new to this duck thing
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For sexing I think you're going to have to wait for plumage or voice.
Males will bicker. They may bite at each other or chase each other around but you shouldn't have a problem until they are breeding age. (Note: breeding age is not when males start to mount females, it’s when the females are producing eggs. Male ducks have a tendency to practice before they can actually do the deed.)
They are plenty old enough to vent sex but if you are not sure how to do that voice sexing is the next best thing. Drakes feather out with hen plumage so you would need to wait for a molt before you will see drake feather patterns. The drake feather curl in the tail is the last thing to show up.
Drakes are easier to vent sex as long as their boy part pops out. The two bumps on the hens are harder to see but you can find them with practice.
Voice sexing is a sure thing for hens while vent sexing is a sure thing for drakes. I have missed a boy part before and later had the duck quack so using a combination of techniques is good once they are old enough to voice sex.
I have voice sexed 2 day old ducklings with 100% accuracy but then you are listening for the pitch of the peeps and that is a bit harder. I am a singer so that helps, lol.
Edited by Duck Drover - 4/13/16 at 4:17pm