Just throwing this out here. But two of my chickens ate and drank right up to maybe 3hours before death with mareks disease.
No one on here can tell you what's wrong but only give possibilities and opinions, I do hope you find out exactly what the cause is. Best of. Luck
unfortunately if it was mareks there is no cure if your flock was in contact to the rooster they may have contracted it. Look out for these few signs (these are the symtoms shown in my flock, some birds show NO signs of it at all)
Falling wing or wings, lameness, falling over, being all fluffed...
I'm just across the water in Northern Ireland. I have just had another outbreak of mareks disease so that's why I thought I'd mention it, most symtoms sound like it although none of my chickens shivered with it