Why do chickens lay eggs with chicks in them, instead of giving birth like all other animals?

Question: Why do chickens lay eggs with chicks in them, instead of giving birth like all other animals?
- Keywords:
  chicken,
  egg
Asked by cookieland to Katie, Sam on 21 Jun 2011.
- Katie Marriott answered on 20 Jun 2011:
  
  This is a characteristic all birds share but I am unsure why! Sorry I am not good at biology, I am much better at chemistry!
- Sam Tazzyman answered on 21 Jun 2011:
  
  Actually birds, reptiles, fish – most animals lay eggs – it’s really only mammals that give birth to live young. But I am not sure why the change occurred in mammals – even some mammals lay eggs, for example echidnas and platypusses (platypi?).

Question: Why do chickens lay eggs with chicks in them, instead of giving birth like all other animals?