Sadly, my money would be on Marek's as the likely cause too, especially since they were young birds, but without a necropsy, we can only guess.
Marek's is one of the most common killers of chickens under a year old. It often lies dormant for weeks or months after they are infected and then breaks out when the bird is under stress. That can be the surge of hormones at point of lay, or unwanted attentions from an overly amorous cockerel, or even a dramatic change in the weather perhaps requiring them to be confined when they were used to ranging. Each bird will react to stresses differently, so you don't necessarily get birds all showing symptoms at the same time. The disease can also cause a huge variety of symptoms, so often people rule it out because their bird doesn't exhibit the classic ones, but some birds don't suffer any obvious neurological symptoms and instead get secondary infections like coccidiosis or respiratory disease, due to the Marek's virus compromising their immune system, whilst others get the more classic leg or wing paralysis or ocular changes.
Marek's is one of the most common killers of chickens under a year old. It often lies dormant for weeks or months after they are infected and then breaks out when the bird is under stress. That can be the surge of hormones at point of lay, or unwanted attentions from an overly amorous cockerel, or even a dramatic change in the weather perhaps requiring them to be confined when they were used to ranging. Each bird will react to stresses differently, so you don't necessarily get birds all showing symptoms at the same time. The disease can also cause a huge variety of symptoms, so often people rule it out because their bird doesn't exhibit the classic ones, but some birds don't suffer any obvious neurological symptoms and instead get secondary infections like coccidiosis or respiratory disease, due to the Marek's virus compromising their immune system, whilst others get the more classic leg or wing paralysis or ocular changes.
