Evolving for flight may have changed the shapes of bird eggs | Science News for Students

Evolving for flight may have changed the shapes of bird eggs

Scientists might have cracked the mystery of why eggs look so different
Jul 17, 2017 — 7:10 am EST
bird eggs


Bird eggs come in many different shapes, sizes and colors. A new study shows a link between how birds fly and the shape of the eggs they lay.



Bird eggs come in many different shapes, sizes and colors. A new study shows a link between how birds fly and the shape of the eggs they lay.


Bird eggs come in many shapes, from the brown hawk owl’s almost perfectly round egg to the raindrop-shaped egg of the least sandpiper. The reason for all these different egg shapes has long puzzled biologists. New research suggests an answer. As birds evolved for different types of flight, changes to their bodies may have helped shape their eggs. Stronger fliers tend to lay eggs that are more stretched out, for instance.

Eggs all have the same job: to protect and feed chicks. So the reason they come in so many shapes “was a question that we found intriguing,” says Mary Caswell Stoddard. She’s an evolutionary biologist at Princeton University in New Jersey and one of the study's authors.

Previous studies had suggested many ways the different shapes might help birds. Maybe cone-shaped eggs are less likely to roll out of the nests of cliff-dwelling birds. Or perfectly round eggs might resist damage in the nest. But no one had tested these guesses across a wide range of birds.

So Stoddard and her team decided to take a stable at it. They analyzed almost 50,000 eggs. These represented1,400 species, or about 14 percent of all the bird species. The researchers reduced each egg to its two-dimensional silhouette. Then they used an algorithm to describe each egg using two variables. One was how elliptical or spherical the egg is — is its silhouette more of an oval, or a circle? The other variable was asymmetry — is the egg pointier on one end than the other?

Story continues below image.

bird eggs
Researchers examined the eggs of 1,400 bird species for two variables: ellipticity (how round or oval-shaped the egg is) and asymmetry. Each orange dot represents the average egg of one bird species.

Next, the researchers looked at how these two traits vary across the bird family tree. One pattern jumped out. Species that are stronger fliers tend to lay more elliptical or asymmetrical eggs. (The researchers used wing shape to measure how strong a flier each bird was.) Stoddard's group shared its results June 23 in Science.

Study coauthor L. Mahadevan is a mathematician and biologist at Harvard University in Cambridge, Mass. He cautions that the data show only that egg shape and flying strength are linked — not why.

But Stoddard's group has one idea.

As birds evolved to fly, their bodies adapted by becoming more streamlined. Their reproductive tracts might have become more streamlined and narrow too. That would have limited the width of an egg that a female could lay. But eggs provide nutrition for growing chicks, so shrinking eggs too much could harm developing birds. Long, stretched-out eggs might have been a compromise, Stoddard says. That shape could give eggs more volume without increasing their width. Asymmetry can increase egg volume in a similar way.

It’s hard to prove that adapting to flight changed egg shapes long ago. “Of course we can’t replay the whole tape of life again,” says Claire Spottiswoode. She’s a zoologist at the University of Cambridge in England. Spottiswoode wrote a commentary that was published in Science along with the study. Still, Spottiswoode thinks the evidence is compelling. “It’s a very plausible argument,” she says.

Santiago Claramunt isn’t convinced that flight adaptations drove the evolution of egg shapes. He’s the associate curator of ornithology in Canada at the Royal Ontario Museum in Toronto. Claramunt says that how streamlined a bird is has more to do with its feathers than the shape of its body.

“High-performing fliers can have rounded, bulky bodies,” he argues, which wouldn’t give elongated eggs an advantage. He says frigate birds and swifts are good examples. Both of these birds make long-distance flights but have fairly broad bodies. “There's certainly more going on there,” Claramunt says.

Some groups of birds showed a much stronger link between flying and egg shape than did others. And certain factors — such as where birds lay their eggs, or how many they lay at once — weren’t srongly related to egg shape across all birds. Still, they might be important within certain branches of the bird family tree.

Power Words

(for more about Power Words, click here)

adaptation     (in biology) A process by which an organism or species becomes better suited to its environment. When a community of organisms does this over time, scientists refer to the change as evolution.

algorithm     A group of rules or procedures for solving a problem in a series of steps. Algorithms are used in mathematics and in computer programs for figuring out solutions.

asymmetry     Not symmetrical, such as not the same shape on the left and right sides.

birds     Warm-blooded animals with wings that first showed up during the time of the dinosaurs. Birds are jacketed in feathers and produce young from the eggs they deposit in some sort of nest. Most birds fly, but throughout history there have been the occasional species that don’t.

coauthor     One of a group (two or more people) who together had prepared a written work, such as a book, report or research paper. Not all coauthors may have contributed equally.

commentary     (in science) An opinion piece, often written to accompany — and add perspective to — a paper by others, which describes new research findings.

curator     Someone who manages a collection of items, for instance in a museum, library or art gallery. This person’s primary job is to design exhibits, organize and acquire collections and to do research on the artifacts included in the collection.

elliptical     Having the shape of an ellipse, which is an oval shape.

evolution     (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the particular conditions in which it developed. 

evolutionary     An adjective that refers to changes that occur within a species over time as it adapts to its environment. Such evolutionary changes usually reflect genetic variation and natural selection, which leave a new type of organism better suited for its environment than its ancestors. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.

evolutionary biologist     Someone who studies the adaptive processes that have led to the diversity of life on Earth. These scientists can study many different subjects, including the microbiology and genetics of living organisms, how species change to adapt, and the fossil record (to assess how various ancient species are related to each other and to modern-day relatives).

factor     Something that plays a role in a particular condition or event; a contributor.

nutrition     (adj. nutritious) The healthful components (nutrients) in the diet — such as proteins, fats, vitamins and minerals — that the body uses to grow and to fuel its processes. A scientist who works in this field is known as a nutritionist.

ornithology     The scientific study of birds. Experts who work in this field are known as ornithologists.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

tract     A particular, well-defined area. It can be a patch of land, such as the area on which a house is located. Or it can be a bit of real estate in the body. For instance, important parts of an animal’s body will include its respiratory tract (lungs and airways), reproductive tract (gonads and hormone systems important to reproduction) and gastro-intestinal tract (the stomach and intestines — or organs responsible for moving food, digesting it, absorbing it and eliminating wastes).

trait     A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.

variable     A factor that can be different in different individuals, establishing variations among them. (in experiments) A factor that can be changed, especially one allowed to change in a scientific experiment. For instance, when researchers measure how much insecticide it might take to kill a fly, they might change the dose or the age at which the insect is exposed. Both the dose and age would be variables in this experiment.


Journal: M.C. Stoddard et al. “Avian egg shape: Form, function and evolution.Science. Vol. 356, June 23, 2017, p. 1249. doi: 10.1126/science.aaj1945.

Journal: C.N. Spottiswoode. “The most perfect thing, explained.” Science. Vol. 356, June 23, 2017, p. 1234. doi: 10.1126/science.aan2517.