If the intent of the mirror is to make the space feel larger, you may be able to "break the image" enough to pacify the birds while still retaining a reflection of the garden scene by hanging something like a fishnet over the front of the mirror. Something with a 1/4 inch mesh or so might make the reflection sufficiently less "in your face" for the birds. And the birds will bounce better when they fly at the reflection.

Of course a few nice ferns would work as well, but netting grows faster.

The mirrored wall deco thing has been touted in a few places in the US too. The idea is to make it blend into the garden, and it can be hard to "soften" the reflection enough to avoid the "invasion of the techno monster" effect.

John