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.
Etymology
The word down comes from the Old Norse word dūnn , which had the same meaning as its modern equivalent.
Powder down
Powder down is a special type of down feathers. It occurs in a few groups of apparently unrelated birds and thus is probably an example of convergent evolution. These down feathers differ from the usual down as they are exceptionally fine and produce a dust between the fronds. The bird takes this dust from its powder down feathers and coats all its external feathers with the dust. This dust mainly provides the bird with some waterproofing for the main flight feathers so it can fly in the rain. It also conditions the feathers and keeps the barbs of the flight feathers interlocked to provide good lift. It may also be useful in suffocating feather lice and other ectoparasites by clogging their tracheae.
Powder down is found in birds such as herons or the Kagu. Larger species of white cockatoos (such as Sulphur-crested Cockatoo) also have powder down, located on their flanks (sides of the body).
Powder down feathers are probably seen most easily in the cockatiel. In this popular cage bird, they are located at the lower back beneath the outer tail feathers' bases. When used up, they are pulled out by the bird allowing new ones to grow. They are like a fluffy ball and when rubbed between the fingers the powder from the down will cover the fingertips.
Birds with powder down usually have a reduced uropygial gland, but not all birds with vestigial or missing uropygial glands possess powder down.
Dinosaurs
Feathers found in amber in western France suggests that some dinosaurs may have had primitive, down-like feathers.
Human use
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 (in 3 /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 in 3 /oz (230-520 cm 3 /g). Down rated 500-600 in 3 /oz (290-360 cm 3 /g) is warm enough and light enough for most conditions, and 800-900 in 3 /oz (460-520 cm 3 /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.
Methods of harvesting
Down can be collected in a variety of ways. Birds which provide the feathers may be used for other purposes, for example to provide meat. Some birds are killed solely for their down, while some birds (particularly some geese) are periodically live-plucked of their breast feathers. Some birds, such as the eider duck, line their nests with down, and such down is harvested safely after the young leave the nest.
Animal welfare groups consider the live-plucking of down to be a painful procedure and cruel, particularly since birds must undergo down collection repeatedly.
References
- ^ " 7 down (noun)". Merriam-Webster Dictionary . http://www.merriam-webster.com/dictionary/down . Retrieved on 2009-05-29 .
- ^ Owen, James (11 May 2009). "Dino-Era Feathers Found Encased in Amber". National Geographic News . http://news.nationalgeographic.com/news/2008/03/080311-amber-feathers.html . Retrieved on 29 May 2009 .
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