I think Mrs. K has a very good handle on the finer points.
We had a female Dalmation that got into similar problem eating rats at the neighbors hog farm. Neighbor gave us a heads up and dog was treated by vet promptly.
That is an uh-oh. Did you cut open intestines length-wise to look at lumen? I hope you find bleeding there. If not, then you may have a bigger problem.
I would check anterior and posterior intestine as well. Gizzard may not grind glass properly. The intestines might show extensive bleeding from internal bleeding caused by rodenticide.
What does the rodenticide look like? You might be able to see chunks of it in the crop. Or find mice...
Eating a bunch of glass would be the only type of hardware disease that I think would hit multiple birds at once with relatively rapid death. Otherwise you are still on track for a poison.
Have you considered doing autopsy to look for extensive internal bleeding?
Potential valuable lesson for grand kids. Show them and explain. Have them help you find potential poison deposits the chickens are eating.
Does rodenticide label indicate emergency treatment if consumed by nontarget animal?
Another possibility. Anyone breaking light bulbs where chickens can get at shards? I have lost a couple to what appeared to be hardware disease. They passed on roost with a lot of food in crop and digestive tract overall.