Superbowl

I went looking for a diagram of the follicle to show how interesting that structure alone is, but this one goes above and beyond. The report it came from is pretty intriguing as well, as it looks into possible evolution of feather follicle and mammary glands being similar. It's a bit over my head though, but still very interesting.

The images in B show the follicle structure. There's some sort of hormonal signal IIRC that triggers growth of a new feather. Follicles can become damaged by injury, in which case feathers can't regrow. If this occurs over a large area, such as a burn, infection, or persistent plucking, flight can be compromised.

In E (a-d) you can see cross sections of the pin feather through various stages of development. The rachis is the shaft of the feather, that you would think of when you picture a quill pen. Everything branches off of there. Barbs come out first, and then those branch off into tinier barbules. The Barbules link together like velcro, and that forms the 2 vanes of the feathers. If you run your fingers through a feather against the grain, you will feel resistance and that sensation of pulling apart velcro (though not near that tight!) and that is the barbules separating. When birds groom, one of the things they are doing is setting those barbules back together nicely. This is what gives them a semi-rigid structure, so that when they flap, the air doesn't just pass right through.