Usually if they've eaten quite a bit of something unhealthy for them..they will have diahhrea..last winter my girls were pecking creeping buttercup around the outside of their fencing. Our acreage has quite a few places with it. It was green..it was winter. They had a big area to forage but had pecked it down. If you are worried about poisonous weeds get a weed foraging book. It has poisonous mimics and edible stuff..I forage myself.. it's been a while. When getting into farm animals knowing what to eat and what to leave be is good. I doubt it was a plant that did this..