This morning I thought let's look at the air filter since I haven't done that in a few weeks. I opened up to find a bunch of bird seed in there. Maybe 2 tablespoons total. My wife puts same seed out for the squirrels and chipmunks. I have to assume a chipmunk though this would be a nice place for a winter nest. I should've taken a pic but didn't have my phone on me. I cleaned it all out though.