This is an article from the Press of Atlantic City, about a young woman who had a bird mite infestation after birds nested near her air conditioning unit:
Chickadees nested in a protected area adjacent to her air conditioner outside the window of her Brigantine apartment. It was pretty cool getting a close-up view of chickadees raising their brood.
But when the fledglings took flight and the chickadees abandoned the nest, the bird mites that thrived by biting the birds suddenly went looking for a new blood meal.
The nearest animal was the young woman.
The mites had no problem crawling through the tiniest cracks around the window casing, since including legs and head they’re no bigger than the period at the end of this sentence, and their bodies are smaller still.
Bird mites are not very common, but when there is a bird mite infestation, this is a possible reason why.
The solution is to verify the problem is bird mites (you can use tape to pick up samples on a window ledge, for example). And then get a pest management professional to treat it.