Not necessarily a bird identification, but trying to get some ideas of what could be using my bird box. We noticed some fluff inside a couple of weeks ago but today we went out and found it overflowing with fill material. Any ideas on what birds or other animals would completely fill a birdhouse like this?
What is filling my birdhouse
If I took a guess, I would expect it to be some sort of ground squirrel or chipmunk making a house out of it. They tend to well insulate their nests so they are plenty warm and dry. That would be my guess!