The other option, if you have the stomach for it, is to de-frost the bird and open it up for a do-it-youself autopsy. You can post pictures here of anything odd you find, and maybe people here can help you. At a minimum, you can cut open the intestines to check for worms, and look a the condition of the liver.
My gut instinct is that it is some type of toxin -- maybe a bad batch of feed or something they found in the yard. It's not impossible that it is something like worms, but worms usually don't kill that fast, with no warning. Still, it's something to consider.
Sorry you are going through this, and sending positive thoughts that your remaining birds stay healthy.