Can a drake mallard sense if my duckling is a hen?

[FowlLanguage]

I have a BIG mallard named Bill (pun) who has gotten more, well... 'Aggressive' with my chickens since I put my little ones out in the coop. The birds can see each other, but not interact. Duckling was born mid-April and I do not know the gender yet.

Also, at what age would a female mallard be able to have lil' ones? I am pretty sure that it is sooner than it would lay infertile eggs.

 

[CoopCrazy]

I would think he would know before you would.. Im sure he can smell or hear something in her that would let him know