Hi again,
sorry for being stubborn, but after more research I still believe it's a Greater Scaup. According to the identification guides I have seen the most reliable field mark for swimming birds is the shape and relative size of the head and bill.
Greater Scaup has a rounded head with the highest point in front of the middle of the head. It has also a relatively larger head and a larger bill with a larger black tip compared to the Lesser Scaup.
Lesser Scaup has a relatively smaller head with the peak of the head behind the middle of the head (in normal, relaxed state, so 100 percent conclusive). The black tip of the bill is narrow and small.
A bold white ear patch would indicate Greater Scaup (specially visible in spring and summer), while a bird without it could be either species.
The plumage seems to vary greatly between different individuals.
As your bird clearly has a big, rounded head with the top towards the front and large bill with extended black tip I would say that it is a Greater Scaup.
Here are some pictures as examples:
http://www.pbase.com/slisch/image/82148724 has similar body and bill pattern (with a wide black tip) as the bird in your picture.
The female Greater Scaup at
http://www.avianweb.com/scaup.html clearly doesn't have any white ear patch
Note that the female Lesser Scaup at
http://galleries.xenogere.com/gallery/5 ... JbJL7-A-LB has a smaller head with the peak behind the middle of the head and only has a narrow black tip of the bill.
Note also picture J in my link above
http://www.biology.eku.edu/kos/scaup.htm that the only visible difference between the Greater and Lesser Scaup females is the size and shape of the head.
Regards,
Thomas
