Most common cause is probably coccidiosis if it wasn't a toxin. They can die from it even with poop looking fine i think. We move our chickens to new grass every day and have had no cocci deaths in years, cross fingers. They get it when kept on the same area day after day, especially in rainy weather.
Our county sprays herbidide along our road (just did the other day) and they ignore our big signs saying organic farm, don't spray. It doesnt seem to affect the birds but i still hate seeing them do it. We use boiling water to kill fire ant mounds and wasps, plus our favorite phoebes help eat all the wasps too and nest under the porch every year.
