So sorry for the loss of Megatron.. it's never easy. Companion animals teach us the value of life in the smallest things, and in turn to be kinder to all those around us. It's such a hard lesson for children to learn, and just as hard for adults to whom children look for guidance on how to behave when faced with grief and loss.
When thinking about separating birds (which will cause them stress too) consider how long have your flock been together, and whether they came from the same breeder. The question is whether they have spent a month in the same environment already, which would make it likely that they all have whatever the others have.
Given that there are no symptoms (changes to poop or behaviour) it sounds unlikely to be a disease. I can't think of *anything* that would kill within 8 hours in an asymptomatic bird outside of physical defects leading to an event like stroke/seizure/heart attack, ingestion/inhalation of toxic substances, or a night fright leading to a head or spinal injury as a result of hitting cage bars.