Wxguru - of course the neighbor shouldn't trespass - but sometimes talking to the person is a better avenue than calling the cops immediately. There's really no going back from escalating to the police.
Unless maryhsc has had conversation with the neighbor before, it sounds like they have escalated it already by calling animal control. At the same time, conversations work wonders... As do free eggs.
@maryhsc , what are the zoning laws? Like so many have asked, are you allowed to even have chickens in your area? I'd be interested to know.