You should use every field mark at your disposal when you identify a bird. When identifying scaup, you must use every field mark, and use them carefully. Here's how to identify these Lesser Scaup.
Head shape: The word rounded is too imprecise when dealing with scaup head shape. The question is, where is the peak of the head, and what happens behind that? On Lesser Scaup, the peak is rear of the eye, and often the head looks flat-topped, and often there is a pointy peak at the rear of the crown. The left bird, sleeping, looks perfect for Lesser Scaup. The right bird's head shape is different - and you have to learn to be okay with that, the scaup change their head shape depending on what they're doing - but ask yourself, where is the peak of the head? Answer is a little behind the eye, and the top of the head is flat. Greater Scaup often gives a look where there is a peak in the height of the head over or even in front of the eye, and then the top of the head slopes downward towards the rear from there.
Size: If you were tempted to call these two different species because of the different head shapes (and the species do mix freely), consider this: they're the same size. Side by side, Greater looks 10-20% bigger than Lesser, because it is.
Bill: the bill of the right bird looks small enough for a Lesser.
Back: the barring on the back of Lesser is coarser, and they're also more prone to having dark vermiculations on the flanks, looking less clean white than Greater.
Where: These two were in the center pool of the South Cape May Meadows, which is relatively shallow and a relatively small body of water. This leads one to think Lesser Scaup, which on average like smaller water. This is an imperfect field "mark" to be sure, since both can be on big or small water, but I watched these birds dive here. They were happy on the small water, comfortable. Greaters might wind up on small water, but like it less.
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