It's hard to say from looks on ducks so young. Do the quack test that I told you about. If the one is quacking, it's definitely a girl. If the other is still peeping, is it a hoarse sounding peep? Like it is loosing it's voice? If so it's most likely a male. I didn't notice the color changes much on my rouens to know when the drakes get their colors, but the quack test always works.
