Different sources provide different answers about how many things birds do with their feathers, but all of them agree that insulation is important. The snow piling up on Mom's back last night provided a clear picture of the insulative properties of feathers: Mom wasn't losing enough heat to melt the snow away from her back. I wish my roof was so efficient.
Feather Anatomy
There are two basic types of feather: vane feathers, which cover the birds exterior, and down feathers - looser, fluffier feathers underneath the vane feather. All true feathers are branched: they have a central rachis with rows of barbs on either side. However, down remains light and fluffy, which traps air and helps insulate the bird, while flight feathers hook together like a zipper, to form a continuous 'vane'. Personally, I think of it as dressing in layers: the vane feathers form a sort of 'overcoat' underlain by the soft, insulative down feathers. Given that snow also has insulative properties, Mom was quite warm and cozy under her blanket.
So how else do birds use their feathers? According to 'Ask A Biologist', they help birds fly, keep warm, control body temperature, provide weather protection, aid in swimming, diving and floating (waterbirds and piscivorous birds), snowshoe (Ruffed grouse), toboggan (penguins), brace, feel, hearing (owls, harpy eagles), making sounds, muffling sounds (owls), foraging, keeping clean, aiding digestion, constructing nests, transporting water, escaping from predators, sending visual signals, and camouflage. I think I'm jealous!
Since feathers do so many things, it is unsurprising that they come in more than one type.
Types of Feathers
The feathers we find dropped in the woods most often are tail and flight feathers. They seem the same but are actually a little bit different. Tail feathers are balanced evenly left and right of the rachis, while flight feathers have a wider and narrower side. This helps them cut through the air with very little drag. Of course, wing shape also influences flight, but that is for another post. The downy feathers that are good for warmth are shown on the far right. Semiplume feathers are also insulative and help water birds float (although some diving birds are more concerned with going deep - their feathers become waterlogged to help them sink). We think the bristle feathers, found around eyes, nostrils, and sometimes the mouth, help protect those sensitive areas. Filoplume feathers are can be found around the tail and flight feathers. They are thought to be used to sense when the flight feathers need to be maintained.
Although feathers seem light, all of them put together weigh roughly two or three times more than a bird's skeleton does. They also require a lot of maintenance. Most birds have a preen gland near their tails. This gland secretes oil which they spread over their feathers with their beaks. Preening helps remove dust, dirt and parasites from feathers and also aligns them properly. Even with care, feathers eventually begin to suffer damage and must be replaced through molting - an itchy-looking process that renders some birds (but not eagles) flightless. However, the benefits of feathers - flight, protection, insulation, display - far outweigh their costs. We sleep warm in our beds with the benefit of furnaces and blankets. All Mom and Dad need are their feathers.
For more information on feathers, I suggest this website: http://askabiologist.asu.edu/explore/feather-biology. I also took some information from Colin Tudge's book The Bird.
