Not just rat poison; look for D-Con too, or any of the knock-off rodent killers on the market.
Also, if any people nearby are on coumadin as a blood thinner/high blood pressure medicine and it's possible the chickens have gotten into thrown away medicine (or in their water supply?). Coumadin is rat poison.
Also, if any people nearby are on coumadin as a blood thinner/high blood pressure medicine and it's possible the chickens have gotten into thrown away medicine (or in their water supply?). Coumadin is rat poison.