I think that you need to send a politely phrased note back to them stating that there IS a leash law, requiring that their dogs be on-leash and under the owner's control (quoting the law would be good). That your bird range on your own property at unpredictable times when you can watch them...
You need to inform the nice neighbors that you want your birds to be able to roam in your own yard without being in danger from loose dogs. That you understand that a dog may accidentally escape from a vigilent owner, but that having dogs running loose on a regular basis is simply not...