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The down of is a layer of fine found under the tougher exterior feathers. Very young birds are clad only in down. Powder down is a specialized type of down found only in a few groups of birds. Down is a fine thermal insulator and padding, used in goods such as Down jacket, bedding ( and ), and . The discovery of feathers trapped in ancient amber suggests that some species of non-avian dinosaur likely possessed down-like feathers.
Female wildfowl use down feathers plucked from their own breasts to line their scrape nests. This process performs the dual function of helping to insulate the Bird egg and exposing the female's brood patch—an area of bare skin, rich in blood vessels, which transmits heat very efficiently. Of the various items birds use to line their nests, down feathers provide the most effective insulation, though only when dry; wet down is the least effective insulator. Down may also help camouflage the eggs when the female is away from the nest, particularly as the birds often draw the feathers over their eggs before leaving.
Because a bird can eliminate heavy metals in its feathers and because feathers can be collected non-invasively and stored indefinitely, down feathers can be used to check for evidence of metal contamination in the bird's environment. Studies have shown a high level of correlation between the level of metal contamination in a bird's diet and the level found in its feathers, with the proportion of the chemicals found in its feathers remaining relatively constant (and relatively high for some metals).
Anglers who Fly fishing have long used the down feathers of waterfowl for the construction of artificial fly. Because they are white, down feathers can be easily dyed a variety of colours. They are used to make the bodies, tails and wing casings of the flies. Historically, much of the down used for the flies came from the wing and tail of the marabou stork; these feathers were also popular with the millinery and dressmaking trades. Though it is still called by the same name, today's "marabou" (sometimes spelled "marabout"), comes from domestic turkeys, as marabou storks are protected.Hellekson (2005), p. 91
For centuries, humans across the globe have used down feathers for insulation. Russian documents from the 1600s list "bird down" among the goods sold to Dutch merchants, and communities in northern Norway began protecting the nests of eider ducks as early as 1890. (1997). 9783540613572, Springer-Verlag. ISBN 9783540613572 Eiders are still "farmed" by people in Iceland, Scandinavia and Siberia. The birds are provided with nest sites and protected from predators, and down is collected intermittently during the nesting season without harming the nests or female ducks. The first collection is made roughly halfway through the incubation period, when some of high quality down is removed per nest. When the eggs have hatched and the young have left the area, the remaining down and breast feathers are gathered, typically resulting in another of lower quality feathers per nest. In general, 50–60 nests will produce about one kilogram (2.2 pounds) of down feathers. No more than four of eiderdown is collected from wild nests each year. Approximately 70% of that harvest is from Iceland. According to the International Down and Feather Bureau, eiderdown contributes only a small, but lucrative, portion of the world's total down production, estimated at 175,000 tonnes annually. An eiderdown duvet requires from 600 to 1,600 grams of eiderdown fill; a simple 800 gram duvet sells for about 640,000 Icelandic króna (US$5,116) as of 2021.
In Iceland and Scandinavia, colonies of more than 5,000 birds sometimes develop in "farmed" areas, while in some protected areas of the Novaya Zemlya archipelago, nest densities exceed . On the other hand, the more nomadic indigenous peoples of Arctic Canada did not see down production as a reasonable source of income and tended to overexploit eider nests with "indifferent" collection of down feathers from the nests. Although the down feathers of various species of wildfowl, and other have historically been used for insulation, most now come from domestic geese. Some 70 percent of the world's supply comes from China, typically from birds killed for their meat. Most of the rest comes from Europe and Canada, from birds harvested for meat or pâté.
A portion of the world's supply of down feathers are plucked from live birds, a practice condemned as cruel by animal welfare groups. The precise percentage of down harvested in this manner is uncertain; while some references report that it is only a small fraction of the total (less than 1% in 2011), a 2009 Swedish documentary reported that it might be as much as 50–80% of the total supply, a figure supported by IKEA and an industry representative, but disputed by organizations within the industry. The documentary also shows birds lying on the floor with large flesh wounds from the plucking, after which the wounds were stitched using a needle and thread without anesthetic. Although live-plucking is illegal in Canada, the United States and Europe, it is known to occur in two European countries (Poland and Hungary) and in China. Public sentiment against the practice has, in some countries, been strong. IKEA and clothing manufacturer Patagonia have altered product lines to eliminate the use (or possible use) of live-plucked down.
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 (in3/oz). To measure fill power, an ounce of down is placed into a graduated cylinder, and a small weight is dropped in on top of it; the volume below the weight indicates the fill power. Eiderdown has the highest fill power, at 1200. However, even down with a fill power as low as 550 still provides reasonably good insulation. 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 . Down rated is warm enough and light enough for most conditions, and fill is used for very lightweight and/or very cold-weather gear.
Down is warm, lightweight and packable. If well cared for, it retains its loft up to three times longer than do most synthetics. However, 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.
Down feathers tend to become more allergenic as they age, as they become contaminated with molds and . Pillows provide the most common source of exposure, though mattresses, , outerwear and upholstery can also cause problems. A Finnish study has shown that true feather allergies are rare, with most issues caused by dust mites.
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