http://www.sciencemag.org/news/2019/04/hair-raising-feather-mite-vacuum-cleaner-birds

Some things can’t be unseen. One is the final meal of a feather mite, a bloodsucking parasite that lives in the hollow quills, downy tufts, and outer layers of bird feathers. Or so scientists thought. Now, researchers have realized these tiny arachnids aren’t parasites after all, but may be beneficial cleaners that vacuum fungi and bacteria off bird feathers.

More than 2500 species of feather mites live in and on the feathers of almost every type of bird. Some of them—those that nest in the contour feathers that form the outer covering of a bird’s body—are thought to be so harmful that pet owners, zookeepers, and veterinarians try to remove them using special sprays or dusts.