(noun) Soft, fine feathers that help insulate birds but that have no structure to support flight. Very young birds develop down feathers long before their mature plumage develops, but most birds have at least some down as insulation all their lives. Waterfowl and cold-climate birds have more extensive layers of down as an adaptation for their environments. Eiders, in particular, are well known for their luxurious down.
The down of young birds is often called natal down, and precocial birds have these feathers when they hatch. Altricial birds may have very limited natal down upon hatching, but will develop more shortly after birth. On adult birds, these insulating feathers are known simply as down or body down.
In addition to the insulating properties of down, these feathers also help waterfowl control their buoyancy by trapping additional layers of air close to the body. Mature birds can manipulate these feathers to adjust that buoyancy for diving or swimming if desired.
Photo – Duckling © andy