Monday, February 7, 2011
Click on picture(s) for a larger view.
Answer by Peter Wilkinson and Tony Leukering
As on last week's quiz, one of the regular respondents provided a good rationale for his ID, so we'll start with Peter Wilkinson's words.
"Aha! We have a dark-backed, large white-headed gull (LWHG) with yellow legs. The yellow legs rule out Great Black-backed, Slaty-backed, and the darker Western (wymani) gulls, as all have legs of varying shades of pink. The LWHGs with yellow legs, or at least yellowish legs, on the ABA list are Yellow-footed, Lesser Black-backed, and Kelp gulls, and, perhaps arguably, Yellow-legged Gull, which just about gets to the paler end of Lesser Black-backed but not really as dark as this. This is clearly a heavy-set bird, with a large and deep bill; Lesser Black-backs, though they match the colour of the back (apart from graellsii, which is paler than this), simply don't match the build. Kelp Gull has greenish rather than bright yellow legs.
So, we are left with Yellow-footed, which appears to fill the bill. It is difficult to be sure from the photograph, but the color of the orbital ring appears also to support an ID of Yellow-footed. LWHGs are, of course, a bit of a minefield, where even experts sometimes have to agree to differ, but there does not seem to be any reason not to believe that this individual is identifiable, and no reason to start thinking of hybrid origin."
What Peter didn't specify is the color of the orbital ring on our quiz bird. As this can be a crucial ID feature, it is incumbent upon us to get this right, particularly as it can be a tricky feature to assess correctly in the field, to say nothing of a digital photograph. At least, though, the bird isn't moving its head around making it even more difficult. The picture below is extracted from the quiz picture and shows the bird's yellow orbital ring -- the same color as the bill, which is different from the red of Great Black-backed, the orange of Kelp, and the orange-yellow of western.
The very heavy bill, orbital-ring color, and the obvious skirt (for a refresher on skirts, see Mr. Bill Mystery Quiz #335 one year ago), among other features, rule out Lesser Black-backed Gull.
Of course, had you been standing with me when I took this picture at Puerto Peñasco, Sonora, Mexico, on 21 February 2006, you would have been able to be pretty sure of the ID of Yellow-footed Gull just on the location!
Incorrect species provided as answers:
Lesser Black-backed Gull - 6
Kelp Gull - 1
Congratulations to the 24 of 31 getting the quiz correct:
Answer: Yellow-footed Gull