Birds are endothermic tetrapod vertebrates, and some birds fly and some birds swim, but all birds have feathers. Given these facts, some aviary species fly and others swim but, all endothermic tetrapod vertebrates share what trait among the following options:
- warm-blooded
 - Weight
 - fur and fat
 - sweating
 - Epidermis
 - feathers
 - retina
 - four limbs


 

      feathers   