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Ditto - typically, a protein deficiency. I used to use dry cat food when I saw this problem - just throw a handful into the pen eveyr few days for a week, then once a week, until you see the behavior stop. I now feed a bit higher protein, and add Calf Manna at the rate of 10% to the feed, making sure to provide oyster shell at all times for the gals that are laying.
There are lots of things you can do break boredom as well. Things as simple as a milk crate in their run to jump around on - move it randomly around each day. I have "chicken trees" - trees that have died, I dig a hole in the run & "plant" the tree, adding horizontal branches (screwed on) so they can fly up & down, and hop around. I also suspend suet cages from the tree "branches" for them - just at head height - then fill with dark, leafy greens - a most excellent treat, and it takes them awhile to get it all pecked out!