Before I begin this post, let me just say that the following comments on Greater vs Lesser is purely from a newbie and humble perspective. I don't claim any expertise with the species and only share what I understand in the hopes it might help others.
There are those days where I simply go out into the field and pick a species to spend some time to study and review what I have learned. On the 31st of March, I ventured out to Jamaica Bay Wildlife Refuge to put in some time looking at Scaup. Just a few weeks ago there were well over 5,000 Scaup on the West Pond and it made sorting through them quite the task. However, as I have learned from being in the company of very experienced birders, like Shai Mitra, Patricia Lindsay, Tom Burke and Gail Benson. One must remain "focused" while tackling large flocks of birds. So, with this huge raft of Scaup, I tried to apply what I have learned (from some of the best) and in doing so chose to go through this exercise by taking small amounts of ducks at a time. It was fun and I took some documentation digiscoping shots to show the comparison of the Greater vs Lesser Scaup.
Quite a few diagnostic features stood out, such as the head being taller in a Lesser Scaup showing the obvious corner at the rear. The body or flanks of the Greater Scaup being much whiter than a Lesser. All of these features are well covered in the Sibley's Field Guide To Birds of Eastern North America. There were some other things I noticed, one item in particular that I need to get additional photographs of Greater and Lesser Scaup to see if there is any consistency, so this might be a post that I circle back to at a later date. Hopefully the photos from this post provides a realistic look at what is presented live in the field and helps in nailing down the ID.
Tags: Jamaica Bay Wildlife Refuge, birding, Greater Scaup, Lesser Scaup