Chicks need to break through the egg shell to escape from the egg. They do this by tapping with the point of the beak (the egg tooth) against the inside of the shell. To be able to develop enough force to break through the shell, they have a special muscle in the neck, the so-called pipping muscle. This muscle swells up in the days before hatching, than is used for the hatching process itself, and in the hours/days after hatching disappears again.

This means that just after hatching, we can find a swelling in the back of the neck of the chick. This is not oedema or gas forming from bacteria, but a normal finding in every newly hatched chick.