Down feather

Down feather

The down of birds is a layer of fine feathers 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 jackets, bedding, pillows and sleeping bags. The discovery of feathers trapped in ancient amber suggests that some species of dinosaur may have possessed down-like feathers.

Description and etymology

The word down comes from the Old Norse word dūnn, which had the same meaning as its modern equivalent.^[1]

Body down

Body down is a layer of small, fluffy feathers that lie underneath the outer contour feathers on a bird's body.^[2] Down feathers have a short or vestigial rachis, few barbs, and barbules which lack hooks.^[3]

Like many precocial hatchlings, domestic chickens are already covered with a downy coat of feathers when they hatch.

Natal down

Most species of birds are covered with down feathers at some point in their early development. The exception are the Megapodes, which are already covered with contour feathers when they hatch.^[4]

Powder down

Powder down is a special type of down. It occurs in a few groups of apparently unrelated birds and thus is probably an example of convergent evolution.^{[citation needed]} In some species, the tips of the barbules on powder down feathers disintegrate, forming fine particles of keratin, which appear as a powder among the feathers. These feathers grow continuously and are not molted.^[5] In other species, powder grains come from cells that surround the barbules of growing feathers.^[6] These specialized feathers are typically scattered among ordinary down feathers, though in some species, they occur in clusters.^[3]

All parrots have powder down, with some species (including the Mealy Parrot) producing copious amounts.^[7] It is also found in tinamous and herons.^[3]

Birds with powder down usually have a reduced uropygial gland, but not all birds with vestigial or missing uropygial glands possess powder down.

Function

The loose structure of down feathers traps air which helps to insulate the bird against heat loss.^[2]

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 (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.

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.^[8]

Dinosaurs

Feathers found in amber in western France suggests that some dinosaurs may have had primitive, down-like feathers.^[9]

References

Notes

Cited texts

• Campbell, Bruce; Lack, Elizabeth, eds (1985). A Dictionary of Birds. Carlton, England: T and A D Poyser. ISBN 0856610399.