Posted by Dave Irons (205.188.197.172) on December 18, 2001 at 07:47:45:
The top two birds are Greater Scaup based on bill size and head shape. On Greater Scaup forecrown is always the highest point on the head. These two birds clearly show what I consider a typical Greater head shape. The white crescent behind the eye on the female bird also points to Greater. The bills on both of these birds look to long for a Lesser Scaup.
The bottom bird is a Lesser Scaup, even though the hindcrown is rounded and doesn't show the 'squared off' look typical of a Lesser. On this bird the highest point of the head is not the forecrown it is farther back. The bill is shorter and slighter than a Greater.
One secondary characteristic I use when the head shape is not obvious is the angle change between the slope of the forehead and the slope of the bill. Lessers have a steep forehead angle and the upper ridge of the bill goes out from the head at on a rather horizontal line.
On Greaters the line of the upper ridge of the bill and the slope of the forehead show a much flatter angle. In pure mathematical terms this angle on a Lesser is closer to 90 degrees and the angle on a Greater is closer to 180 degrees. I hope this makes sense. Another minor characteristic is eye location on the face. In Greaters the eye is very high on the face and on Lessers the eye is lower on the face. These birds show characteristics that match the species I think they are.