Tuesday, December 2, 2008

Quiz #275 (2008-4-09) Answer

Answer by Tony Leukering and Tyler Bell

The number of respondents this week was well down from typical, so I imagine that some number of folks were scared off by the gull wing. I don't blame 'em; this quiz was somewhat difficult. Tyler Bell provided an answer that dealt with many of the critical features and I'll use it below. However, I'm going to do things a bit differently this time and intersperse his comments (always in quotes) with mine, though some of my more editorial bits are enclosed in brackets within his quote.Thanks, Tyler.

"Lariphobia! So, what can we see with yet another partial bird?" The bird has a "white tail, so it is an adult, but the [outer] primaries aren't fully emerged,yet, which could trick one into thinking that it's something else."

The secondaries contrast darkly with the white wing lining, so our bird must be reasonably dark-mantled, say at Least Lesser Black-backed in coloration; paler-mantled gulls would show much paler secondaries that would not contrast like those of our quiz bird. Also note that the white tips to the secondaries occupy well less than the distal half of those feathers, a feature that can be critical in identifying darker-mantled gulls.

"The critical primary, p10 [the outermost; which is only half-grown] has extensive white on the distal portion, with only a small black spot near the tip."

Most white-headed gull species show a white tip to p10 that is separated from the p10 mirror by a black strap. All that exists of our bird's p10 strap is that single small black spot. As Tyler noted, this is a critical feature.

"The bird's p9 has a good deal of white on it as well."

That is, p9 has a white tip and a wide, black strap, but also has a sizable mirror (the vaguely leaning-snowman-shaped white patch distal to the tip of p10). Looking back at the secondaries, note that the white trailing edge is broken near the base of the wing -- that is probably a result of one or two of the innermost secondaries not being full-grown, yet. That will not come into play
in identifying the bird, but it does fit well with the fact that the outermost primaries are still growing -- in most birds, those are the last flight feathers to complete in the pre-basic molt. I point it out, though, because the tertials typically grow in before the innermost secondaries so that when Tyler notes that...

"... there's also a thin tertial crescent..."

... we can be sure that those inner white-tipped remiges are the tertials because of that very gap. The width of the tertical crescent, which is made up of the white tips to the longest tertials, is a feature useful in identifying white-headed gulls, with individual species having either thin or wide tertial crescents and all species with wide tertial crescents also having wide white tips to the secondaries and thin-crescented species having thin white tips to the secondaries.

"Hopefully, these details add up to Great Black-backed Gull!"

And, of course, Tyler is correct. The dark-mantled Pacific-Rim gull species (Slaty-backed, Western, and Yellow-footed) all have wide tertial crescents, wide white tips to the secondaries that occupy at least half of the length of the visible parts of the secondaries, and typically sport a complete strap on p10. Lesser Black-backed Gull has an even thinner tertial crescent than does Great Black-backed Gull, and an even smaller p10 mirror, which means a wider p10 strap. It also has a smaller p9 mirror than does our quiz bird. California Gull has more extensive black on each of the outer four primaries such that nearly the whole feather is black, unlike on our mystery bird, which has black on only the distal fifth or so of each p8 and p7; this creates the much larger black wingtip so typical of that species.

I have included the whole picture of the bird (below), which I took on 22 October 2008 at Cape May Point, Cape May Co., NJ. In the picture we can see an additional character useful in identifying the bird: its dull flesh-colored feet that no other adult large white-headed gull has.

One answer was received on Monday morning, after the deadline.

Tallies of incorrect answers for quiz species:
Ring-billed Gull - 1
Osprey - 1
Western Gull - 1
Mew Gull - 1
Lesser Black-backed Gull - 4
California Gull - 1
Herring Gull - 4

The 8 of 21 respondents answering correctly:

Tyler Bell
Ann McDonald
Christian Nunes
Peter Wilkinson
Matthew Schneider
Aaron Brees
Andrew Spencer
Nick Komar

Answer: Great Black-backed Gull