Down is the fine undercoat of birds under the feathers, and is often used as insulation in clothing. Mammals have an undercoat called downhair.
The down of birds is a layer of fine feathers found under the tougher exterior feathers. Very young birds are clad only in down.
The loose structure of down feathers traps air, which helps to insulate the bird against heat loss. For centuries, humans across the globe have used down feathers for insulation.
In the United States, Federal Trade Commission regulations require that any product labeled "100% Down" must contain only down feathers, while products labeled simply "Down" can contain a mixture of fiber and feathers. In addition, products labeled as "Goose Down" must contain at least 90% goose down, 10% goose feathers. Down insulation is rated by fill power, which is the number of cubic inches displaced by a given ounce of down.
Benefits of Down: Down is warm, lightweight and packable. If well cared for, it retains its loft up to three times longer than do most synthetics.
Negatives of Down: when it is wet, the thermal properties of down are virtually eliminated. Down forms clumps if exposed to dampness or moisture, and will mildew if left damp. In addition, it will absorb and retain odors.
Learn about women's down jackets.