How Do Baby Birds Breathe Inside Their Eggs?

You might not realize it while you’re frying them sunny side up on a Sunday morning, but chicken eggs (and those of other birds and reptiles) offer up some pretty sophisticated pieces of packaging.

Hard-shelled bird eggs contain albumen, or “egg white,” and a yolk. The fertilized egg cell, or embryo, develops within the yolk and feeds off it, as well as the albumen. The baby bird has shelter, food, almost everything it needs inside the egg; except, it seems, a little fresh air.

Animals that develop inside their mothers, like mammals, get their oxygen from mom through the umbilical cord. A bird egg doesn’t have as obvious a way to take in oxygen and get rid of carbon dioxide, but the egg, again, takes care of things.

Directly under the egg’s shell are two membranes. When the eggs are laid by the mother, they’re very warm, and as they cool, the material inside the egg shrinks a little bit. The two membranes pull apart a little and create a small pocket or sack of air. As the baby bird grows, it develops an allantois, which is a hollow pouch that grows out of the bottom part of the chick’s gut. From there, the pouch fuses to the second membrane (the chorion) that’s around the yolk, forming a chorioallantoic membrane. Working similarly to lung tissue, it connects the baby bird’s circulatory system to the air outside the egg.

As the embryo develops, it breathes in oxygen via that membrane and exhales carbon dioxide (CO2). The hollow part of the allantois can also be used to store waste containing nitrogen.

Several thousand microscopic pores all over the surface of the bird egg allow the CO2 to escape and fresh air to get in. These pores also allow moisture to get into the egg to keep developing bird and the egg parts from drying out, which is why hard-boiled eggs always feel a little heavier than raw ones.

What becomes of that chorioallantoic membrane after the baby bird hatches? It stays attached to the eggshell. Your chances of seeing it in an unfertilized egg—like the one you’re scrambling for breakfast—are pretty low, since it exists only when a fertilized embryo begins developing. If there’s no baby bird inside the egg, you won’t be getting it as a morning side dish.

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A version of this article was originally published in 2013 and has been updated for 2024.

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