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Fluffing for Warmth
When temperatures drop, a hummingbird fluffs it feathers to trap small pockets of air that are separated from the outside and warmed by the bird's body heat from within. The trapped air insulates the hummer from the cold. The combination of trapped air and feathers is a much better insulator that feathers alone.

Hummingbird: Fluffing for Warmth
Amy Evoniuk