Bird feathers are dead, like your hairs. (This is good: if your hairs were alive, getting a haircut would be a much more traumatic experience.) A bird’s ability to control its feathers comes from the fact that the base of each dead feather sits in a follicle—a little hole—in the bird’s live skin. Each follicle grips the base of its feather with muscles to prevent the feather from falling out. Sometimes, stressed birds will relax those muscles, causing them to lose some feathers in a “fright molt”. This may even be adaptive: if you are scared because, say, a cat has grabbed your tail, it might be a good idea to drop those tailfeathers and escape.
The follicles are connected to each other through a network of tiny muscles in the skin. These muscles are what allow the bird to move its feathers: through muscles acting on the base of the feather, the bird can raise or lower the feather. By contracting muscles between follicles, some birds can also pull feathers closer together.
There are a lot of reasons a bird might want to move its feathers. Moving feathers allows a bird to change its temperature, its aerodynamics, and the visual image it presents. Birds don’t have the right muscles to make different facial expressions like we do, but a lot of the same messages—I am angry! I am happy. I am worried…—can be communicated by the position of a bird’s feathers.
I use this in my day-to-day field work: when we handle birds, we want to make sure that the bird isn’t getting too stressed out. One of the signs of stress we watch for in juncos is whether the feathers on the top of the junco’s head are raised. When a junco starts raising those feathers, he is quite stressed out, and we quickly release him so that he can calm down.
It’s easy to assume that a bird’s wing feathers are moved by the large muscles that move the bones of the wing; after all, that’s what we mean when we say a bird spreads or flaps its wings—but that isn’t the whole story. Birds also move individual flight feathers, fanning them to increase the surface area of the wing, or folding them to decrease it. The bones of the wing manage the large-scale movements, but the bird does lots of fine-tuning by moving the individual feathers, allowing it to optimize the shape and area of its wings second-by-second as it flies.
Birds even have specialized sensory feathers, called filoplumes, that allow the bird to sense the position of adjacent feathers. These seem to be mostly related to flight, since flightless birds lack them. Filoplumes transmit any disturbance at their tip to the sensitive cells in the follicle at their base, the same way that a cat’s whiskers transmit touch information to the cat. Filoplumes let the bird sense the position of its feathers and may also help the bird detect its airspeed when flying, which could influence how the bird wants its feathers to be positioned.
Gill FB. 2007. Ornithology, 3rd ed. New York: W.H. Freeman & Company.