I don't use poisons because I don't want my chickens eating poisoned mice and I don't want my walls filled with dead mice. I use regular old snap-traps. The safest way I've found to use them is to place a box over them with a few holes cut in the sides. That way the mice can have easy access...
Mice aren't a physical threat to your hens but they can and do spread parasites and disease. My Cornish hens take care of any mice that venture out during the day and I throw my cat in the coop for a few hours every night to get the rest. I didn't think it was possible but my Cornish hens...