i have had the same problem. they do fine till right around 3-4 weeks and then they start tail picking. I keep these chicks in an outdoor brooder house that measures 8ft long x 3ft deep x 2ft tall with a divider in the middle so i can keep to batches at one time. i have one door on each end and i supply each side with a feeder, waterer, and clamp light with a 100w bulb for this time of year. higher if still early. the bottom is 1/2in hardware cloth and the side are wood, the top is tin and the front is chicken wire. at right around 3-5 weeks mine started picking at each others butts until the pickee died. well the first time this happened i moved them to to a larger cage on the ground, where they could pick at natural stuff. this time i dont have the room so i read that it could be from several reasons: too crowded, too bright, not enough protien, not enough food, bordom. so i tried to fish out the problem. so i covered the front of the brooder with cardboard and spray painted the lightbulb black and then put it back in. so now the only other light is coming through the wire floor. also i put a hand-full of fresh grass clippings in with them for them to pick at. i have had no picking problems since.