I'll let you borrow my English Springer Spaniel...no more chickens, no more problems.
(Sorry, I couldn't help myself.... :>)

Seriously, CI has the right idea. You owe it to your neighbor to at least talk to him about what's going on. Hopefully, he'll keep his chickens on his property. If not, he must understand it will be unacceptable for this to continue. You have rights and options and hopefully it won't get to that point.

Good Luck