If birds are warm blooded, reptiles cold blooded, how did the transition take place?
The transition between warm blooded and cold blooded animals would have taken place as a result of selection and the environment favouring one type of animal over another. A warm blooded animal is able to be more active than a cold blooded one, so an animal that was even slightly able to make its own heat would be able to get more of a limited food supply than a cold blooded one.