It has long puzzled scientists how large flying animals first took to the air. They have wings, but not segmented legs and shells like insects. They don’t swim in the water and secrete slimy mucous all over themselves like amphibians.
They lay eggs, but have feathers instead of scales like reptiles. They are warm blooded, but don’t suckle their offsprings like mammals.
They don’t seem to quite fit the bill for any other major group of animals with which we are familiar.
