It isn't a regulation. A hunter must be able identify which birds he himself bagged. If they leave your possession before processing they have to be identified as to how many, what species and sexes, and the hunter's identification.
^That is paraphrasing from memory. Go to the usfws for the transportation requirements. (Also, still in the us, you need the head and/or one fully feathered wing left attached.)
Where guys run into trouble is when they can't tell which individual bird they bagged. Pile pic, hanging on a sign, thrown together in the boat or piled up at camp, etc. Green jeans rolls up he is going to want to know who killed what. If Bill's birds are there but Bill isn't, green jeans is going to assume you are over the limit (if more ducks present than an individual specie, sex and/or daily limit.)