Outpost may I ask?
Did you spray a weed killer in your garden?
Did you spray an Insect killer in your garden?
These things all can weaken the chickens systems if not mame or kill them.
To be sure, please take at least one of the dead ones to your county ag person for testing. You never know. The fact is your ground may be contaminated and you will not be able to have chickens for years to come in some cases of disease. Have their water checked for bacteria too. When we just dont know what it is, sometimes it is best to go to the county extension office. Ours county ag office is in direct contact with VIrginia Tech, so they can help. Good luck, I hope you find what is causing this destruction of your birds soon. Here is hoping everything works out.
Edit, I had a problem with predators and brought that under control. On to my garden,...had sprayed weed killer and thought it was japanese bug killer. Then sprayed bug killer after I killed my garden. Then I had a problem with predators again. If the weed killer did not take them down then the bug killer did not get them, some were snatched by predators. Right under our noses! Bold hawk!
No use of chemicals for weed or insect killers here. Thanks for the advice on checking the water and ground as well. Contacting the extension office is a good idea.