All birds are plagued by parasites, as is every other species. Here’s a scholarly paper about parasites in wild birds. Some of the parasites are internal, and there is very little the affected bird can do about it. Others are outside, on the bird’s skin or feathers. Getting rid of those is a major part of a bird’s daily business. Here’s a House Finch (Haemorhous mexicanus) male on a twig on the north shore of the park. He spent at least 15 minutes beaking his skin and feathers front, back, and all around. My video excerpts less than a minute. It does show, I think, the intensity and concentration of the bird’s effort to rid itself of these tiny plagues.