At around 4:30 AM I hear a clear whistle from my backyard. It's a single note- a perfect Bb, it lasts about 3 seconds and once in a while, it repeats right away. That's it- only one per morning. It sounds like a high note on a child's recorder like they get in music class- or that of a tin whistle. My neighbors are all asleep. In fact when I first heard it last year, I thought it was one neighbor nextdoor blowing a small metal whistle to call her cat as she lets it out at night. Their house is empty and for sale now. This has confounded me for 2 years. The only thing I can think of is a chickadee. It's similar to the two-note song that they repeat, but only the first note. I've never heard a chickadee sing at night though! Any ideas?