Monday, May 10, 2010

Quiz #348 (2010-2-06) Solution

Click the picture for a larger view.

Answer by Kevin Kerr and Tony Leukering

Kevin Kerr provided a fun and fairly complete answer, so this week's solution starts with his response.

"This week's quiz birds look to be of sandpipers of the genus Tringa. Despite the recent expansion of the genus, the bright yellow legs in combination with the white spots on the backs and wings leave us with few species to consider: Lesser and Greater yellowlegs.

"When the birds are mute, I find the bill to be the most reliable feature to separate these two species. The quiz birds feature relatively long bills (particularly compared to the size of their heads), with thick bases that are fairly pale in colour. They are also slightly upturned. All of this is typical of Greaters, whereas Lessers tend to have shorter, blacker, and more needle-shaped bills.

"Since this is a quiz, it is worthwhile to double-check any would-be overlooked species. Since all of the birds are comparable in size, we need only scrutinize the flock for outlying species that could be a good match in size. Really, Common Greenshank is the only contender, but all four birds show bright yellow to almost-orange legs, eliminating the Greenkshank as a possibility.

"As for aging the birds, I would expect adults in alternate plumage to have darker throats, but the the black feathers dotting the scapulars and tertials suggest to me that these birds, at the very least, are molting into their alternate plumage. Unfortunately, I'm not familiar with the timing of molt in yellowlegs to decide if that is a reasonable conclusion.

"One last scan for grebe heads poking out of the water... nope, none."

Thanks, Kevin.

I found it interesting that multiple respondents used the very same characters to support their contention that the birds were Lesser Yellowlegs, though most providing Lesser Yellowlegs as a species present thought that both species were present. Also interestingly, I specifically took this picture thinking that viewers might get sucked into the optical illusion that the front bird was smaller than the back birds. It was not.

In researching this answer, I was more-than-a-bit disappointed in the various shorebird field guides in their treatment of alternate-plumaged yellowlegs. It's no wonder that folks still have trouble with these two species, as the guides just don't do them justice! While some mention the average differences in underparts pattern -- Greater with more and more extensive marking below, particularly on the belly and flanks -- none mention something that I've noted over the years: Greater's scaps and nearly entire upperparts tend toward gray in spring, while Lesser tends to brown. Both species sport a variable number of black-based alternate feathers in spring, but the retained basic feathers are usually more numerous and their coloration imparts the color differences that I've noted. I also note in the large number of yellowlegs pictures that I perused on Flickr, few photographers take pictures of alternate-plumaged yellowlegs!

To summarize, all birds are nearly the same size and all have bills that are obviously (at least, to me) longer than the head is deep (from bill base to nape). Additionally, the bills have noticeable to extensive pale bases, even the front bird, though due to the glint there, this bird's bill pattern is difficult to assess correctly. However, "embiggening" the picture does show that there is an extension of pale below the cutting edge of the mandible past the glint; Lesser Yellowlegs certainly would not show such. The left bird's underparts are probably too extensively barred to be within the range of variation of Lesser Yellowlegs and the back right bird's legs look quite thick to me -- another feature useful to experienced observers.

I took this picture of four Greater Yellowlegs in North Cape May, Cape May Co., NJ, on 9 April 2010.

Incorrect species provided as answers:
Lesser Yellowlegs - 13
Solitary Sandpiper - 1

Congratulations to the 9 of 23 getting the quiz correct:
Kevin Kerr
Al Guarente
Robert McNab
Richard Jeffers
Chris Warren
Marcel Such
Joel Such
Judi Owens
Claire Mix

Answer: Greater Yellowlegs