I'm so sorry for your loss! I understand how devastating something like this can be. It does sound like poisoning. I would certainly consider the neighbor possibility. We have a dreadful neighbor who poisoned our beloved German Shepherd and her puppies. It's unbelievable that people could be so cruel, but they can indeed!
It sounds like you have been given some great advice & information to help you try to get to the bottom of things! At the time I know you are sad & bewildered but it's also the most critical time to take action to ensure this does not recur.
Have you had any conflicts with your neighbors? Has someone complained about your chickens or have they wandered into anyone else's yard? It may be well worth the small cost to have your feed checked out and possibly get an autopsy on one of the birds. If someone did do this, they should be punished for it. It's a terrible thing, but it would be even worse if it happened again.
Until you know what happened, please do not use the same feed for your remaining hen & chicks. I wouldn't store it anywhere that anybody else can gain access to. I'd take a good sized sample of the water and the food and then dispose of it all carefully. It wouldn't hurt to check all ventilation sections and doors for any evidence of tampering and take a good walk around the pen. You may find a footprint or cigarette butt, etc that could help figure out if this was indeed a deliberate, malicious act.
If there's other neighbors with poultry, you might check with them to see if they have experienced any problems or losses. I'd go further to suggest adding a concealed game camera placed high enough to prevent the birds from setting it off.
Sincerest condolences and please update us with your findings!
God Bless!
I'm sorry for your losses. If mama hen hasn't been getting off the nest much she may not have ingested as much toxin if that's the case or have been up and moving to be exposed to other agents.
