https://phys.org/news/2019-03-feather-mites-birds-plumage.html

There are more than 2,500 species of feather mites that live inside the hollow quills of feathers, in the fluffy down, and on the vanes of wing and tail feathers. Until this study, vane-dwelling mites have often been assumed to be parasitic, like feather lice, which are harmful to birds.

Alongside undergraduate student Arnika Oddy-van Oploo, Proctor conducted a rigorous investigation of what vane-dwelling feather mites eat. The pair examined the gut contents of 1,300 individual mites representing 100 different species, collected from 190 bird species. Co-authors augmented these data with genetic analyses of the diets of a smaller number of feather mites.