Japanese Bantam Cockerel Deaths?

[SeramaShelly]

Hi! Approximately three months ago, my Campine hen, Stella, went broody so I purchased a mixed lot of hatching eggs (5 Black Tailed Japanese Bantams5 /Quail D'Anver Bantams) for her to hatch. On June 9th, Stella hatched out three of the Quail D'Anvers (one cockerel and two pullets) and four of the Japanese Bantams (two cockerels and two pullets). The weather has been warm so I moved Stella and her chicks to my small coop outdoors on July 15th. Everything had been going well until July 29th. That morning, I noticed one of the Japanese cockerels appeared to be lethargic and not interested in eating. Later that evening, I found him inside the nesting box, dead. The next day, the other Japanese cockerel exhibited the same symptoms and that night, I found him dead inside the nesting box as well. Stella and all the other chicks are doing very well. The remaining chicks appear to be extremely healthy and are thriving. None of the other chicks have exhibited any signs of illness but they remain quarantined from my other birds in their separate coop. Stella and all her chicks drank the same water and ate the same food -- nothing different than any of the other chicks.

This was my first experience with Japanese Bantams. Is there something I wasn't aware of about that breed's cockerels? Is there some type of genetic problem I'm not aware of? I'm just confused as to why both cockerels reached a certain age and died when all the other chicks did not appear to be affected by anything.

Anybody have a clue as to what happened? I'm at a loss