Simply dissappearing birds would put OP in the wrong. Notifying authorities is the way to go. Document presence of birds and damage they cause. Also document chicken owner's investment in containment. Also do a quick search on your state and county statutes as almost certainly details you are looking for are present. Their will be loopholes that can be closed by your documentation. I did similar a couple of years back where trespassers was a dog (did not go after stock but caused other issues that had to be documented). Some documentation realized by simply calling authorities. I would also indicate to chicken owner you are going the legal process as that will close another potential legal loophole where he / she did not now you had an issue.