Fact 1: Birds are endothermic tetrapod vertebrates.

Fact 2: Some birds fly and some birds swim, but all birds have feathers.

Given the two facts above, answer the question "Some aviary species fly and others swim but, all endothermic tetrapod vertebrates share what trait?" with the following options: 
- warm-blooded
 - Weight
 - fur and fat
 - sweating
 - Epidermis
 - feathers
 - retina
 - four limbs


 

      feathers   