Look to the bird's wings for egg shapes

Owls’ are spherical, hummingbirds’ are elliptical and sandpipers’ are pointy. All bird eggs have the same function — to protect and nourish a growing chick. But they come in a brilliant array of shapes. This variety has puzzled biologists for centuries. Now, in the most comprehensive study of egg shapes to date, published June 22 in Science, a team of scientists seems to have found an answer.

The researchers catalogued the natural variation of egg shapes across 1,400 bird species, created a mathematical model to explain that variation, and then looked for connections between egg shape and many key traits of birds. On a global scale, the authors found, one of the best predictors of egg shape is flight ability, with strong flyers tending to lay long or pointy eggs. “This paper is remarkable because it creates a wonderfully unified theory for the variety of egg shapes we see in nature,” said Claire Spottiswoode, a bird ecologist at the University of Cambridge, UK and the University of Cape Town, South Africa who did not participate in the research.

Focussing on the membrane

In the new study, the authors conducted a multistep investigation that brought together biology, computer science, mathematics and physics. They first wrote a computer program, named Eggxtractor, that classified eggs based on their ellipticity and asymmetry. Elliptical eggs are elongated and round on both ends, like cucumbers, and asymmetric eggs are pointier on one end, like mangoes.

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 California, USA. “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. 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, USA and an author of the study.

Finally, the researchers looked into why egg shapes might be so spectacularly diverse. One popular hypothesis centred 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.

Practical solution

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 flyers and have asymmetric eggs, as do least sandpipers, which migrate long distances. Wandering albatrosses are one of the most far-ranging flyers and have elliptical eggs.

Eastern screech owls rarely move beyond their small territory, where they tend to fly in short, low-powered glides, and have almost spherical eggs. “Perhaps, evolutionarily, birds stumbled upon this very natural, geometric solution, which is to increase the ellipticity and asymmetry of their eggs,” 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,” Mary said. “In something as familiar and common as a bird egg, we are still discovering new truths.”