I find that by 3 weeks I can start to hear happy girl noises (but you have to know what to listen for) when they eat. Unfortunately, I can never tell which duckling is doing the happy chortle. 
At 4 - 5 weeks I can start to hear some real "quacks" in the pen and can start to tell who. Sometimes at that age you can pick them up and the girls will quack. Mostly they still all peep though. At 6 weeks, its pretty easy to hear an indignant "quack!" when you pick up a girl. Have a couple of pens handy and sort quacks into one and peeps or raspy voices into the other. You can't be 100% sure on all of them, but if you hear a definite "quack", you definitely have a girl. At 8 weeks on, you start to get adult coloring in the boys (green heads in the breeds that get them, for instance) and the curly boy tail feather. After your first batch, you'll know well before then who's a girl and who isn't though. By 12 weeks it should be abundantly clear who is who.
They don't normally breed until next spring, though some might be precocious and breed later this fall. It can be hard to sell the older ducklings, depending on your area. I'm planning on trying to vent sex the ones I have in the incubator so I can sell some day olds. I have tons of eggs in there and only room for a few female ducklings!
At 4 - 5 weeks I can start to hear some real "quacks" in the pen and can start to tell who. Sometimes at that age you can pick them up and the girls will quack. Mostly they still all peep though. At 6 weeks, its pretty easy to hear an indignant "quack!" when you pick up a girl. Have a couple of pens handy and sort quacks into one and peeps or raspy voices into the other. You can't be 100% sure on all of them, but if you hear a definite "quack", you definitely have a girl. At 8 weeks on, you start to get adult coloring in the boys (green heads in the breeds that get them, for instance) and the curly boy tail feather. After your first batch, you'll know well before then who's a girl and who isn't though. By 12 weeks it should be abundantly clear who is who.They don't normally breed until next spring, though some might be precocious and breed later this fall. It can be hard to sell the older ducklings, depending on your area. I'm planning on trying to vent sex the ones I have in the incubator so I can sell some day olds. I have tons of eggs in there and only room for a few female ducklings!
