Why are sleeping bags and parkas stuffed with down?

Question

(Sleeping bags that are designed for use in very cold weather)

Down is the soft under plumage (feathers) of birds

Answer 1

Down is a good insulator because it creates many air pockets.  It is actually these air pockets rather than the down itself that hold in heat.

Answer 2

JasiusGG... is right, but there is more. Down is, of course, a very light material - the lightest of the commonly available insulators, but more importantly it is very resilient and compressible. That means that you can crush it, even compact it quite tightly, and when released from its containment, it springs back into its open form, with a little fluffing, within less than an hour.

These three qualities: the way it traps air, the way it compresses and expands, and its intrinsic weight make it an excellent choice for sleeping bags. It's only drawback, and the reason that one might choose synthetic fibbers even when price is not an issue, is that it does hold dampness, and when damp is a much less effective insulator.