When is this happening? If at night, most likely a predator. If it is a predator, and you see no signs of digging, it must be something small enough to get through the large wires. I suggest that your first line of defense should be to put smaller wire over the large rectangular wire. Hardware cloth is best, but even chicken wire over the larger wire would be a major improvement. If the injuries occur during the day, it could be picking, although that seems pretty severe. If it is daytime when this is happening, you need to start spying on your chicks from a distance so you can find out what they do when you are not around. If it is the chiks, you will start having to troubleshoot why it is happening (Nutrition, crowding, bad habit, food competition, etc)