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. Powder down is a special kind of down found only in a few birds, being exceptionally fine in natured with a dust between the frons.
Down feather

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 it can be packed into a very small space.

For outdoor equipment, down is considered to be the single best insulating material available due to its light weight, compressibility, and heat retention. Down insulation can be quite expensive, so alternatives, known as synthetic insulation, are available. Synthetic insulation types generally cost less and are usually not as lightweight or as compressible as down. However, synthetic insulations work better when wet and are easier to dry, whereas down insulation does not work at all when wet and takes a very long time to dry out. Thus people who expect a significant amount of rain when camping will either bring a down sleeping bag with a water-resistant shell, or a bag with synthetic fill.

Down insulation is rated by fill power, measured as the number of cubic inches displaced by a given ounce of down (in3/oz). Higher fill-power downs will thus insulate better than lower fill-power downs of the same weight. Insulation in most outdoor equipment ranges from about 400 to 900 in3/oz (230-520 cm3/g). Down rated 500-600 in3/oz (290-360 cm3/g) is warm enough and light enough for most conditions, and 800-900 in3/oz (460-520 cm3/g) fill is used for very lightweight and/or very cold-weather gear.

When wet the thermal properties of the down are virtually eliminated, making it a worse insulator than most equally wet synthetic fills. Compressed down is also a very poor insulator, and thus sleeping bags insulated with down require the use of a sleeping pad to provide insulation from warmth that would otherwise be conducted into the ground.