Bird is seeing his reflection and sees the reflection as another male muscling in on his territory. Shine a light out of the window to eliminate the reflection. As the sun moves in a few more weeks and the mating season ends, this problem will self correct and you can turn the light off.

Other options include hanging objects in front of window to break up reflection. Some even smear some soap on the window and then wash it off after the nesting season is over.

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