Wow thanks guys, never even thought of the bird problem!

But I was actually thinking of having the mirror low down and long rather than upright.

There is a wall at the bottom of my garden so I was going to have a 2" x 6" mirror run along the back of the plants so it would reflect the plants and the garden. Not sure birds would see it with it being so low.

I'll try and post a picture to better explain.

Thanks