If you have feed available to your chickens 24/7 I would start by changing over to a scheduled feeding program, maybe twice per day so that all food is eaten by the time they go to roost. I generally feed my chickens table scraps from the previous day in the morning, they free range during the day, and are fed their ration of layer feed when I get home around 5. By the time it gets dark around 7PM they have cleaned up all of the feed, nothing left to attract rodents. By switching over to this feeding schedule I have cut my feed bill in half, the chickens are encouraged to forage more during the day before they are fed, egg quality has increased due to increased natural foraging as a food source, and my chickens seem healthier. I believe the feed use has been reduced because I am not feeding the rodents and my chickens are getting more food by foraging on their own.
I always keep a few snap traps set in the coop, just in case. I recently found some, a bit pricey though ($4 each?) , that have the snap mechanism enclosed in a plastic housing so the chickens don't get caught in them accidentally. My biggest problem with the conventional snap traps in the past is that the hens would consider the dead mice as easy pickin's and run off carrying the trap, with the mouse, out into the pasture (greatest game of Keep Away besides giving them grapes!) and I would be constantly buying more traps. Since these new plastic housing traps keep the snap mechanism and the resultant dead mouse contained in the housing I can easily check the traps, empty the mouse out (you push down on a lever that releases the snap), and reset it for the next time.
Besides the fact that rodents carry undesirable diseases that neither you or your chickens should be exposed to (hanta virus for humans, too many to mention for animals) I have little tolerance for rodents in my hen house. Also, be careful when cleaning your coop or piles of rodent droppings. Always wear a face mask when creating a dusty environment (such as raking/cleaning out the coop) and wet down any droppings or bedding before cleaning to keep the dust down. The viral particles are generally found in the dust created by the droppings.
That being said, I was just out to check on the chicks in the coop brooder before it got completely dark out tonight and I saw a mouse run across the floor of the coop. OK, rodents are a fact of life when you have livestock, but keep them down to minimal levels.
