Type peepers into the search engine here on BYC & on Google & you'll get more info, but basically, peepers (a.k.a. pinless peepers)are little "sunglasses" (
...that's what they look like, they're really just blinders) that clip to the birds nostrils on its beak. The "lens area" is opaque plastic & blocks the bird's frontal view, so it can only see peripherally. This inhibits attacks and feather plucking on other birds. Works well. Some say it's a bit painful for the bird when applying them & if they hit them on something, but the bird adapts & its better than having a tormented or dead bird, IMO.
After a couple weeks, you remove them & see if the bird's behavior has changed towards it's flock mate. If not, back on they go! I've even heard of people using them permanently. But, at that point, maybe a cull of the flock would be better... Dunno.
All I know is I have a feather plucker right now... Likes to pluck the neck feathers on two of my other birds & I'm about to order these & see if they work!
ETA: Just bought some off eBay. Got 10 for $4...$6.20 w/ shipping. Also found them online in pkg of 100 for $20. Hope they get here soon...my chickens are gonna need scarves if they don't! lol...