Whenever the neighbors cat tries to cross my yard, all 7 ducks will walk over to the property line and keep pace with him, escorting him past our borders. He walks around my yard now. I have no idea if they ever had a fight, but he has no interest in the ducks.
They challenge the other neighbors dog regularly. Its a little Pomeranian yipper, but if she sees the ducks outside she will charge. The ducks will charge right back and meet her at the property line, get all puffed up and flappy. The dog backs down every time and runs for her door.
I've watched them chase off a hawk even. I was looking out the window when it flew in and landed in the tree right above where they were napping. I started for the kitchen door to save them when the sentry duck sounded the alarm. I noticed them all wake up, stand up tall, stare right at the hawk, flap like mad and quack for all they had. The hawk left after about 30 seconds.
When they first moved outside we lost one to an unknown predator before we had our system down completely. I think they had a flock meeting after that and decided to start up their own neighborhood watch. The biggest problem I have now is that they are getting cocky and trying to expand their territory. If they start charging the other neighborhood animals protection money, I hope I get a cut.
Wait... they just started laying, so I guess I do get a cut.