We use poison here for rodents. I personally haven't seen any secondary poisoning that I'm aware of.
Corn, or rations in metal cans in the heat can mold. Aflatoxin comes to mind. Not saying that it, but I do notice in summer any feed in metal cans gets moldy quickly if you don't keep an eye on it.
Heat can kill over 80 degrees sometimes.
What are you feeding? Fatty treats can lead to fatty liver which is deadly. Hens can rupture and bleed out fairly quickly internally after a strenuous activity like laying.
The only diseases that kill multiple birds quickly tend to have obvious...