There's a strange thing going on since a few days ago in the office. Suddenly we recognized a blue tit flying like crazy up and down on our window. It looks quite average, a beautiful bird, not like somehow sick or something. Still, the first day we thought: "Okay, this poor thing sure is ill." We didn't bother anymore since we thought it's going to die soon.
The next day however, the blue tit flew a few charges again. Always flying to the window just to go up and down on it then. This lasts for about 5 minutes before the bird moves away again.
The behavior is shown frequently for about the last 5 days. Approximately the tit comes every 5 hours trying to storm our window. This doesn't mean, that it's always exactly 5 hours. Sometimes it's only there once per office-day and sometimes it comes back after an hour.
I've never seen something like this. What's wrong with it? What causes this behavior?