*I* would ask your neighbor to keep their birds home and/or consider improving your own barrier fence line (if you have one) at the property line. Bio-security concerns come into play here especially with water fowl. While these may be domestics being kept by the neighbor, the may be coming into contact with wild water fowl via any pond they may have, etc and can easily carry pathogens to your birds. At the moment, AI is a primary concern and has been linked most strongly to wild water fowl.
I guess I'll add that our neighbors are our landlords so technically our property is theirs, I've never actually seen the ducks (only heard about them) until about a week ago when they showed up. Are they attracted to the feed or something else
Domestic ducks are extremely sociable and crave interaction with other birds, and I assume Mallards behave similarly. The food probably attracts them too though.
For a while we only had one duck, who deeply fears people and strange things. One time he followed one of the chickens into the house though, because he wanted company so much. It was both hilarious and sad.
Are the neighbors ducks, drakes or hens? I've heard that lonely drakes will attempt to breed chicken hens. Duck anatomy is much different than that of a chicken. Breeding a chicken hen could damage her severely or kill her. With that in mind, I would NOT want his ducks around at all if they are males.
The ducks are hens, it really doesn't bother me really, like I said they are our landlords and they are very kind to us I don't want to start problems over a duck they were nice enough to let us have chickens in the first place