I have made some of these out of string from the feed sacks and small coins, dangle then above the birds etc and they do move on there own accord, round in a circle for hens and back and forward for a cock.
They even work on the rabbits, chickens, guinea pigs, dogs and humans but not 100% accurate on the pigeons but very close :-/
Anyone else use them and how accurate have you found them :-/