Did the birds look wet like that on their heads and wings and torso when you found them? Kinda looks like a snake got ahold of them, and couldn't swallow them past a certain point, so spit them out. Might be worth a close inspection for injury and a posting (or cross posting of this thread) on the Predator Forum. When my POL pullets/hens died of a (probable) heart attack (had one egger and a couple cornish cross meat birds drop dead on me), it just looked like they were lying down with necks stretched out, on tummy or side. No wetness, injuries, wings against body. No blood or other injuries.
Did you have adequate grit available to the chicks? Could be they got ahold of something and that caused an issue. I'd carefully inspect the inside of the crop, the inside of the gizzard, and every inch of the intestines to see if you can identify a blockage or toxic substance like a piece of metal, or who knows what. Also check the internal organs for swelling and deformity (liver, crop, heart, kidneys, etc.). The Meat Bird Forum has a lot of info on identifying healthy and unhealthy internals of a chicken. The search function can probably help you out there. "healthy chicken, signs of disease, healthy organs, etc" might be some good search terms. The Merck Veterinary Manual site can also be helpful. Maybe research signs of a heart attack - I don't know if there are any, but MERCK manual would know.
Before you start your autopsy, I'd look for bruising or wounds, punctures, etc. on the outside of the chicken commensurate with predator attack. Might be harder to see on AC, but worth a try. There's probably pictures on here somewhere for a guideline. In normal chickens, there's swelling and darkening of the area around an injury/bruise (black, blue, green), and that stuff happens before the chicken dies, so it can stay in place after they're dead. I'd also look for any sign neck has been broken or bones dislocated - this might be hard to tell for a beginner.
You may have to wait until rigor has passed to autopsy, or you'll miss a broken neck or bone because the muscles are holding everything together. Or you can do internals now, and come back in a few days to check the neck and such after rigor passes. Rigor can pass in 3 days or less depending on size and age of the bird, probably less in this case. Just keep lots of ice in the cooler, without trying to actually freeze. I think 40F is a good target temperature for chilling. And these birds are so small you should be able to chill them rapidly, so i'm not worried the internal organs will "go bad" in that time enough that you can't tell what's going on. [meat birds without organs in, full size 7 lbs carcass take anywhere from 3-5 days for the muscles to loosen up again after rigor. Your birds are 1 lb or less, so will take less time.]
So sorry you lost your chicken!
