Answer the following question: 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, some aviary species fly and others swim but, all endothermic tetrapod vertebrates share what trait?

Select from the following.
[1]. warm-blooded;
[2]. Weight;
[3]. fur and fat;
[4]. sweating;
[5]. Epidermis;
[6]. feathers;
[7]. retina;
[8]. four limbs;
Answer:
[6].