Feathers and body temperature

Birds are warm-blooded animals and their feathers play a crucial role in regulating the body temperature – especially in sub-zero temperatures.
The winter plumage of most birds not only has a higher density of feathers but in cold temperatures the feathers can also be raised by special muscle cells.
Warm air is trapped in the spaces between the feathers – thus producing an optimum insulation layer. This often gives small birds a spherical shape, also reducing their relative surface area and keeping air exchange and heat lost to a minimum.

Mute Swan (Cygnus olor) / down feather / 40mm