The other day I heard a tapping coming the garden in my courtyard. It was a wren defending its territory from its mirror image.
I had placed to mirror there to direct more sunlight to some plants. But as soon as my resident wrens saw the “other” birds in the mirror they started defending their territory. They were so intent on doing this I was able to get within a yard or two of them.
The odd thing was, even though I should have been perceived as the bigger threat the birds completely ignored me.