Monday, April 11, 2011
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Answer by Margaret Smith and Tony Leukering
Wow, if shorebirds scare respondents off, it's quite obvious that gulls do an even better job at that. While last week's Monday response total was only nine (compared to 25 the week before), this week it took until Wednesday to get the ninth response and with only 11 responses by Sunday!
Margaret Smith provided a pretty good explanation of her tack to the correct answer, so this week's solution will start with her words.
"My initial jizz ID was "gull," based on long, thin wings and a white tail with a dark end band. The dark band practically forces the gull to be a juvenile. This bird has a relatively uniform-width tail band, unlike Herring Gull or Ring-billed Gull, which have a fatter dark part in the tail center and thinner on the edges. This bird has dark outer halves on the rectrices. Only two species are shown so in Sibley: Laughing Gull and (Common) Mew Gull. Laughing Gull has a broader dark band than does that form of Mew Gull, very much like the quiz bird’s. The quiz bird also has dark feet, good for Laughing Gull and not for Mew Gull. The other dark-legged gulls, Black-legged Kittiwake, Heermann’s Gull, and Franklin’s Gull, look very different from the quiz bird as juveniles. Heermann’s is too dark, Franklin’s has white outer rectrices, and Black-legged Kittiwake has a concave tail tip, not a convex one. However, what’s with the white tip only on the center rectrices? The gull should have white tips on all the dark tail feathers."
Margaret is right, first-cycle Laughing Gulls should have white tips to all of the rectrices, not just the central two. However, there is an easy explanation for that, but it requires more-careful scrutiny of the individual feathers and it is based partly in how worn those rectrices are. They're trashed. Big chunks are missing from every feather, probably a result of some stress factor (illness and poor or insufficient diet being the leading possiblities) when they were being grown. That stress caused fault bars, which then makes the feathers susceptible to breaking at those bars. However, the two central feathers did not escape that fate, they have simply been replaced with new juvenile-like feathers with nice, clean, unfrazzled white tips.
For more explanation, Heermann's Gull was probably provided as an answer because the color of the feathers anterior to the black tail band were construed as gray, rather than white. If that were true, Heermann's would be the only possible candidate. However, note that the pale color, as Margaret implied, extends onto the inner webs of the rectrices, which it never does in Heermann's Gull. Except for thin white tips, that species sports solidly black rects in all plumages. The apparent grayness of the rump and base of the tail is due to the lighting, which is identical to that on the trailing edge of the wing, which is of essentially the same color. And Heermann's Gull never shows a gray trailing edge.
Lesser Black-backed Gull can be remarkably difficult to rule out, given this view, and if we cannot trust the foot color. However, ageing the bird will greatly help us in this regard. The quiz bird appears to have some gray mantle feathers. If that is true, then the bird would have to be in its second plumage cycle if a LessBack. Given that, our bird's very pointed outermost primaries prove the case against LessBack, as second-cycle gulls have wider, more-rounded outer primaries. First-cycle LessBacks have dark barring at the bases of the outer rectrices, which this bird lacks, ruling out that age of the species.
A final couple notes on foot structure and wing formulae, since those topics have come up. Our quiz bird's toes on its left foot are easily discernible: one on each side and one down the middle of the webbing. The totipalms -- or birds with totipalmate feet (all four toes facing forward and all connected by webbing) -- formerly all resided in the order Pelecaniformes, and all members of that order were totipalmate. This odd mishmash of pelicans, cormorants, tropicbirds, and others has been broken up into many different orders recently and the Sulidae -- the family of gannets and boobies -- has come out of that massacre being the type family of the new order Suliformes.
"Wing formula" (plural formulae) is a representation of the relative lengths of the primaries of a bird's wing. This feature is used by many birders -- though many don't know the term -- to identify some birds, like Lesser Nighthawk and Mississippi Kite; both species have their outermost primary relatively shorter than those of similar species. We also use the feature in identifying gulls and golden-plovers, with this feature being important on the folded wing. Most bird species have the outermost primary shorter than, at least, the next outermost. Waterbirds, particularly long-distance-migrant waterbirds and those that spend a lot of time foraging on the wing (such as gulls), buck that trend and have the outermost primary the longest on the wing. Despite the aerial nature of even Peregrine Falcon, there are no raptor species that have the outermost primary being the longest. Our quiz bird has 10 primaries and the longest is p10, the outermost.
I took this picture of a first-cycle Laughing Gull in Villas, Cape May Co., NJ, on 17 August 2010.
Incorrect species provided as answers:
Heermann's Gull - 2
Northern Gannet - 1
Rough-legged Hawk - 1
Lesser Black-backed Gull - 1
Congratulations to the 15 of 20 getting the quiz correct:
Answer: Laughing Gull