I was watching my welsh harlequin ducklings (4 days old) yesterday. I can already tell their sexes by their bill color. However, I noticed that the females were peeping loud and clear and the male's peeps were raspy and a bit quieter. I think this might actually be a way to sex them by their voice too when they are only ducklings! I mean it held true for the welsh harlequins. Anyone else want to try it out?