is a special type of down feathers
. They occur in a few groups of apparently unrelated birds and thus are probably evolutionary homoplasies
. These down feathers differ from the usual down as they are exceptionally fine and produce a dust between the frons. 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 still 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.