I had a similar encounter with a sparrow last spring. While it was wonderful to have him right at the window and singing all day (in between attacking the 'enemy/rival' reflected in the window), I became concerned when I realized he was spending all day every day at the window in battle. I looked up this behaviour online and found out that male birds will defend their territory against their own reflection for weeks or months, neglecting their other duties, not eating and also being so focused on the perceived threat that they don't pay attention to real threats like cats. Anyway, I ended up taping sheets of paper to the outside of windows he was focussed on so there was no longer a reflection and he went back to taking care of his little family.
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