Monday, February 23, 2009

Quiz #288 (2009-1-09) Answer

Click the picture for a larger view.

Answer by Tony Leukering

Back to gulls! Aaarrgh!

There are four of 'em this time, and they look to be of fairly similar size and build; perhaps there is only one species present -- that would be nice, eh? All birds are gray-mantled, with most having obvious white tips to the outer primaries, which tells us that they're adults in definitive plumage (or very nearly so; see below). Though the lighting could be throwing off our estimation of eye color, the birds whose eyes we can see have dark irides and that, combined with the plumage and the fairly short and thin bills, greatly reduces the options for us. In fact, there are only two that we ought to seriously consider: Mew and Thayer's gulls (Laughing Gull lacks such strong white tips to the outermost primaries.)

If we had other species for direct comparison of size, we should have no probably separating Mew and Thayer's gulls, but we don't have such, so we'll have to figure out some other way to make that determination. A very suggestive feature is that the birds are riding fairly high in the water with their wingtips and rear ends elevated above the horizontal. This is a feature of the smaller gulls; larger gulls having these parts more nearly parallel to the water's surface. Note that I wrote 'suggestive' and not 'definitive.' That is because Thayer's Gulls are fairly small for large gulls and smaller individuals often show a non-parallel upper surface.

But, there is a feature that is definitive illustrated in this picture: the undersides of the far wingtips. This is a feature that is quite helpful in gull ID and the middle two birds are positioned to show the feature at least moderately well. On both birds, what we can see is a large white terminal half of the visible part of the outermost primary (p10) and a black basal half. Thayer's Gull sports a mostly pale underside to p10, with only the outer web being dark, not the basal parts of both inner and outer webs.

I took this picture of four Mew Gulls at the Everett, WA, sewage ponds 6 February 2005. As noted by a couple of respondents, one or more of the birds might be not-quite adults. The dark bits on the a few of the birds' bills might suggest that (or just be typical markings of many basic-plumaged birds). However, note that the center front bird seems to have larger white primary tips than do the back and right-most birds. Such smaller tips provide a strong suggestion that checking the birds more carefully could tell us that the birds are a bit delayed in obtaining definitive plumage and, for these Mew Gulls, would suggest that any such birds are in their 3rd plumage cycle. Most Mew Gulls should have obtained definitive plumage by their 3rd cycle (and be indistinguishable from older birds), but a fairly substantial minority don't quite get there and obtain their first definitive plumage in their 4th cycle. The left-most bird is at such an angle to preclude any definitive assessment of this feature.

One respondent's correct answer was precluded from being correct for the competition as it was submitted in the form of a plural. Please, all, the rules say that the answer must be the accepted name of the species, and those are not in plural form. For the same reason, one incorrect answer would have been precluded had it been correct.

Tallies of incorrect species provided in answers:
Herring Gull - 3
Thayer's Gull - 1
Laughing Gull - 1
Lesser Black-backed Gull - 1

The 22 of 28 providing the correct answer:
Brandon Percival
Nick Komar
Ann McDonald
Andy Dettling
Aaron Brees
Bryan Guarente
Peter Wilkinson
Mark Dettling
Chris Warren
Peter Burke
Thomas Hall
Mark Gabriel
George Cresswell
Tom Parchman
Robert McNab
David Hill
Tyler Bell
Joe Bens
Joel Such
Marcel Such
Chishun Kwong
Victor Germain

Answer: Mew Gull