This behaviour is not uncommon. He sees his reflection and thinking it is another male, he is trying to drive away this rival from his territory. Not a potential mate as suggested above. The behaviour will stop as soon as the serious business of actually nesting is really underway.
Based in Sussex, I garden to encourage as many birds to my garden as possible.
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I have seen blue tits tapping my bedroom window - thought they were just winding up the cats!
Yes we have had birds attack the wing mirrors on our cars too. And they nibbled the rubber around the side windows on ours and our neighbour’s cars.
This behaviour is not uncommon. He sees his reflection and thinking it is another male, he is trying to drive away this rival from his territory. Not a potential mate as suggested above. The behaviour will stop as soon as the serious business of actually nesting is really underway.
An old neighbour had a pied wagtail attacking his wing mirrors so often, he took to putting socks over them.
I used to have mallards peck the windows to encourage me to feed them.
I agree with Dove. The chickens at my workplace tap on the glass too, probably thinking there's another chicken. Quite funny to watch.