I'm getting in on this a little late but I've been thinking about it...the rat bait boxes are tripping red flags for me. I know that you said the chickens can't get at them, but have you considered that the rodents might be bringing bits of it IN where the chickens are? A small or juvenile rat, or even a passing mouse, might be able to get through the chicken wire and may drop little bits of that rat bait in the run. We all know how chickens are - if there's a tiny kernel of something new and they spot it, they'll devour it first and ask questions later. If, during scratching around Hen A finds a bit of it and eats it, death can be swift. Then a few weeks later Hen B finds another morsel, and so on. Just a thought, and I might be way off base, but small rodents can squeeze through some pretty small openings.