What birds have feathers that are useful to humans?

The number of feathers a bird has is largely influenced by a) its size (although this does not mean that the largest bird has the most feathers), b) where it lives and c) which season of the year it is. Feathers provide warmth and insulation, birds in colder climates have more feathers. That's why "moulting" occurs during the warmer months, when excess feathers and downy, underlying feathers drop out. The bird with the most feathers is the Whistling Swan Cygnus columbianus which can have up to 25,000 feathers during Winter. The bird with the smallest number of feathers is the Ruby Hummingbird Archilochus colubris with only 940 feathers altogether.