First Appearance: Hvs.tCF 123
A chicken has a life cycle of 16 turns. They spend three turns as a fertile egg to be hatched, spend 10 turns growing up and then laying 2 infertile eggs a day, and spend 3 turns roosting a fertile egg. When the egg finishes hatching, the mother hen will briefly inspect the chick and then depop. A fresh chicken corpse prepared for meals will then appear in the larder of the owning side's nearest city. If the fertile egg is destroyed before the turn it is due to hatch, the chicken's cycle will reset back to 10 turns of laying infertile eggs.
Real World References
See the Chicken Wikipedia article.