Which characteristics evolved independently in mammals and birds?

Answer: endothermy

Explanation:

This term describes the ability of some animals to use an internally generated heat to maintain body temperature. Their body temperature tends to stay steady regardless of environment. When it's cold outside such animals increase their metabolic heat production to keep their body temperature fixed. For this reason, the internal body temperature of an endotherm is more or less independent of the temperature of the environment.

The adaptation to different temperatures

Explanation:

Mammals and birds share one common feature - they are vertebrates meaning that they have the vertebrata column or simply put, a backbone. In addition, unlike other animals, say the amphibians, these have the ability to regulate the body temperature through homeostasis. This is the ability to thermally regulate the body's temperature. This is done by the flow of the blood to the different parts of the body. Thus, the mammals and birds have evolve to share the common features.