The fully developed embryo breaks open the shell when it is time.
Since the beak and the claws are not fully developed, birds have an “egg tooth.”
In birds, the process of breaking open the eggshell is referred to as pipping.
When a chick becomes too large to absorb oxygen through the pores of its eggshell, it uses its egg tooth to peck a hole in the air sac located at the flat end of the egg. This sac provides a few hours’ worth of air, during which the chick breaks through the eggshell to the outside. The egg tooth falls off several days after hatching