When we did up a room in our house some years ago, we took down a couple of mirrors that were there since when we bought the house. I fixed them to the back wall of the garden. Until this year, the birds didnt pay much attention to them. During the summer, a juvenile dunnock would spend time every day confronting its reflection, and now, a young blue tit has taken excelption to the mystery rival that can match it’s every move. When I was filming this, I could hear the knocking noise of the bird pecking the glass (it’s not very loud in the video). I’ll take down the mirrors this weekend. It’s probably not useful for the birds to waste energy on chasing imaginary rivals – they’ll need to save all their energy dealing wit the goldfinches that have arrived back at the feeders, looking for trouble.