Feathers are to birds as hairs are to mammals .. but not quite.

Here are some feather facts to ponder.

• Feathers, like hair, are dead structures that have no nerves and cannot change or heal themselves if damaged.
• Our hair grows continuously.  Feathers grow to completion and then stop, so they must be replaced when worn out.
• The follicles that hold feathers in the skin have muscles that grip the feathers so they don’t fall out.  Anyone who’s plucked a chicken knows these muscles are strong.
• In some birds, such as nightjars, the follicle muscles let go when the bird is frightened suddenly.
• A new feather literally grows under the old one and pushes it out of the follicle.
• Contour (body) feathers are symmetrical and so are their follicles.  Flight feathers are lopsided: narrow on one side of the rachis (shaft) compared to the other.  Flight follicles are lopsided too.
• The same feather follicle can produce differently colored feathers at different times of year — for example colorful feathers for the breeding season and drab ones for basic plumage.  Imagine if our hair could do that! We could automatically change our hair color in the spring.

Resources: Anatomy: Parts of a Feather.

(Inspiration for this Tenth Page is from page 90 of Ornithology by Frank B. Gill. Photo from Wikimedia Commons; click on the image to see the original)