How does air trapped in feathers slow down heat loss from the body of birds?

The base layer of feathers on birds contain small, fluffy, and extremely soft shafted feathers called the down. These are the feathers used in making pillows, comforters and clothing. They provide the insulation needed to protect and maintain their body temperatures during cooler temperatures.

When temperatures heat up, however, birds can often be seen "sunning" themselves. By opening their wings and spreading their feathers apart they are able to produce a cooling affect by increasing air circulation. Turning their backs to the sun with the feathers in this position will reflect the sun's rays rather than absorbing them. This also acts to shade their legs and feet which are not insulated.