Running down the list of possibilities, we can then rul out the feed, and the water. I don't think you need to be concerned with Marek's as it doesn't become symptomatic this quickly. Cocci also rarely affects chicks so new. (Unless you got them from a disreputable source.)
Chilling is probably the most common killer of chicks. They could have been chilled as they were shipped to your source. I'm not clear on that. Did you pick them up directly from a hatchery or breeder? Or did you get them from an intermediary such as a feed or pet store? That would entail shipping.
Chilling causes shock but the chicks don't necessarily show signs of it right away. It can take a few days for them to sicken and die. As I already pointed out, some chicks, after you got them, may not have been able to warm themselves under your setup. Have you looked at that?
If you rule out all these things, if you want to know what killed these chicks, a necropsy is the only way.
Chilling is probably the most common killer of chicks. They could have been chilled as they were shipped to your source. I'm not clear on that. Did you pick them up directly from a hatchery or breeder? Or did you get them from an intermediary such as a feed or pet store? That would entail shipping.
Chilling causes shock but the chicks don't necessarily show signs of it right away. It can take a few days for them to sicken and die. As I already pointed out, some chicks, after you got them, may not have been able to warm themselves under your setup. Have you looked at that?
If you rule out all these things, if you want to know what killed these chicks, a necropsy is the only way.