Fact 1: Birds are endothermic tetrapod vertebrates.

Fact 2: Legs define Tetrapods.

Given the two facts above, answer the question "Birds are vertebrate endotherms with what?" with the following options: 
- fat
 - Energy
 - legs
 - like some sharks
 - Tail
 - invertebrates
 - arms
 - four


 

   legs      