How do bird eggs hatch?
Bird embryos use an “egg tooth”, not a real tooth but a small sharp temporary structure on the top of their bills, to cut through their shells from the inside. With the help of a special muscle in the back of its neck an embryo rubs its egg tooth against the shell, which has already been thinned and weakened as the growing embryo absorbed calcium from it for its bones. Continued rubbing in one spot cuts a small hole in the shell (see below).
