Quick call the RSPCA!
My advice would be (if you already done so) is kindly ask your neighbour to secure them, and give the reasons why, and then make note of it.
If they were to fly/enter in your garden and your dog kills them/injure them, your neighbour has the right to bring you to a small claims court, however this very rarely happens (especially over chickens, unless they are prize stock, but then the right owner would have better knowledge not to make them wonder) This would also cost your neighbour money, and lots of it too. So the chances are very slim.
If your dog entere