A baby bird doesn’t start breathing until just before it breaks out of its egg. How then does air get inside the egg for the bird’s first few breaths?
Though you can’t see them, there are tiny holes in the shell of a bird’s egg. These holes allow air to fill up part of the egg.
So, when a bird is ready to hatch, it takes a breath, sucks in the air that was inside the egg, then breaks out of its shell.