White collar on my duckling?

[Frogchick617]

I have 4 Rouen ducklings about 5 weeks old, first time keeping ducks after hatching. One is starting to quack, and another is developing a white collar. They were not sexed when I bought them. Is the one with the white collar possibly a drake? I also have one with a poof of feathers on it's head. I can post pictures this evening, but was wondering if anyone had thoughts?

 

[HollyDuckFarmer]

Might be.

 

[Oel99]

Could be. But 5 weeks seems a little young to be able to tell the sex by their plumage. Sounds like you have one crested duck.

 

[Frogchick617]

The top photo has all 4the ducks, one with the poof is in the back. Bottom photo is the developing white collar.

 

[ShayBaby]

I'm going to grab the popcorn and watch this one. One of my young mallards has developed a Mohawk of sorts down the back of her head/neck, as well, and I'm curious what it may mean, as I've never seen it before and it's clearly not a crest.
Sorry I can't be of help. I do know that mallards (so presumably Rouens as well) can be "mismarked" and occasionally have bibs and such, so perhaps that's what's up with the ring?
Edit: On second look, it seems your poofed one actually is just crested. I'll have to look for mohawked mallards elsewhere.