Sexing Mouskovy ducklings

Our ducklings are approximately 6 weeks old now. According to some info on the net if duckling get their permanent feathers between week 4 and 6 they are most certainly females. Our duckling got their first permanent feathers at week 4 and as you can see in this new picture their wings are almost entirely covered with feathers :) Since I would like to increase this flock for the next year I am very much looking forward to get 3 more females. 

Apparently males start getting permanent feathers later but they start forming the red skin around their face sooner than the females.