Cracking the mystery of egg shape

Cracking the mystery of egg shape

Not all eggs are shaped like a chicken’s—now we know why.

Every egg rolls a different way.

For centuries, scientists wondered why egg shapes are so different from one bird to the next. Now, we think we’ve finally cracked the mystery.

To start, we need to know what egg shapes are even possible. Luckily, hobbyists and natural history museums have been collecting and cataloging bird eggs for hundreds of years.

Then, what kind of birds are linked with the different shapes?
And finally, how might their special traits—from nesting habits to body size—change the shape of their eggs over evolutionary time?

Over the years, bird lovers and scientists have come up with all sorts of hypotheses for egg shape—most are related to the life history of the bird or the environment in which they live.

Extracting the data

Scientists started their quest by looking at photographs of 49,175 bird eggs collected from nests, burrows, and colonies all over the world for more than 100 years.

Each photograph—supplied by the The Museum of Vertebrate Zoology at Berkeley—has two key elements: an egg (or eggs) and a ruler. The team used a special computer program to scan each picture, get the egg measurements, and then figure out the complete range of egg shapes in birds.

They collected pictures from 37 bird orders. That’s all of the major bird orders.

1400 species

That’s 14% of all 10,000 bird species.

Clutch size

The number of eggs in a clutch could influence egg appearance, with some shapes optimized for sharing the warmth.

Calcium conservation

Spherical eggs have less surface area, which could help conserve calcium in places where the mineral is rare.

The roll factor

Spherical eggs could easily roll off a cliff. Conical eggs, however, roll in a tight circle, making them perfect for cliff-nesting birds.

A motley crew

Next, researchers looked at two features: asymmetry, or how pointy the eggs are, and ellipticity, or how much the eggs deviate from a perfect sphere.

Eggs are all over the place. Some are elliptical. Some are asymmetrical. Some are both at the same time. And some—like the perfect sphere of the hawk-owl—are neither.

Egg extremes

The hawk-owl has the most spherical egg. But other birds are off the charts, too. Shorebirds like the murre and sandpiper produce super asymmetrical eggs. The murre’s egg is also one of the most elliptical.

A happy medium

The most common kind of egg is like that of the graceful prinia (Prinia gracilis)—not quite the chicken egg most of us conjure up when thinking about egg shape.


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