Ever wondered how baby birds can breathe in their hard egg-shells? Well, it’s all down to some nifty egg-ineering.
Hard-shelled bird eggs contain egg white and a yolk. When the egg cell is fertilized, the embryo develops within the yolk and feeds off of it and the white. The baby bird thus has food and shelter, so what else could it need? Well, maybe some fresh air.
Humans and animals that develop inside their mothers get their oxygen through the umbilical cord. Birds, on the other hand, don’t have such an obvious way to take in oxygen and get rid of carbon dioxide, but the egg, again, takes care of things.
It’s all down to some nifty engineering involving two membranes within the eggshell that effectively act like lung tissue, connecting the chick’s circulatory system to the outside world. When the eggs are laid by the mother chick they’re very warm, and as they cool down somewhat, 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 developing bird grows, it breathes in oxygen from the air sack and exhales carbon dioxide. The oxygen diffuses through several thousand microscopic pores all over the surface of the egg which also allow the CO2 to escape.
Here’s a cool video explaining the process.
The pores also allow moisture to get into the egg to keep developing the bird and the egg parts from drying out. That is why hard-boiled eggs always feel a little heavier than raw ones.