Birds and mammals share one of the following characteristics as a common feature.

Birds and mammal are warm-blooded. This helps them maintain a constant body temperature in different weather conditions.

Pigmented skin is present in Mammals not in birds.

Pneumatic bones are present in birds which help birds to have a light body and help in flight.

Viviparity is a characteristic of mammals as they give birth to young one but birds are oviparous which means they lay eggs.