The Brilliant Design of the Egg

Written By: Sarah Girton

We’ve talked a lot on this blog about the beauty of birds. Their plumage, their songs, their extravagance. But beauty in the bird world starts before they are even hatched.

A bird’s first home is their egg. Carefully created by their mother’s body, and laid in a nest, eggs have almost as many shapes, sizes, and colors as the birds themselves. What’s more impressive, they are a self-contained life support system for a developing bird. The yolk is their food source, the amnion acts as a cushion to protect the embryo from outside movement, and the chorion (not labeled) holds all of the most important components together in a sac. One more important feature to note are the chalazae, which is what holds the yolk in place.


Pennsylvania 4-H


Now let’s talk a bit about shape. We are all familiar with the most common shape of an egg; rounded at one end and pointed at the other. This design is not by accident — it’s so the egg rolls in a circle and is less likely to roll away from the nest. This is especially handy for cliff-dwelling birds. Birds who nest in cavities on the other hand, tend to lay more rounded eggs.

By Sheila1988 – Own work, CC BY-SA 4.0,

By the way, how are all those beautiful colors and patterns made? Specialized glands in the mother bird’s oviduct (the pathway from ovary to cloaca where the egg is slowly developed) apply splotches and colors. It is usually the final touch before the egg is laid. Cornell posted a very informative article back in 2017 all about how the egg is formed, and how colors, spots, squiggles, and blotches, are added. You can read this article here


And what about the composition of the shell? Egg shells are highly porous, filled with thousands of tiny holes that allow for the passing of carbon dioxide and oxygen. Oxygen eventually fills the larger end of the egg, and it is from this air sac that a baby bird draws its first breath. The shell itself is composed mainly of calcium carbonate. A lot of bird parents will ingest the shells after their chicks hatch for an extra calcium boost.


Over millions and millions of years of egg evolution, nature has perfected this little miracle so that a baby bird can have all the tools they need to develop into a strong and healthy chick.

Hopefully you have learned something new, and have gained some newfound appreciation for the fascinating, extremely specialized, and beautiful device that is the egg.

2 thoughts on “The Brilliant Design of the Egg

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  1. Yes I did learn some fun facts, like how the egg is no round to keep the egg in the nest, so much goes into just a cute little bird egg. Thanks.

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