they tend to fly in short, low-powered glides, and have almost spherical eggs.

Photo

An egg from an Eastern screech owl.

Credit
Harvard Museum of Comparative Zoology

“Perhaps, evolutionarily, birds stumbled upon this very natural, geometric solution, which is to increase the ellipticity and asymmetry of their eggs,” Dr. Mahadevan said, since doing so allows for greater volume without increasing girth. This explanation requires further research, he added.

Ultimately, this study shows that “we can challenge old assumptions,” Dr. Stoddard said. ”In something as familiar and common as a bird egg, we are still discovering new truths.”


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