|The terror bird Titanis|
So here we go: a post about a bird! Gasp!
Birds, despite being warm-blooded, air-breathing vertebrates, are, of course, not mammals, or even synapsids. Instead, they are diapsids, part of an evolutionary line that parted company from the synapsid/mammalian line over 300 million years ago, and which today also includes most, perhaps even all, of the reptiles. (The position of turtles is, to the best of my knowledge, still slightly contentious - they don't look anatomically like diapsids, but there are reasons to think that might be misleading. This is, however, a subject well beyond the scope of this post).
As a result of this long evolutionary separation, birds and mammals are substantially different in a number of ways. The most obvious is perhaps that birds have feathers, and mammals (usually) have hair, but there are many more differences 'under the skin'. For example, the wings of bats and those of birds, while performing the same function, have quite a different skeletal structure to them.