Which of the following characteristics is not shared by birds and mammals?1. Breathing using lungs2. Viviparity3. Warm-blooded nature4. Ossified endoskeleton

To determine which characteristic is not shared by birds and mammals, let's analyze each option:

1. Breathing using lungs: Both birds and mammals have lungs and use them for respiration. Therefore, this characteristic is shared by both.

2. Viviparity: Viviparity refers to the ability to give birth to live young. While most mammals are viviparous, birds are oviparous, meaning they lay eggs. Therefore, viviparity is not shared by birds and mammals.

3. Warm-blooded nature: Both birds and mammals are warm-blooded, meaning they can regulate their body temperature internally. This characteristic is shared by both.

4. Ossified endoskeleton: An ossified endoskeleton refers to a skeleton made of bones. Both birds and mammals have an ossified endoskeleton, so this characteristic is shared by both.

Therefore, the characteristic that is not shared by birds and mammals is viviparity (option 2).