I use old CDs as signalling mirrors in GHBs ..etc. I have also taped some at home windwos to ( hopefully ) scare away nesting pigeons. They didn't scare the pigeions , so I hope they would work as signalling mirrors. LOL

For those with pigeion problem , you may try hanging the CDs from thins strings instaed of taping them to the window glass. May be if they are moving with the wind, they would be more effective than being staionary.