If you want to be sure or just learn how to vent sex, it's really not that hard. I watched a video by Metzers. Then when I mistakenly got a 5th duckling with my delivery a couple of weeks ago, I could tell it was female.
I did not know they had a video. Only that they say males have dark bill and females have light with dark tip. Was not sure about the middle one with a strip running down bill. I will go back to Metzer and look for video.
Thank you for info. Linda