A large number of creatures live within each bird’s feathers including lice and mites, bacteria, and fungi. These are whole worlds of eating, feeding, and reproducing. Mites for instance feed on bacteria and fungi that feed on keratin, the protein substance feathers are made out of. They also feed on accrued debris. It is an entire community of creatures (some unique to each species of bird) and usually ends up balancing the system, the pterosphere which makes for a healthy bird at least in terms of fully functioning feathers.
If you want to dive a little deeper into a pterosphere study, here is an interesting recent investigation into mite behavior on feathers at night: https://doi.org/10.1002/ecy.3550