Birds are the only living organisms with feathers. Feathers are marvelous structures. They are flexible, yet strong; they help keep a bird warm and dry, yet can help keep birds cool and hold and transport water. Feathers keep birds protected from injury, and allow them to send signals to their friends and warnings to their enemies. Of course, feathers also allow birds to fly.
Feathers are composed of keratin. This keratin is similar to the substance that makes up most mammals' fur, reptiles' scales, and such different things as a rhinoceros' horn and our own fingernails.
Feathers come in several forms, but they are all made up of the same basic parts, though those parts may be absent or rearranged a bit, depending on the main function of the feather.
Each feather has a main shaft, or rachis, that supports the whole structure. While the feather is growing, the rachis has blood vessels within it that carry nutrients to the growing parts of the feather (AKA blood feathers). When mature, these blood vessels die and the rachis is sealed at the base, leaving the feather shaft hollow. This helps to make the feather very light.
Branching off the rachis are barbs. These barbs each have branches called barbules, and the barbules have branches called barbicels. These three parts make up the vane of the feather, which gives the feather its "feather-like" shape. The barbicels are very tiny, and you'll need a good magnifying glass or microscope to see them. They are generally hook-shaped, and interweave with each other. They hold the vane of the feather together, sort of like Velcro or a zipper. If you've rubbed a feather the "wrong way" and then smoothed it back to its original shape, what you've done is un-hook and re-hook the barbicels. What I like to call "zipping it back together". The barbicels can hold the feather vane together so tightly that water cannot go through. This is actually what keeps water off a ducks back, not the oil they apply to their feathers. In fact, the oil is only used to keep the feathers clean and in good condtion, not to coat them for "waterproofing".
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