Monday, May 25, 2009
Click the picture for a larger view.
Answer by Bryan Guarente and Tony Leukering
Bryan Guarente supplied an answer that covered the bases on this quiz, so I'll continue with his answer after some minor candidate-reduction efforts of my own.
Our quiz bird is swimming and sleeping and appears virtually all dark. So dark, in fact, that we might as well call it blackish (at least). The number of virtually all-blackish swimming birds on the ABA-area list is quite small and we can probably consider only the scoters, cormorants, and guillemots. Cormorants can be eliminated by our bird's large round head and short tail. Even though we might suspect that the white on alternate-plumaged guillemots could be hidden on our bird, the tail is too long and the head too rounded for that genus. Take it away, Bryan.
"So, I have been deliberating over this one for a while (Black versus White-winged Scoter), thinking over and over that this bird is a Black Scoter. That ID comes from the bird's overall shape (more circular than elliptical), its rounded head, and the extension of the primaries off of the back. All of these somewhat subjective characteristics lead me to Black Scoter.
"Now for that darn whiteness on the wing. I have no idea what it is and cannot account for it, unless I say this bird is a White-winged Scoter. However, all the other characteristics point to Black Scoter. If this bird were a White-winged Scoter, I think you would be able to see some white around the eye, which might just barely be visible over the bird's back. So that just puts me in the Black Scoter camp.
"How did I rule out Surf Scoter? Well, the head is too round to be that of a Surf Scoter, and in all ages of Surf Scoter, there should be white [or whitish] somewhere on the head, and most of those places are visible on the bird in this image."
Thanks, Bryan. Despite the seeming incongruous small white bit of white in the wing, he is correct as to our quiz bird's ID. Head shape is a great separator of Black vs. the other two, but the complete lack of any paler color on the head is the clinching factor -- Bryan is correct about the placement of the bird's eye and the occurrence of white on the head of adult males of the other two scoters. Additionally, females of any scoter species would have some paler color on the face, so those options can be ruled out. For those that ID'ed the bird solely on that little bit of white in the wing, remember -- one field mark does not an ID make, as in that situation, one would not be able to separate Red-eyed Vireo from Black-crowned Night-Heron.
Finally, I don't know what that apparent white bit is, but suspect that it's either an artifact of sharpening the original image or a bit of misplaced down.
One correct answer was received 7.5 hours after the deadline.
Tallies of incorrect species provided in answers:
White-winged Scoter - 9
Gadwall - 1
Hooded Merganser - 1
The 9 of 20 providing the correct answer:
Answer: Black Scoter