Susan Skylark
Songster
Sudden unexplained death in multiple neonates is most commonly associated with toxin, environmental stress, or certain nasty bugs. You’ve ruled out the toxin possibilities pretty well as well as addressing possible heat stroke. The nasty bug list could be something like an e. Coli, salmonella, or clostridium, a virus would likely have higher mortality and morbidity (sick chicks). If it is a bug, it is basically an overgrowth of a normal environmental or gi bug that produces a nasty toxin that kills the host with no time to get sick. You are doing great with sanitation and cleanliness so it isn’t contaminated water, rather something throws off the natural balance of the normal bacteria allowing the pathogen to over grow and overwhelm the host with the toxin. Think clostridium difficile in hospital patients. There isn’t much you can do to treat or prevent it except sanitize well after this batch and keep young birds separated from older birds for several months until their immune system is mature. Maybe the heat stress made them succeptible? Don’t blame yourself or get upset, it is a fluke thing and happens even in the best managed herds and flocks and all species. Hope the rest do well!