Monday, June 14, 2010
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No ABA-area rarity is present in this picture.
Answer by Tony Leukering
I put this week's quiz picture up to talk about a specific field mark, primary coverts, that provide a great clue in separating a couple of similar species. The best plans of mice and men -- I wasn't counting on most of the incorrect answers being for an altogether different species. And now, I have to revamp the answer that I had written in my head.
All respondents got to the smaller gulls, probably due to the strong pattern of black and white on the upperparts, with the dark coloration in a vague 'W' pattern ('twould have been an 'M' pattern if the bird were heading away from us). With the red-phosphor caveat provided with the quiz picture, we don't have to worry about things like Gray-headed Gull, leaving us with just Bonaparte's and Black-headed gulls and Black-legged Kittiwake.
The field mark I had intended discussing, and still shall, is the color of the greater primary coverts. Though all birds have greater primary coverts (and, by extension, lesser primary coverts), there aren't that many groups for which the lessers are particularly visible in the field, so we tend to shorten things to simply primary coverts (covs). But, gulls are one of those groups that sport obvious lesser primary covs, hence the need for the modifier, 'greater.' This tract of feathers can be quite useful in separating immatures of Bonaparte's and Black-headed gulls, which can otherwise be quite tricky. Bonaparte's Gull sports a mix of black and white in those feathers, with the black much the more abundant on the outers and white more noticeable on the inners. By contrast, Black-headed immatures show nearly all-white greater primary covs.
While a number of respondents selected Black-legged Kittiwake as their answer, juveniles and immatures of that species have the entire leading edge of the hand (the wing from the wrist to tip) black or blackish, rather than the mix of black and white so obvious on our quiz bird. Particularly, note that the quiz bird's lesser primary covs are nearly entirely white, which would be a radical departure for a kittiwake. Also note that our bird's alula and outermost primary (p10) are also a mix of black and white, two feathers that are all black on young Black-legged Kittiwakes.
I photographed this immature Bonaparte's Gull at Villas, Cape May Co., NJ, on 22 April 2010. At this date, even immature Black-headed Gulls ought to be showing at least a hint of the adult bill color.
Incorrect species provided as answers:
Black-legged Kittiwake - 4
Black-headed Gull - 1
Congratulations to the 11 of 16 getting the quiz correct:
Answer: Bonaparte's Gull