Blogger's note 1/13/09: See comments to this blog entry. I always listen to the opinions of David Ringer. His thoughts and insight into bird identification are thorough, and he always asks the right questions. After reading his comment on this post, I investigated. The comments that struck a chord with me were not about the "nail" (Sibley guide says there is much overlap), but the one about a feeding, threatened or alert Scaup.
From http://www.biology.eku.edu/kos/scaup.htm , I quote: "Images E and F depict how head shape can change with attitude. Lesser Scaup typically compress their head feathers when feeding eliminating the peak to the crown which gives them a totally different appearance. They may also do this when alert or threatened. They can appear very similar to Greater Scaup when assuming this attitude, particularly when showing a green gloss to the head."
Perhaps the Scaup of this post is a Lesser, and the three Scaup of the previous post are Greaters. Or perhaps all are Lesser. I've learned a lot about Scaup through these sightings and follow-up blog entries.
Birders have come down on both sides of the fence on the original post called "The Scaup Scoop". Perhaps I should return to Robbins and Easterla's comment, "the only reliable field mark for distinguishing these two species is the extent of white on the upper side of the primaries."
Well, today I seemed to have run into another rare Greater Scaup in the Ozarks. This individual did not have the striking green head of last Thursday's birds, but the gizz sure seems right to me. Check out the wide head on the bottom pictures. What do you think?