Unscrambling the shape of bird eggs

Researchers tie the massive array of different egg shapes to birds’ flight ability

Researchers analyzed the shapes of nearly 50,000 eggs from 1,400 different birds to find out why and how different egg shapes evolved. The results, published in Science, suggest that birds that are strong fliers tend to lay asymmetric or elliptical eggs and that the stretchy egg membrane, not the hard shell, is responsible for the massive diversity of egg shapes we see in nature.

We spoke to lead author Mary Caswell Stoddard of Princeton University about the work.

ResearchGate: What motivated this study?

Mary Caswell Stoddard: My colleagues and I were intrigued and puzzled by the great variety of egg shapes we observe in birds. Some birds, like owls and trogons, lay near-spherical eggs. Other birds, like loons, lay elliptical eggs, and still others, like murres and guillemots, lay extremely pointy eggs. Why do birds produce such a variety of eggs, given that the egg must serve a similar function in all birds: to protect and nourish the growing chick? We wanted to understand why eggs have different shapes — and how these shapes arise.

RG: What is it about the shape of bird eggs that has fascinated researchers?

Stoddard: Many scientists — from field biologists to mathematicians and engineers — have been fascinated by egg shape. Biologists have wondered why different egg shapes evolved and what functions they serve, while mathematicians have asked how best to quantify egg shape variation. Engineers have explored the mechanical properties of different egg shapes.

RG: Can you tell us how all these different scientific disciplines came together? How did you analyze all of the eggs?

Stoddard: This was an interdisciplinary collaboration of evolutionary biologists, computer scientists, mathematicians, and physicists. I think bringing together diverse perspectives on egg shape, from formation to function, allowed us to understand the evolution of avian eggs in a new way.

We analyzed eggs from the Museum of Vertebrate Zoology at the University of California, Berkeley. Using a digitized version of their egg collection, we were able to analyze almost 50,000 eggs representing 1,400 species — about 14 percent of all birds. We developed custom computer software to extract the eggs from images and to analyze each egg’s shape in terms of two variables: asymmetry and ellipticity.

RG: What did you find?

Stoddard: When we mapped all of the egg shapes for the 1,400 species, we discovered that the eggs of birds are indeed very diverse in shape — spherical, elliptical, asymmetric, and almost everything in between. Some bird orders, like the Charadriiformes (shorebirds), occupy a much larger portion of the egg shape space than others. Next, we propose a new mechanistic model that can describe egg shape formation in the oviduct. In this model, it is the eggshell’s flexible membrane, not the hard shell, that contributes to different egg shapes. Finally, we investigated egg shape in an evolutionary context, and we tested different hypotheses about egg shape variation. We discovered that, at a broad scale across birds, egg shape is related to flight ability in birds.

Birds that are strong fliers tend to lay more elliptical or asymmetric eggs. Surprisingly, penguins lay pointy, asymmetric eggs — even though they are flightless. We believe that penguins’ bodies may be adapted for powerful swimming, and so perhaps the same processes that influence egg shape in flying birds are at work in swimming birds as well.

RG: What can your results tell us about bird evolution?

Stoddard: Our results reveal a link between two characteristic features of birds: flight and eggs. Our results suggest that birds that are adapted for strong flight have undergone morphological changes which have ultimately affected the egg-shaping process. Birds that are strong fliers tend to lay eggs that are more asymmetric or more elliptical. To maintain sleek, streamlined bodies for flight, we think birds have evolved elliptical and asymmetric eggs to increase egg volume without increasing egg width — this is advantageous for narrow, streamlined bodies.

RG: What’s next for your research?

Stoddard: We are excited to look at variation in eggshell membrane properties across birds, from ostriches to hummingbirds. We would like to see if what we observe in these membranes is consistent with the predictions of our mechanistic model. In addition, we are eager to explore how eggs changed shape at the dinosaur-to-bird transition. We believe that both egg asymmetry and flight evolved roughly during this time, raising the intriguing possibility that the emergence of powered flight is associated with egg shape variation.

Featured image courtesy of Rodger Evans.