Probably if you had introduced the mirror when they were a couple of days old, they would have loved it. Chicks, or chickens for that matter, are scared of change. I remember the first time I dug worms for my three-week old chicks and put them in the brooder in their sand box. All six Australorps piled up in the corner, with their necks stretched as high as they would go, all staring with fright at the five or so earth worms in their beloved sand box. I waited and waited, but they wouldn't come out of the corner until I removed the worms, then they didn't go close to the sand box until a couple of days later. You would have thought those earth worms were fire-breathing chicken hawks!
When they were much older, and had been outside in the coop for a while with a make-do roost, I decided to build a nice new one. They refused to go in the coop for two days and nights, choosing to sleep outside in the run.
They are now getting close to laying, so I put new nest boxes in the coop. Same thing. Sigh...
