Hello.
@JB, I had an experience like this when I bred rabbits, what I did YOU can do also. I am not a felon, but I have some experience with the law.
You said, "My SIL waited until dark and he got a ladder and went up in the rafters of the barn and one at a time collected the chickens and took them back home to their hen house." I assume this means he went on your property at night without permission? If this is correct, you DO have a Legal Leg to stand on.
Here is what happened to me, I had a rabbit while I lived in Post Falls, ID. A man came by and broke into the rabbit hutch, to take care of the rabbit because, "I was abusing it by not feeding it and not cleaning it." This was NOT true. I called Post Falls Police for advice, and when I told them the story of what happened, They said that man was guilty of breaking and entering, they sent an officer to our house and we explained the situation. He went to the mans house and firmly told him he was NOT to come on my property anymore.
My advice is to do what I did. Call your towns Police Department and explain the WHOLE situation. Explain what is going on and explain to them that he is going on your property without permission at night. Explain the whole story.
If he does go on your property without permission, tell them this, and more than likely they can talk to him about breaking and entering and they will tell him NEVER to do it again. If he cant control his chickens going onto your property, Animal Control might get involved with him also.
Send me a tree mail if you want to talk to me further.
Jared