So the new house has lots of big windows, especially in the den; it's one of the reasons we bought this house.
What I didn't realize is that these are actually windows of death. Bluebird death, specifically. They keep flying into the big windows; I guess they are defending their territory from what they don't realize is their own reflection in the window.
Luckily they are usually just stunned, not dead, and they recover and fly off in a few minutes.