Which one of the following characteristics is not shared by birds and mammals?

Birds are oviparous while mammals are oviparous (prototherians) and viviparous (metatherians and eutherians). Oviparous- producing young by means of eggs which are hatched after they have been laid by the parent, as in birds. Viviparous- bringing forth live young which have developed inside the body of the parent.

The only oviparous mammal is platypus.