The down of birds is a layer of fine feathers found under the tougher exterior feathers. Very young birds are clad only in down. Down is a fine thermal insulator and padding, used in goods such as jackets,Bedding, pillows and sleeping bags.
Down offers excellent thermal properties, and has good lofting characteristics. This means that the down traps small pockets of air efficiently. The small pockets of air provide the thermal barrier. Down has the added property that is can be packed into a very small space.
When wet or compressed the thermal properties of the down are far deminished. In cold situations sleep mats provide the insulation from the ground for those in a down sleeping bag. In the rain it is necessary to wear a waterproof jacket to stop the down getting wet.
Birds that provided the feather may be being used for other purposes, to provide meat for example, or being killed deliberatly in order to remove the down. Alternatively, some birds line their nests with down, and such down can be harvested safely after the young leave the nest.