You are presented with the quiz "Some aviary species fly and others swim but, all endothermic tetrapod vertebrates share what trait?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "birds are endothermic tetrapod vertebrates" and "some birds fly and some birds swim, but all birds have feathers". 

So, what's the best answer to the question?


 

      feathers     