That is a possibility. There is a product called MouseX that is supposed to not be harmful to livestock if they accidently eat it or a mouse that died from eating it. I wouldn't leave it where my chickens can get at it but it's safer than your standard mouse poison. I use it when mice get in my...
I'm so sorry that you lost them. 😢 Did you see any other symptoms such as unusual coloring in combs or wattles, any signs of congestion (wheezing, runny nose or sneezing), anything else unusual with either of them? When they are out in the yard do they have access to a compost pile? Moldy/rotten...