The mystery of the 6 new ducklings

I haven't been up to the boat yard for a while so I could have missed them, but today was the first time I spotted these six youngsters.

Photography by AnnMackieMiller: copyright 2011
Ducklings

They look to be about 2-3 weeks old and they are larger than your usual mallard ducklings.  When I saw them there weren't any adults near them.  Obviously they are all from one brood.  One is a 'splotched' Blue Swedish Duck, 3 are white - must have been yellow - and 1 is a very dark brown with mallard-like markings, but much much darker.  When I came back up the canal they have gone onto dry land at the boat yard and both TweddleDum and TweedleDee the Aylesbury ducks were with them as was 1 of the Blue Swedish Ducks.  The adult spoltchy Swedish Duck was up at Greenhill.  #
Now here is the question _ where did they come from?  Are they crossed from Aylesbury Ducks and Blue Swedish Ducks? But I thought all of those were male.
Or - did a female mallard produce them?  I certainly saw a several female mallards being harrassed and mated by both the big white ducks and by the black Swedish duck.  We haven't seen the other black Swedish duck around for weeks.  Has it been sitting on eggs?  I could only see one black adult today so I can't tell for sure.  Needs investigating though.  I don't know why I find different colourings so fascinating on ducks and geese, I just do, and I can't wait to see what they turn out to be in adulthood.  These youngsters have to be about 3 weeks old, as they are mostly white now, the yellow they would have been when they hatched is almost completely gone. 

"let's see who can hold their breath the longest"