It is true with hen mallards and black ducks but they are very close to the same species, different species are fairly easy for me though...spent more time in the marsh than I did in school growing up lol.
I do remember early on in my hunting days, in fact, it was my labs first hunt, I was 15 and I limited out on Widgeon in like 15 minutes and I had never seen them before. Dog retrieved all the ducks and went back in after the decoys. (fixed that pretty quick though)
I had to check out with the game warden at the check station and wasn't sure if I was going to be written up or not lol.