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Cracking the egg mystery: what makes eggs round or pointy?

Cracking the egg mystery: what makes eggs round or pointy?


It has to do with the bird’s flight skills, according to a new study

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Scientists may have just cracked the mystery of why bird eggs come in all shapes and sizes: it has to do with the bird’s ability to soar up in the sky — and the body changes wrought by years of evolution.

Egg shapes are incredibly diverse among bird species: hummingbirds lay eggs in the shape of Tic Tacs; owls make globe-like eggs; and sand pipers lay pointy eggs shaped like raindrops. But how did different egg shapes evolve, and why?

Through the years, scientists have come up with many explanations: birds that make their nests on cliffs tend to lay pointy eggs because if a pointy egg is bumped, it will spin in a circle instead of rolling off the edge of the cliff. Birds with a diet low in calcium make round eggs because they require the least amount of shell material. Eggs have different shapes so they can better fit all together in a nest — allowing all eggs to be incubated equally.

Hatching gull chick.
Hatching gull chick.
Photo by MC Stoddard

But these hypotheses have never really been tested. So a team of researchers from Princeton, Harvard, and other institutions around the world decided to unscramble the mystery. The results are published today in Science. “We had a pretty good puzzle on our hands,” lead author Mary Caswell Stoddard, an assistant professor of ecology and evolutionary biology at Princeton University, tells The Verge. The team analyzed the shapes of nearly 50,000 eggs representing 1,400 bird species. These eggs came from all over the world and were collected by naturalists in the late 19th and early 20th centuries. (The researchers didn’t actually study the physical eggs — only their photographs stored in an online database at the Museum of Vertebrate Zoology at UC Berkeley.)

First, the team built a custom computer program that allowed them to analyze how pointy and how elongated each egg was. With this information, they built a map of egg shapes — ranging from the round eggs of owls to the raindrop-shaped eggs of sandpipers, and everything in between (with one exception, Stoddard says: no eggs shaped like hot-air balloons exist). The researchers then compared egg shapes across different bird lineages, taking into consideration what the birds eat, where they make their nests, how big they are, and how good they are at flying. 

Their results: the egg shape isn’t influenced by what the bird eats or how big the clutch is, Stoddard says. Instead, it seems to have to do with how good at flying birds are. Their analysis suggests that birds that are good flyers — like swifts, which spend most of their lives up in the air — tend to lay eggs that are pointier or more elongated. Why? The bodies of these sleek birds evolved to be smaller and their organs more compressed so they’re more streamlined for flight. These body changes affect how the egg is formed.

A map of egg shapes.
A map of egg shapes.
Chart by G. Grullón / Science

Every bird creates its eggs in a stretchy tube called the oviduct. Here, a stretchy membrane is deposited around the egg white and yolk before the eggshell is formed. And it’s this membrane — not the eggshell — that gives an egg its shape, the study says. Just try dissolving the shell in acid and you’ll see that even without the eggshell, a membrane retains its shape. (You can do this at home using a chicken egg and vinegar, by the way, according to Stoddard.) 

Birds that are better at flying have a narrower oviduct — so when the membrane is deposited, it can’t create a wide egg. The eggs have to be pointier or more elongated, so the chick can fit and still be healthy. “Birds may face a trade-off,” Stoddard says.

“Birds may face a trade-off.”

The findings are intriguing, but “this discovery will be far from the final word,” conservation biologist Claire Spottiswoode, who did not take part in the study, writes in a perspective published alongside the Science paper. It also raises a lot of questions: why do birds that aren’t as good as flying — like flightless ostriches — lay rounder eggs? And do pointer and more elongated eggs have disadvantages? Do they make it harder for the chick to hatch, for instance? “We have more questions now than we did at the start,” Stoddard admits. “I think that’s the surprising wonder that came with the study.” 

Stoddard wants to answer all these questions in future studies, including understanding how eggs have changed shape as certain dinosaurs evolved into modern birds. This is not only important for solving some key evolutionary conundrums, it also puts into perspective the beauty of breakfast. Scrambled eggs and omelettes will never look the same.

This story was originally published on June 22, 2017 and has been updated to include video.