just the way that the feathers are arranged. You wrote "The
blue shade is due to the heavy blue cast as it was overcast when this shot was
taken". But the front of the wing, from the shoulder
to the wrist, is pale blue in these birds. Looking at a picture of the bird in
flight, in a field guide shows that. What you have there as blue is correct and
not because of any lighting effect.
I don't see any white patch. The pale patch
that I think you are asking about, is the blue front of the wing. It is only
confusing, if at all, because the brown feathers of the flank ("body feathering") have come up over the wing, and thus
break up (cover) the front outline of the wing. If those brown feathers of the
flank were flush with the body or still under the wing, (as would be the case if
the bird had recently opened its wing), the blue front of the wing would be
quite obvious. When I look at the swimming picture in Pizzey's book that shows
the brown feathers of the flank completely covering the front of the wing.
At the Jerrabomberra Wetlands today there was one female
shoveler and two males (so, Geoffrey, it was a case of a pair and a spare)
The males spent a lot of time jousting as in the images attached. I am
intrigued by the “white” patch on the front duck. (The blue shade is due
to the heavy blue cast as it was overcast when this shot was taken) It is
not an artefact because it shows in the reflection as well, and you can detect a
feather pattern. Question is, is the patch of white feathers part of a
wing or body feathering?