How do birds maintain their body temperature?

In cooler weather, rather than increasing their metabolic rate, birds can save energy by using environmental heat to raise their body temperature passively. ... Birds have evolved the ability to maintain their body temperature at a somewhat lower level during periods of inactivity or in response to food deprivation.

Explanation:

Birds can save energy by using environmental heat to raise their body temperature passively. They evolved the ability to maintain their body temperature at a somewhat lower level during periods of inactivity of in response to food derivation.