If it is a federal/state flyway or hunting area then it is protected
against low flights... Better check with the DNR before going low...
Usually 1500 feet or more AGL is required...
If it is not a protected game area under federal or state regs then
basic open country FAA rules apply - 500 feet from buildings, people,
etc... It doesn't have to be 500 vertically, just 500 feet away...
If the ducks are present, then think about how you will handle taking
one through the windscreen, before screaming across the fields..
A bit of thinking it through ahead of time will save a lot of hassle
later...

denny