Tuesday, December 2, 2008

Murky shades of 'Slaty'.

Slaty-backed Gull - sub-adult (Kodak 9.5 - 11.5) with adult Great Black-backed Gull (Kodak 13 - 15), Windsor Landfill, CT. November 28th. The scoring system follows Howell & Dunn in 'Gulls of the Americas'.

This fine sub-adult Slaty-backed Gull, found by Nick Bonomo at the Windsor Landfill, CT on November 28th, has come under plenty of scrutiny since the images first appeared on the North American Stop Press page on Surfbirds.

Most of the debate has centered around the color of the upperparts and whether or not the mantle tone is dark enough for a 'pure' Slaty-backed Gull. Gary Rosenberg opened up the debate on the Identification Frontiers forum, resulting in an extremely interesting thread which you can follow by clicking here. I must confess that I came away from the Windsor Landfill with the feeling that I'd seen a pretty normal Slaty-backed Gull, ie. a large dark-mantled gull showing all the criteria that one might expect for a solid 'Slaty' claim by one of Connecticut's sharpest birders.

So what's going on? The big question seems to be orientated around the judgment and appearance of gray tones in digital images,
taken by a range of photographers using different photographic equipment and viewed on computer screens. This comes as a big surprise, because I'd assumed that the 'dangers' of trying to accurately assess gray tones both in the field and from digital imagery had been widely accepted for some time. Very unfortunately, the assessment of mantle color by the field birders on the ground seems to carry little weight in the argument and questions over the apparent 'paleness' of this bird continue.

The Kodak Gray Scale has become the standard scoring system for the very difficult job of attempt to assess the back color of gulls. Just focusing (for the moment) on the scores used in the most recent publication on gull identification "Gulls of the Americas" by Steve Howell and Jon Dunn, I thought it would be a useful reminder to compare back (mantle and scapulars) colors with several of the Slaty-backed Gulls that I've seen over the last two years to see how they measure against more familiar species present in the same images.

Howell and Dunn offer the following rough guide;
Kodak 3 - 6 is pale gray.
Kodak 6 - 8 is medium gray.
Kodak 8 - 10 pale slaty gray.
Kodak 10 - 12 slaty gray.
Kodak 12 - 14 dark slaty gray.
Kodak 14 - 17 dark slaty black.

They state that "most taxa are fairly consistent in the gray shade of their upperparts" and only vary by 1 to 1.5 Kodak values. They offer several useful tips such as "bluer hues tend to make grays look lighter and browner hues can create darker tones", and advice on the best conditions for 'gulling' and back color assessment; "high overcast or partly cloudy skies". So with these factors in mind, here's an image selection drawing comparisons between familiar and rarer species and the Kodak Gray Scale scores referenced in Howell & Dunn 2007.

Lesser Black-backed Gull L.f. graellsii (Kodak 9 - 11) adult at rear with adult Ring-billed Gulls (Kodak 4 - 5), Amherst, MA. October. My personal assessment of the back color of the CT Slaty-backed was about the same as an average graellsii Lesser Black-backed Gull. Some birders present at the scene thought it was a tad lighter, but not by much. NB. A score of Kodak 9 would be an extreme light graellsii.

Lesser Black-backed Gull L.f. graellsii (Kodak 9 - 11) adult/sub-adult with American Herring Gulls (Kodak 4 - 5),

Slaty-backed Gull - sub-adult, Nome, AK. June 26th, 2008. (Kodak 9.5 - 11.5).

Slaty-backed Gull - second/third-cycle, St. Paul Island, AK. June 28th, 2008. (Kodak 9.5 - 11.5)

Slaty-backed Gull - third cycle (Kodak 9.5 - 11.5) with adult Mew Gulls (Kodak 6 - 7.5), Nome, AK. June, 2008.

Vega Gull - second/third cycle (Kodak 7 - 8), Nome, AK. June 2008. Back color rather darker than American Herring Gull (Kodak 4 - 5).

Slaty-backed Gull - adult (Kodak 9.5 - 11.5), Rochester WWTP, NH. January 10th, 2007. Sub-adult Great Black-backed Gull ((Kodak 13 - 15) at rear, surrounded by adult American Herring Gulls (Kodak 4 - 5). Taken on a bright sunny mid-winter morning.

Slaty-backed Gull - adult winter (Kodak 9.5 - 11.5), Rochester WWTP, NH, with American Herring Gulls (Kodak 4 - 5). Same as above, January, 2007. Compare with the Massachusetts SBGU below, one year later in January 2008.

Slaty-backed Gull - adult (center). In this shot appears almost as dark the two swimming adult Great Black-backed Gulls. t Using Kodak scoring system this Slaty-backed should represent the extreme dark end(K 11.5) of the range of variation to appear so close in tone to these adult Great Black-backed Gulls (extreme light would be K13). January 2008. Compare with the New Hampshire SBGU above. This SBGU was photographed on an overcast day.

Slaty-backed Gulls - worn adult (Kodak 9.5 - 11.5) lower left in both shots, with Glaucous Gulls presumed barrovianus (Kodak 4 - 5), and third cycle Slaty-backed Gull (top left in both shots). Nome, AK, June 2007.

Kamchatka Gull - adult/sub-adult (Kodak 6.5 - 8) with Ring-billed Gulls (Kodak 4 -5). Rhode Island, January 2006.

In summary, I remain happy with my field assessment of the back color of the CT Slaty-backed, placing it as being extremely close to an average graellsii Lesser Black-backed Gull. This would give the CT bird a score of around K 9.5 or 10, perhaps light for some SBGU but certainly not enough to cast doubt over the identification based on a single feature. Looking at my images from Massachusetts last winter, and some of those coming from Newfoundland, it's also rather clear that some variation in plumage types (light and dark) is evident in the vagrants turning up in the North-east. At the moment, variation in SBGU seems to be poorly understood with question marks being placed against extremely light birds as well as extremely dark birds, but birds which otherwise shows a perfect suite of characteristics for our current understanding of SBGU. The variation needs to be explained and I'm hoping to follow up on this fascinating story when more clear information comes to light.

Happy gulling,



Nick Bonomo said...

Nice James, very instructive set of images and information.

- Nick

Angus Wilson said...

The different impressions of mantle color are certainly thought provoking, especially when comparing images of the same bird! One quick question: The Kodak scores you've listed in the legend to each example, are these the range for each species quoted in H&D or you rough estimate for the bird in question?