Monday, July 30, 2012
The picture is not of an ABA Code 5 species.
Solution by Tony Leukering
What's a day when one doesn't learn something? I like to avoid such days and, thanks to Peter Wilkinson, I have avoided that pitfall today.
This week's quiz bird is obviously a small gull (plumage and small, dark bill) and the caveat with the quiz photo precluded Gray-hooded Gull from being the solution. Once amongst the small gulls, we should check wingtip pattern to greatly reduce the number of options. In this view, we cannot see much of anything of the wingtip pattern, but we can see enough. The enough of which I write is the underside of the outermost primary (p10) on both wings and on both wings, those feathers are nearly entirely white. That means that this cannot be other than one of the species with white wedges in the wingtip. (As an aside, studying the underside of the wingtips in most gulls can be quite useful in arriving at an ID and one can usually manage such on a perched or swimming gull.)
Differentiating Black-headed and Bonaparte's gulls can be a mite tricky at times, but this view is not one in which it should be, and I note that no respondent went with Black-headed, though some missed the obvious separator. Bill color is often useful in discerning between the two species, dark reddish in the former, black in the latter. However, this color can be tricky to assess in some pictures and I would certainly be hesitant to rule out Black-headed on the strength of bill color on this picture! Throw in the difficulty in assessing mantle color (Black-headed's is paler than is that of Bonaparte's) and I would have been left with a single -- though excellent -- differentiating character. Our bird's nape is obviously gray; Black-headed sports a white nape (and crown) contrasting with the gray mantle.
This is where that new knowledge bit comes in. Peter Wilkinson pointed out that there is another clue, in the shape of the meeting of black tip and white body of p10 in the two species. Bonaparte's shows a smoothly curved meeting extending from the inner margin of the feather down toward the tip and to the outer margin of the feather (as on the quiz bird; see inset, below). With Peter's suggestion, I looked into the feature a bit and found that the meeting in Black-headed is rather straighter, cutting across the feather or, even, slightly back up the feather, such that where the white meets black on the inner margin is at the same distance from the tip -- or closer -- than where it meets on the outer margin. As Peter noted, I doubt that the character would be useful in the field, except under the most beneficial circumstances, but it seems to be readily assessable in photographs given an appropriate view.
I took this picture of an adult Bonaparte's Gull in North Cape May, Cape May Co., NJ, on 11 March 2010.
One respondent's answer was precluded from being correct for the competition as it neglected the "e" in the name of the species and another's was precluded as the "g" went uncapitalized.
Incorrect species provided as answers:
Ross's Gull - 1
Black-legged Kittiwake - 1
Congratulations to the 18 of 20 getting the quiz correct:
William von Herff
Answer: Bonaparte's Gull