Just about any poison in a pellet or coated grain form would look very attractive to a chicken. Poisons come in many forms, but things that would kill mice, voles, rats, gophers ect could kill a chicken. Pest control people use different things depending on what species they are trying to kill- insecticides- for termites, ants, ect might harm a chicken if they got exposed to enough- but they can actually tolerate many insecticides fine- if not overdosed. Many chicken people intentionally put poisons/insecticides on their birds (I do) to kill lice or mites (carbaryl, pyrethins ect).
If you still have the bodies- you can look in their crops (home necropsy) & gizzards- and see if there are colored grains present, or poison pellets. Most baits are brightly colored to identify them to people as being poison. If an anti-coagulant was used- there will be blood in places that there should not be blood (like free in the chest or abdomen).
You could send one to your state lab as well if you want actual lab testing.
You definitely need to keep your chickens on your property- fence or coop. It is horrible if your neighbor intentionally harmed them, but people can be pretty insane about their landscaping. People spend $$$ on plants, lawns, gardeners ect and to have a bunch of chickens come over and tear it up (mine are incredibly destructive- and they have been fenced out of our garden and the farmer's fields next door) would make many people very angry. IMO she should have talked to you and given you some sort of warning or ultimatum before doing anything besides chasing them back. Intentionally poisoning someone else's animals is horrible, and I really hope you do not have a neighbor this vindictive- but there are lots of people out there trapping and disposing of cats that dare to poop in their landscaping. I also just read about the people that paid for a trapper to catch up a bunch of neighbor hood ducks that were wrecking their landscaping.
If I had someone else's critters getting into my garden or landscaping I would try to chase them back repeatedly and contact their owners repeatedly. If this did not get results- I might try to catch the animals to take them to the county shelter- but I know it would be next to impossible to catch a free ranging chicken or chickens.
In my area people are being driven crazy over free ranging turkeys coming in and eating and scratching up & pooping on their gardens. People in the cities around here have been known to kill pigeons with grains laced with poison.
I would speak to your neighbor again and explain being sorry for the chickens getting into her stuff, explain you are taking action to confine them, maybe offer to help fix any thing they messed up, then ask again about what was used around her property- if she paid for the service- the bill should have what chemical was used. If it was an anti-coagulant in pellet form, any bird that ate this stuff may die in a day to two weeks. Vit K in high doses is the treatment. Be sure to say you know the birds were where they shouldn't be, and you are not planning to sue her or something, you just want to help your birds if they have been exposed. She does not want to give you the information probably because she if afraid you are looking for proof they ate poison, and will be asking for compensation or something. Frame it right so she knows you are trying to help the birds, not come after her- maybe she will be more forthcoming.
Good luck. Bad blood between neighbors sucks, hope you can figure out what happened and help the remaining birds, and hope fences go up so nothing bad happens again---
