At birth, chicken (birds) have an external protective covering called shells which is made up of calcium. This shell protects the chick until it gains enough strength to survive in the outside world. At birth, a chick (baby bird) has an "egg tooth". With the help of this egg tooth, the chick breaks open the shell when it is strong enough.
Egg tooth is a small sharp thing on the beak of the chick used to break the shell during hatching. It is only found in emerging chicks and lost soon after hatching.