I have a few thoughts for ya -
Do you have a concrete floor? or is it un insulated wood? They retain the cold unless you have 6 inches of bedding down. The air temp cold be plenty warm but overnight the floor could be getting very chilly lowering their temp and stressing the birds. I provide an insulated platform near a heat lamp to get them up off the ground or a perch near one.
If it's not - that it sounds like your little one could be coming down with
Coccidiosis.
Are it's stools still firm? or runny?
I'd recommend seperating it. Maybe even putting it back in the brooder for a few days on medicated feed or water. It does sound like it is sick and they do go down hill pretty fast.
I don't think it would be heat exhaustion... unless it you found your birds panting with winds spred when you came home. If you think that might have been the problem get some "bounce back" from a feed store or any electrolye formula for the water. Pedialyte for infants should work but only use it for a day full strength then water it down by half and quit using it after three days.
Use an eye dropper to help it drink if you think it has stopped drinking. But be careful! It's easy to drown baby birds who don't want to cooperate! Take your time. Tiny amounts at a time a few hours appart.
Could have been any of those things - that's all I can think of. I hope it helps! Good luck!
