So i have five Mallard ducklings and they are all developing brown and black feathers on their wings, tail, and stomach. Not sure if it's too soon to tell or not but judging by the pigment of the feathers they look like females so far.
All mallards start off looking female and then the males molt into nuptial colors later. The earliest way you're going to be able to tell their genders will be from their voices. Females will have a loud and braying quack while males will be low and raspy.
Haven't found him yet and might have to wait until I'm home to get out my old camera card, but here, this is what I mean:
See the colors of the juvenile male? Just like a female. Males only have green heads and collars after molting into nuptial (breeding) plumage. They in fact don't have them most of the year while they're in eclipse plumage even as adults.
Agree with Pyxis... just an added FYI, juveniles can sometimes look like they have a green 'sheen' or shine to the feathers on their heads before their adult molt... that sheen/shine is NOT and indicator of gender... my Crash had it as a juvie and she is a 2 year old DEFINITE female...