My mallard has a black and white butt, what does that mean?

[sscad27]

I have two one month old Mallards, one Crested. They are growing their feathers in and I noticed the crested has black and white feathers, in an almost polka dot pattern, below his tail! The other is just brown. Is this just a unique trait, or could it be a determination for sex? I have attached a photo.