I agree with the above poster on just setting eggs inside the pen. Snakes love them and baby chicks. I would put 6-a dozen in there. If you find them gone its a snake. Mice and rats will leave shell behind, with a snake they swallow the whole thing. I lost a few to snakes also and my first clue was I started to not get any eggs in the morning as usual, then I started missing a couple of chicks every few days. I just happened to walk in and catch one of them about 1000 am laying in a corner in my coop, swallowing one of my silkies; he was so big I thought it was a python at first. Needless to say the enraged chicken farmer came out in me and he died by the shovel. I personally have killed 3 over 5ft and over 5 1/2 feet this year. Another trick is to glue a fish hook on the eggs and tie fishing twine to the cage so you can catch him. There must be NO OTHER ANIMALS in the pen though. This is an old timer trick that does work. You must use superglue to attach the hook to the egg. They swallow the egg and hook and cant get away!