Took a run out and about today to see if the strong south winds we've been having for the last 24 hours brought anything new in to the area. The open water that had been packed with waterfowl was relatively quiet and I saw lots of ducks out in the agricultural fields nearby. I did get to watch a small group of scaup on the water and I spent a good deal of effort trying to see if there were any Greaters in the bunch.
When I was in college my ornithology prof (who I greatly admire) taught us that any scaup we see were most likely Lesser because we were inland (Wisconsin) and Greaters frequented coastlines. Now, 30 years later, I know it's not that simple. There are supposed to be field marks that help differentiate the two but I think the best explanation of the differences between Greater and Lesser Scaup are illustrated on this web page from the Kentucky Ornithological Society. Unfortunately it's never this easy in the field.
The only other hint I ever got from someone was on identifying female Greater Scaup. The light, crescent shaped mark on Greater Scaup females is quite prominent while on Lessers it is faint if visible at all. I don't know how reliable this is but it's a mark I look for in conjunction with other clues.
Someday I'll write about the struggles I still have with other groups of birds and believe me, there are plenty.