Once this spring and now this summer I witnessed odd bird behavior that I am not sure what to think about. It involves a bird precariously perching on or near a window frame and then flying continually at the window panes themselves, as if it is trying to get in. This Spring I saw an American Robin do it. Over and over and over again.from perches right on the frame..and from perches a few inches away, tapping against the second story window. There were plants inside on the window sill. So first I shut off the lights, no change. Then I moved the plants out of sight, no change. Finally I put a towel inside the window frame and the behavior ceased. This morning a Male Common Yellowthroat is doing the exact same thing, at a different window, this one on the first floor. It has been going on for at least 20 minutes, and since my husband saw the bird on a nearby post over the past few days, I wonder if it has been going on for that length of time. Any ideas on what this behavior is and whether I should be worried for the bird? Thanks. Jeannie