Something no one mentioned is toxins they could have ingested. Rotting vegitation, mushrooms and spoiled feed (gotten wet and hot in the summer sun and spoiled) can kill waterfowl quickly.
You said you dewormed, but didnt say with what, how much, how often. Deworming without knowing if they have worms, and or what type is medicating for no reason.
Toxin flushes: https://www.backyardchickens.com/t/700526/flushes-for-aiding-in-toxin-removal#post_9508213
You said you dewormed, but didnt say with what, how much, how often. Deworming without knowing if they have worms, and or what type is medicating for no reason.
Toxin flushes: https://www.backyardchickens.com/t/700526/flushes-for-aiding-in-toxin-removal#post_9508213