Monday, November 9, 2009
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Answer by Tony Leukering
This week's quiz bird is dark above other than some white on the uppertail coverts and buffy-white fringing to most of the wing feathers. We don't really have much help here in determining the bird's underparts color, but embiggening (thanks, Homer) the picture enables me to see a sliver of the bird's right side and that area looks whitish. The wings also look quite long, but the secondaries look fairly short, as they're mostly covered by the greater coverts. One foot looks somewhat pinkish, but the other looks blackish -- lighting?
The two geese provided as answers by respondents would show much more white at the back end of the body and would not show unmarked white sides. The lack of mottling or other internal markings on the wing coverts rules out virtually all of the gull options and the white side rules out the remaining gull option, Heermann's Gull. Jaegers would be another good group to consider, but even Long-tailed Jaeger should show at least some bright white primary shafts among the outer primaries and the shafts of all or most primaries in all three jaeger species are noticeably paler than those on our quiz bird, though they may not be white. With various larids excised from consideration, we really have no good options outside the Procellariiformes, the tubenoses.
"Ugh," one might say, as most of us don't see those things anywhere nearly often enough to be able to nail this bird going away. But, it's not all that difficult to work through the field guides to the correct answer. I mean, only about 50 species have occurred in the ABA area! And if that's not enough to make one cheer with abandon, we can do some wholescale deletions from that list. First off, we can remove the storm-petrels from consideration -- they don't have the wing length or the plumage pattern exhibited by our quiz bird. That's 13 down. Most albatrosses don't show our bird's topside plumage pattern and all of 'em have even longer wings than does our quiz bird; another eight down. Of course, now that we're into the Procellariidae (petrels and shearwaters), we're into a fairly difficult group, and one that accounts for the remaining 29 ABA-area tubenose species!
Even now, though, we can conduct large-scale removals, and we'll start with all of those species that are not both evenly-dark above (there's little suggstion of the typical Procellariid 'M' on our bird's wings, but it could still be present if it were not particularly contrasty, given the angle of the photo) and with white sides. If we also use our bird's thin white uppertail-covert band as a decision character, then we're left with very few possibilities. In fact, there are so few options that we have actually arrived at our answer: Greater shearwater. At this point, one might opine that it would be nice to be able to use some positive characters to ID our bird, rather than simple process of elimination. Ah, but we have. Our bird's thin white uppertail-covert band is a particularly good field mark for the species. Black-capped Petrel sports a much wider white band proximal from the tail and neither Cory's nor Cape Verde shearwaters are so dark above, nor do they have white sides. Northern Fulmars with white bands proximal to the tail are paler above with obvious pale wing panels.
I provide another view of the same bird, below, an individual that I photographed off southern New Jersey on 21 October 2009.
Incorrect species provided as answers:
Wilson's Storm-Petrel - 1
Black-footed Albatross - 2
Parasitic Jaeger - 1
Black-tailed Gull - 1
Brant - 1
Canada Goose - 1
The 13 of 20 providing the correct answer:
Answer: Greater Shearwater