5 months ago, I bought two ducklings from a Tractor Supply Store, one black duckling and one Pekin duckling.

I've been finding eggs in their pin recently (though I'm not sure if BOTH of them are laying or just one).

Neither of them has a drake feather, but the black one mounts the Pekin a lot as if it were trying to breed. I've candled the eggs to see if there were any embryos inside, but found nothing.

The black duck exhibits male breeding behavior, but honks like a female and doesn't have a drake feather.

I'm really confused, how can I know for sure what their genders are?