You do have a nice big pen, and I see your layer boxes, but where do they sleep at night? Do they have an enclosed building or coop, with roosts, to keep them secure from predators and out of the weather?
Oops! Thanks. I did not see anything marked "stomach" in the schematics I saw, the digestive tract went straight from gizzard to intestine. I can't seem to c&p links any more so I can't share what I saw.
And though it's been mentioned, don't forget the grit. They can starrve to death without it. Chickens have no stomachs, the silly things, and no teeth! They have gizzards, a tough muscle that, when it contains grit, uses that grit to grind their food into digestible bits. Aren't you glad you...
It sounds like you are throwing food to them once a day in the morning, which they eat. But they "have no body mass, " are lethargic and dying. It sounds to me as if they simply are not getting enough to eat. They need feed available all day long. Provide all-flock or grower in feeders they can...