I had a lot die like that this Summer. It was not cociddiosis (sp). I think it was some kind of bacteria? Jack lost a lot that way too. We both lost a lot of Wheatens; Jack- Marans, and me - Ameraucanas. We think the breeds with narrow bloodlines are more susceptible to what even it was because my Ameraucana chicks that are Wheaten/Blue crosses all survived but none of my pure Wheatens! I don't know if I want to cross those 1/2 Wheatens back to my pure Wheatens and strenghten the bloodline of sell all my Wheatens. I feel pretty busy with my 2 Cochin projects - Self-Blue and Mille Fleur. I will NEVER give up my Cochins but those Ameraucanas sure are pretty.
I had trouble with Isbars from February until August with the exact same type of problem, I would lose all the chicks one at a time from 3-4 weeks. If I was really careful and dosed them with Tetroxy HCA I might manage to save one out of 10-15. Did OSU testing and found out the hens were passing e. coli bacteria to the chicks through the eggs. Poor things didn't stand a chance. Had to get an RX from a vet to cure the adults, Bactrim. Just as I finished up the Bactrim regimen a neighborhood dog got into my yard, ripped their run door open, and killed my rooster. Of course.