The answer is scales and amniotic eggs.
It is obvious that reptiles have scales, which are very important for their survival. But birds have scales, too. Their claws, feet, legs and beaks are covered by scales.
Reptiles and birds, together with mammals, belong to the group of Amniote, which means their eggs are amniotic. Amniotic eggs are either laid on land or retained inside the mother, unlike anamniotic eggs that are laid in the water.
Some of the listed characteristics are not common to both groups:
- Only reptiles are ectothermic (cold-blooded animals) while birds are endothermic (warm-blooded animals).
- Birds have beaks and reptiles have teeth.
- Birds are the only animals with feathers.
- Birds have hollow bones, and thus, lighter which help them to fly. Reptiles bones are dense and heavy.
