2 different breed ducks. Are my babies male or female?

[Paws2bpampered]

Hello I have 4 ducklings out of my Khaki Campbell hen and Rouen drake.

The 4 babies look identical to dad but im hearing male and female noises out of the group. How do I know which ones are male and female?

 

[sourland]

Drakes may have a more olive colored bill like their father.