Birds and mammals have the following characteristics in common: they are vertebrates, meaning they have an internal skeletonthey are warm-bloodedthey breathe using lungs, so … have full respiratory systems they have circulatory systems. Most mammals give live birth, but there is a sub-group known as monotremes, which are the egg-laying mammals. 2  This group includes just the platypus and the echidna. 3  They are still mammals because, like all mammals, they feed their young on mothers' milk.