Scientists Hatch a New Explanation for How Eggs Get Their Shapes

Flight ability could explain how eggs are shaped

Mary Caswell Stoddard studies eggs at Princeton University. Denise Applewhite

What’s in an egg? Your breakfast omelet aside, the answer is simple: birds. But not every bird is alike—and neither are their eggs. Look closely, and you’ll see a wide variety of egg shapes and sizes, from the tiny elliptical egg of the broad-tailed hummingbird to the pointed egg of the least sandpiper.

Mary Caswell Stoddard, an animal coloration expert and avian ecologist at Princeton University, wanted to know why. And so, along with a team of international researchers, she hatched an ambitious plan: compare the eggs of thousands of bird species with the eventual flight ability of the birds they produce to see if they’re linked. Their results, published today in the journal Science, show a strong correlation between egg shape and birds’ ability to soar through the air.

To study this link, the team examined photos of 49,175 eggs from a vast digital collection at the University of California Berkeley Museum of Vertebrate Zoology. The eggs, which were collected by naturalists around the world during the 19th and early 20th centuries, run the gamut of bird sizes and locations. The researchers plotted egg shapes in terms of asymmetry and ellipticity (how close they are to an ellipse), resulting in a gigantic map of bird egg shapes.

But the question remained: How do they get this way? “If you take an egg and dissolve away the calcified shell, you’re left with a membrane-encased blob that’s shaped like an egg,” Stoddard tells “It doesn’t revert to a sphere. To me, that was a wake-up call that it’s the membrane that is really responsible for generating this great diversity of egg shape, not the shell.”

This physical reality—as suggested by other scientists—inspired the study’s next step. The team created a mathematical model to simulate how a bird’s oviduct—a passageway that spits out eggs like a human Fallopian tube—might create different egg shapes before the shell is formed. When they adjusted the model for different variables, like pressure and membrane thickness, it produced different hypothetical eggs. The result adds to the likelihood that it’s not the shell, but the membrane beneath it, that gives eggs their shape.

Then came a revelation that cracked a longstanding mystery. The team plugged different variables like body mass, diet, nest location and environment into their model. But only when they used a variable called the hand-wing index—a ratio that expresses a bird’s flight ability—did the eggs turn out like they do in real life.

“When I think about birds, flight and eggs come to mind,” says Stoddard. “The discovery that the two may be linked somehow was certainly a surprise to us.” Birds with the most asymmetric and elliptical eggs are the best fliers. On the other hand, birds like kiwis, which have extremely symmetrical eggs, aren’t. In the case of kiwis, they’re flightless.

The new information about egg shape could help scientists figure out how birds evolved and changed shape over time. But the study has some limitations. It didn’t include all of the roughly 10,000 species of birds, and overlooked parasitic birds, which mimic other species’ eggs, entirely.

Some species defy the model. And Stoddard says that the new research doesn’t necessarily disprove other theories about egg shape—like the suggestion that the location of a bird’s nest dictates the shape of their eggs. But it appears they may have less of an influence than previously thought.

Of course, correlation is not causation, and there’s no genetic confirmation of the new idea—yet. But, says Stoddard, “nothing is impossible.” Will science ever truly crack the code of why birds lay different eggs? For Stoddard, that tantalizing possibility isn’t that far away.

And while you wait, she tells, she hopes it changes the way you think. “I hope that when people look at a robin flying and then crack open an egg for breakfast, they marvel at how these two might be linked.”