The birds are another product of many years of evolution. They have 4 limbs (2 legs for walking and 2 wings for flying in most cases), feathers for keeping warm, and a toothless beak. Having been descended from a branch of small theropod dinosaurs in the Jurassic, the birds were usually small, but had still proven worthy to be the rivals of the kings of the skies at the time: the pterosaurs. After all the non-avian dinosaurs died, they diversified so much that they had outnumbered the mammals 2 to 1. As well as this, they grew to large sizes, started to show uniqueness in themselves, and in some cases, went on a terrestrial and carnivorous lifestyle, as shown in quite a bit of the Cenozoic.