I use poison 24/7 and have been doing so for almost four years. No worries and not ever a problem. Once in all that time there was a dead chipmunk on the lawn in the morning when on route to open coop. Picked it up and put in garbage. The key to poison is to use the chunx blocks so it can't be strewn around. We use tamper proof exterior bait boxes too. This way animals can't open the box and get to bait, the bait is secured on pegs so can't be dragged out of box and the box can be put right in the run against the coop wall and along a run wall where natural pathways of rodents would be.
Rodents are the seedy underbelly problem of keeping livestock. It's not addressed enough to new chicken keepers and leads to a lot of grief. Take feed away at night, check. Store feed in metal can, check. These little things I did when first having birds but still was overrun with chipmunks and rats. Went to tamper proof bait boxes and have never had a problem since. We have other pets and I use to live with neighbors that had dogs and would come onto property. Never a poisoning to unintended target- rodents. I keep one bait box in run and one side of house closest to run.
Boxes like this, 2 combo-
http://www.ebay.com/itm/Protecta-LP...hash=item1e58206b5e:m:mhhVWTpsee-S-WWdtWZ_Ocg
I use this chunx-
http://www.ebay.com/itm/Tomcat-All-...302029?hash=item54327cf1cd:g:9l8AAOSwvg9XdaPi
You'll go through a lot of bait in first few weeks and think it's going to drain your wallet. Don't sweat it. After that initial bonanza things go to a stand still until more rodents try to move in. During the off parts of year you'll not use any bait excepting a thoughtful change out due to getting old. I use only 1 chunx per box most of the year and check it once a month. In spring and fall when rodents are on the move fill the stations and check weekly. After intial cleaning up of rodents in your area a 4 lbs pail with last years.