Sunday, March 21, 2010

Fun with Chroicocephalus!

Many might wonder, perchance, what in tarnation is/are 'Chroicocephalus.' In the wholesale slicing and dicing of the genus Larus a while back, we got the opportunity to learn a number of new genera, as that admittedly overlumped genus was split asunder. Interestingly, replacing the easily said and remembered Larus for such common species as Bonaparte's and Laughing gulls were some tongue-twisters, Leucophaeus for Laughing and Chroicocephalus for Bonaparte's. So, the dainty Bonaparte's Gull now has a genus name nearly as long as the beast! Well, in 120-point type, anyway.

The subject of this essay, and if you've been paying attention to the blog recently you may already have an inkling, is Bonaparte's Gull and, particularly, its congener, Black-headed Gull. Of course, Brits must be color-blind, because the head of Black-headed Gull in alternate plumage is chocolate, not black, and that's milk chocolate, not dark chocolate! We don't really have an option to change the name to Brown-headed Gull, because that would just make for incredible confusion with the existing Brown-hooded Gull of South America. Even its scientific name doesn't supply any assistance as 'ridibundus' means 'laughing!'

Anyway, a couple of adult Black-headed Gulls have been unreliably reliable along the southern Cape May County bayshore from the Ferry Terminal north to Norbury's Landing, with the two most reliable sites being the mouth of Cox Hall Creek and the end of Miami Avenue in Villas (an area termed Miami Beach), just south of Norbury's. At Miami Beach, they are most reliable near the low tide.

Though these birds are often found with or near the main Bonaparte's flock, they are also quite often independent of that flock. Of late, it has been easy to tell the two Black-headed Gulls apart from each other as one of them has been molting in its alternate head plumage, while the other has remained steadfastly in basic plumage. In fact, the 'prettier' individual is also readily picked out from the hordes of Bonaparte's because its head is so dark, while the smaller species has, for the most part, not initiated pre-alternate molt.

I spent a couple hours yesterday on Miami Beach hoping to photograph the two Black-headed Gulls, and managed half my goal. I got pictures, twice, of the 'pretty' one, but nothing of the basic-plumaged bird. However, those pictures did offer me the opportunity to determine the rate of plumage change exhibited by this individual, as I had photographed it on the 11th at the mouth of Cox Hall Creek. The picture below is a composite of two photos and one can easily see how quickly the bird's head plumage is changing.

[Click on the image to see a (much) larger version.]

Quite serendipitously, while editing last night some of the pictures that I had taken that afternoon, I chanced across the most recent issue of Colorado Birds (January 2010). In that issue, good friends Jerry Liguori and Bill Schmoker authored a paper on the "Photo recovery of a Harlan's Red-tailed Hawk in Colorado and Alaska." The paper provides the details on an individual adult light-morph Harlan's Hawk that winters in Boulder Co., CO (and which has been photographed extensively there by Schmoker and others, including yours truly). The bird was noted on spring migration in southern Alaska, seen and photographed there by Liguori (who some might remember as having conducted the Cape May hawkwatch a while back and who is the author of Hawks From Every Angle). Though the paper is not available online, the two pictures appeared on the cover of that issue (top image from AK, bottom from CO) and that can be seen on the Colorado Field Ornithologists' website. I read the paper (again) and got back to my photo editing.

While editing some pictures of Bonaparte's Gulls, I noted that one bird had an odd black spot in the middle of its lesser primary coverts. My first thought upon seeing that was, "that will make this bird identifiable as an individual should anyone find it elsewhere!" That is because not only was the shape of the spot odd, but the very presence of the spot is odd, as adult Bonaparte's Gulls should completely lack such things.

Once I finished up editing pix from the 20th, I decided to check out my pictures of the Black-headed Gull from the 11th at Cox Hall Creek with the idea of checking the bird's plumage progression in mind. I had put together the composite picture of the bird and was just glancing through some of the Bonaparte's Gull pix in the same folder and THERE WAS THAT BONAPARTE'S GULL!

[Click on the image to see a (much) larger version.]

This composite nicely illustrates the black spot in question, but more focused ogling provides even more bits that help prove that it's the same individual, including the whitish tip of p5 (the fifth primary, or sixth one counting from the outside; primaries are numbered from the inside out), the long black edge to the outermost primary (this feature is quite variable in the species), and the distinctly-paler-but-still-gray sixth greater primary covert.

So, anyone that finds herself/himself looking through adult Bonaparte's Gulls this spring or summer, check the middle lesser primary coverts for a small triangular black spot! And, by all means, get a photo of the bird if you see it!

[Thanks to Bill Schmoker for technical assistance in linking the map to this blog.]

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