I learn something new everyday. Here is one of my first lessons for today.
Bird hunting is big business, and some hunting preserves breed and raise birds to be set free and subsequently hunted. Pressures to keep costs low are the same for these farms as the farms that raise animals for food. Many house the animals in tightly confined facilities in large groups. Whenever you cram any large group of animals together, they get ornery and fight.
To prevent fighting, some of these preserves place "spectacles" on the birds, as shown below. They cut a hole in the beak perpendicular to the face, stick a metal pen through the whole, and attach a plastic device that limits the vision of the bird. The idea is that if a bird cannot see other birds, it cannot try to fight other birds.