Which Came First? Bird Flight or the Shape of Their Eggs?


Egg shape has fascinated humans for a very long time. Even Aristotle wrote about it. And many  people since then have asked: exactly why are eggs shaped the way they are?  To be sure, it’s an important question when you think about the evolution of the egg and its history with vertebrates leaving the oceans and colonizing the air 360 million years ago.  There have been several theories regarding egg shape in birds; from clutch size (to better fit together under an incubating female) to nest location (pointy eggs are less likely to roll and fall off cliff edges). To answer the age-old question, a new study has found the shape of a bird’s egg correlates with its flight ability.

An international team of scientists led by Harvard and Princeton universities, along with colleagues from three other countries, took on the task of answering the egg shape question. Mary Casewell Stoddard, professor in the Department of Ecology and Evolutionary Biology at Princeton University, and her team began by analyzing 50,000 eggs representing 1,400 species of birds.  They used photographs of eggs from all across the globe that had been collected (mostly by naturalists) in the late 19th and early 20th centuries. “We mapped egg shapes like astronomers map stars”, Stoddard says.


In addition to egg shapes, other information was collected about each species such as diet, nest location, growth, body mass and the type of climate where the species were found. While some of this data did explain a few things, like a big bird can lay a big egg (size), none of the data explained the shape of those eggs. But as Stoddard’s data was examined further, there was one factor that did correlate with the shape of eggs: a bird’s flight ability. Stoddard’s team found (by measuring the wings of birds) that good fliers laid asymmetrical and elliptical eggs.

As it turns out, good fliers have streamlined bodies. And to be a better flier your internal organs need to be more tightly packed within your body. And here is where it gets really interesting. As a bird’s body is compressed, a bird’s oviduct gets more narrow and compressed so eggs become more pointy. Flying narrows the oviduct which affects the membrane within as the egg forms, changing the type of egg a bird can lay. In another article about the study, Claire Spottiswoode  , an evolutionary biologist at Cambridge University says, “Streamlined birds need narrower eggs to pass through narrower pelvis, and the only way to fit a chick into a narrower egg is to make it longer.”


As a result, changes in birds for flying produced a decrease in body size and reduced the overall size of the abdominal cavity. These anatomical changes and the shape of eggs happened at the same time birds were evolving to fly.  And it’s a hypothesis that most ornithologists probably haven’t thought of.  For example, owls have spherical eggs but a better flying owl, such as barn owl, tends to lay eggs that are more elliptical. And indeed, you can see this across other bird groups from hummingbirds to seabirds. This can also be observed in some chicken breeds—though chickens tended to be outliers in this study.  This next picture is of two eggs from my very own chickens. One of the chickens is a very good flier and the other cannot fly, but merely flutters across the lawn.

Can you guess which bird (Gladys or Buttercup) is the better flier based on the shape of their eggs?     fullsizeoutput_2fb9

If you guessed “Buttercup”, you’re well on your way to having a good understanding about bird evolution, flight and egg shape. Good for you!

Nicole Reggia, Director  www.nowthatswild.com




Author: naturebynicoleblog

Nicole Reggia is a master naturalist in Pennsylvania and the director of a teaching website on animal behavior, conservation and evolution.

4 thoughts on “Which Came First? Bird Flight or the Shape of Their Eggs?”

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