I had that problem with the only batch I tried. I ordered 200 and sold most of them, kept 80 for myself. I had them in a 10 foot by 8 foot horse stall with six heat lamps spread across the stall. Instead of spreading around under all the lights, the whole lot of them would pile six or seven deep under one lamp, and the bottom layer would get crushed to death because they would just get warm and go to sleep and not get up and move when they couldn't get enough air under there. By four weeks old, they started attacking each other and all ganging up on one bird, killing it, and then ripping it apart and eating its guts.