when breeding birds, you can place them in a 1 male to 7 hens in a pen of 8'x16'. Unless you have an extremely excessive amount of cover for the hens to get away from the males, they will end up with the back of their heads missing feathers and flesh. When breeding pheasants in pens, you have to collect the eggs everyday because if one breaks, the hens will eat the eggs and eventually get in the habit of eating their own eggs.
During the mating season, male pheasants get largely aggressive. Their two methods of getting out the aggression is clawing and pecking. The salt in the blood is an attractive taste to pheasants, and they will continue to peck at open wounds. Even day old chicks will peck and attack other chicks if they are laying down. I've seen chicks drag each other across a brooder and then all the rest of the chicks mob it, pecking it. If you can't stop them at the time they do it, you will end up with dead chicks.
Anyone who's raised and bred pheasants knows that in the wild they have tons of cover to escape. But in pens, there is limited space and cover, and it doesn't prevent pecking no matter what you do. If you want your birds to be healthy, pretty, and without feather loss of anykind due to pecking or aggression, then peepers is a great solution. Whether you use clip-ons, pins, or rings is up to you. Clip-ons just put little nubs in the nostrils and hangs there, they are prone to fall off, but not get the bird caught in netting or fencing. Pins puncture through the membrane between both nostrils through the beak and clip the peeper on permanently until you take it off. Rings use a clip-on method, but using metal rings like a hog ring. The metal in the nostrils can cause problems with the birds.
Books that state pheasants are cannibalistic by nature and how to manage that nature.
Pheasants, Partidges, and Grouse - By Steve Madge & Phil McGowan
Upland Game Birds, their breeding and care - by Dr. Leland Hayes
Captive Birds in Health & Disease - By John & Margaret Cooper
Gamebird Breeders Book - By Woodard, Vohra, Denton
Here is a paper from the University of Nebraska on Poultry Cannibalism (includes pheasants as part of the study)
http://ianrpubs.unl.edu/epublic/live/g1670/build/g1670.pdf