You can vent sex day old ducklings. I have heard within the first hours it is born is the easiest. I personally never tried it until they where walking. But be warned you can easily injure ducklings doing this. You can damage the reproductive systems and break something if you are too rough.. If done correctly no harm will be done. If you have a little boy duckling a little white wee wee will pop out, if a little girl nothing will come out.... but most times for me nothing pops out and it is still a boy! Very frustrating. So I tend to wait until their voices start to mature. When their voices start to come out of the duckling cry the males stay quiet and raspy while the females get deep loud boisterous voices. It can be hard to tell them apart if you wind up with all of the same sex. And the last way to tell is threw feathering. Males will get a "drake feather" which is a curly feather that develops typically in the center on his tail. Females will not get this. Females tend to stay a bland color such as brown and males will tend to get brighter coloring. Just remember the male mallard how he gets the green head and the female stays a all brown. Not all breeds stay to this though. Take the Cayuga for example, both the male and female look the same! Dark black with a green sheen?? Well look for that curly feather for the male and the loud voice of the female.
Safe sexing and good luck with your adventures!
Another picture of the curl on the tail.
A voice video will follow in the next few days of a male vs. a female. Hope this helped someone find out what they have!! Feel free to leave a comment with any questions!