I lost three nice hens, long ago, before figuring out that it was all about a mite infestation! Definitely a guilt trip, and a learning experience. The wild songbirds who come for the chicken feed in the coop, and who are out there with free ranging chickens, bring them in. So, checking at least a few birds weekly, at night, with a flashlight, is best, so they can all be treated is even one bird has a few.
And here the roosters seem to have most, most often, probably because of their mating behavior.
Mary