Powder down

From Freepedia

Powder down feather are located at the back of the cockatiel just above the tail base beneath the outer feathers,these down feathers differ from the usual down as they are exceptinally fine and produce a dust between the frons,the bird takes this dust from its powder down feathers and coats all its external feather with the dust,this dust mainly provides the bird with some waterpoofing for the main flight feathers so it can still fly in the rain.It also conditions the feathers and keeps the frons of the flight feathers interlocked to provide good lift. Cockatiel down is by far and away the softest down feathers on earth,the powder down feather when beyond use are stripped out by the bird allowing a new one to grow,they differ in appearance in that they are like a fluffy ball and when rubbed between the fingers the powder from the down will cover your fingertips.