Four Dead in Three Days

3 days ago i found one of my 11 weeks old chickens dead. I did not really think much of it because the other chickens in the same tractor looked healthy and thought maybe it froze to death because it was found dead in the run area and not in the warm coop. The next day another chicken was dead and one was acting sick. The hen that was sick had drooping wings and seemed to have difficulty swallowing but very eager to eat. I thought maybe they had worms but could not find any strange poop. I soaked some bread and scrambled eggs in apple cider vinegar and fed it to them and the sick hen was looking much better only hours later. It became difficult to pick out of the group which one was the sick one to begin with because of its "recovery". This morning she and another hen were found dead. I found yellow water feces but that could have been from the scrambled eggs.
I did not notice any wheezing or anything coming from the sick chickens noses. The only symptoms were difficulty swallowing and drooping wings. All of the chickens had a healthy appetite up until they died. The birds had been outside for 1 month. This leaves me with 1 chicken in the tractor which is no where near any of my other birds.
My theory is they ate something off the ground that was toxic because they started dying the day after i moved the tractor.
Does anyone else have any theories or ideas? My 1 remaining chicken from this batch seems to be healthy and has no symptoms. I gave 3 of the chickens from this batch to my brother earlier this week and those also are still alive and seemingly healthy.