I’d look for a clostridium type agent (causes various diseases in various species: tetanus, over eating (calves and lambs), blackleg (calves), red water (cattle), hemorrhagic bowel (cattle and possibly dogs), quail disease…). Unless they were somehow poisoned/toxin, it sounds like a bug that strikes fast and unexpectedly, killing before you know they are sick and if you do catch them it is usually too late to treat. A nasty strain of e. Coli or salmonella could also do it, as could certain viruses, but where did it come from? A virus needs a vector (wild birds in avian influenza, mosquitoes for west Nile and sleeping sickness). Clostridium and certain other pathogens can form spores and remain latent in the environment or body tissue until the conditions are appropriate for growth (varies by species: a wound, damaged muscle or liver tissue, too much starch in the gut…) when it wakes up, multiplies, and produces a nasty toxin that quickly kills the host. You’d need to send your dead birds into a diagnostic lab for any hope of an actual diagnosis (and half the time it comes back inconclusive). There really aren’t any available vaccines for birds. Penicillin type drugs are effective but if the animal is already sick it doesn’t have time to take effect, usually all you can do is treat the survivors prophylacticly. Good hygiene, ventilation, nutrition, and clean water help prevent issues but you can’t prevent everything. Limiting contact with wild birds and rodents can also help but isn’t always practical. Sorry for your loss but don’t blame yourself or give up in frustration, stuff like this is part of raising anything and sometimes we never get an answer. Just do your best, learn from the hard times, and keep going!
