Also, the chicks grow very quickly and like to test their wings. They can get through 2x4 welded wire very easily. When they are away from their mother, they sometimes panic and squawk and have difficulty getting back into the pen. Also, they will fly as soon as they can. Babies will start perching a foot or two off the ground when they are just a couple of weeks old. You might run chicken wire on the inside of your pens. If it's on the inside and a predator reaches in to try to snatch a baby, the wire will prevent their paw from getting all the way into the pen. (If the wire is on the outside, the predator can actually pull the wire away from the pen and still get your chicks.) Several years ago I was very surprised when my chicks were able to get to the 10' perch to sleep under their mother's wings. Two babies started walking the perch like it was a tightrope, and walked right outside the pen. This was a structural beam that the birds like to sleep on. Anyway, these two babies were clinging to the wire, screaming their heads off, unable to get back to mom. Mom was screaming HER head off too, because she couldn't get to her kids. From that experience, I ran chicken wire at the end of every horizontal beam in the aviary. Just in case.