||How do you tell the Scaup species apart? One mark that isn't reliable is the iridiscent sheen of the male's head -- usually green on the Greater (remember G) and purple on the Lesser. Notice that this Lesser looks green on the lower part of his cheek, and purple on his crown. A diagnostic mark, if you get a close look, is the width of the nail on the bill. The Lesser's, visible here, is about half the width of what it would be in a Greater. A more difficult mark is the shape of the head -- the highest point of the crown is behind the eye in the Lesser, ahead of it in the Greater. This is good for both sexes and can work at a distance. But apparent head shape is highly dependent on angle of view, and how the bird is holding its feathers -- as these pictures show. There's also a bill shape difference; see the good discussion in Kenn Kaufman, Advanced Birding.