Hi, they are called peepers or some call them blinders, they help reduce fighting between the two males i have..The male in the pictures(WIZARD) with it on is not nice to the other male.
they have a 100' x 25' x 8' flight pen.. Plenty of females to go around.. He just kept after him, so I put a peeper on him and it slows him down a bit..However he is learning to work around it.
Lots of big pheasant farms will use them to reduce cannibalism, there are many different types and sizes available, this type i have there is no push pin to push through the nostril, they are snap on type and are really easy to remove, also if the bird were to get caught up in the flight top netting the peeper pops right off and you don't end up with dead birds.
Ive seen people use them on some chicken breeds as well.
Thanks
Charlie