Mystery Gull (Possible Slaty-backed Gull)
Obsidian Butte, Salton Sea
Imperial County, California
2 February 1999
Joseph Morlan

The following is based on notes made while watching the bird, mostly through my KOWA TSN4 spotting scope:

When sitting, the bird appeared unexpectedly small; only slightly larger than nearby California Gulls and about the same size as the smallest Herring Gulls. The shape was similar to Herring Gull, although the base of the neck may have been a bit thicker. In flight, the bird appeared larger and bulkier than it did when perched. It was much easier for me to pick it out in flight than when it was sitting or sleeping with other gulls on the rocks. I did not see the bird swim.

The mantle was fairly dark, but not as black as I expected. It depended on the light, but I thought it was about as dark as the northern race of Western Gull (L. o. occidentalis). I would score it around a four on the Munsel Scale shown here on Steve Hampton's web site. The tips of the secondaries and the inner primaries were white, forming a somewhat contrasting trailing edge, but it did not seem as broad or as striking as on photos of adult Slaty-backed. The white scapular crescent was somewhat more conspicuous than the secondary crescent. No pale tips or mirrors were visible on the outer primaries. In strong light a slight suffusion of brownish was visible on the wing coverts.

In flight the underwings were white with brown mottling on the primary coverts. The underside of the primaries were blackish on the outer web and paler on the inner. I expected the undersides of the primaries to be much paler if this gull were to be a Slaty-backed. The undersides of the inner primaries showed a somewhat paler coloration than the outer primaries. The axillars and the underwing secondary coverts were white. The leading edge of the inner wing (marginal coverts) appeared black in flight and were the darkest part of the mantle.

The bill was dull yellowish at the base, with a broad black band across the distal portion of the bill, separating the pale base, from the pale hook at the tip. This black band formed two crescents on each mandible and the black extended inward along the cutting edge of both mandibles almost all the way to the gape. The overall bill-shape was similar to that of Herring Gull, with the culmen gradually curved downward from about midpoint instead of more abruptly arched as it is on California Gull. The gonydeal angle was rather shallow, similar to Herring Gull. A peculiarity of this individual was an apparent small bump or growth at about midway out the culmen on the top of the bill. It is possible this was merely a piece of debris on the bill as other observers have not noted it.

The head and body were white, with fine streaking on the crown and face merging into gray-brown smudging on the nape and the sides of the breast extending all across the lower breast to the insertion of the legs. The chest and lower belly were white. The undertail coverts were spotted with brown.

In strong light, the eye appeared pale, with a yellowish iris, not as white as on most Herring Gulls, but certainly not dark. A dark crescent accented the pale eye on the anterior half of the orbital. This gave the bird the appearance of a burning stare, particularly when it looked forward directly at me. A narrow "crease" was evident in the feathers behind the eye, extending upward slightly, but I did not notice any concentration of dark streaking in this area which would be expected in Slaty-backed.

The legs were relatively long with strong ankle joints. The legs and feet were a chalky whitish color, suffused with pale pink in most light, but occasionally showing a hint of pale yellowish color. In my experience, pale pink and pale yellow are often interpreted differently by different observers. My students, when viewing slides often disagree with me on which name to apply to these colors and I don't think that too much can be made of individual descriptions without reference to a color standard.

The tail was mostly dark, with a very narrow white band at the tip and a ragged white area at the base. One or two rectrices on the mid right side appeared to be all white, but I only noticed this when the bird was in flight.

Discussion

I do not currently know what species of bird this is.

If it is not a hybrid, it might well be a Slaty-backed Gull, although it shows a number of puzzling characters suggesting it might be something else. I would expect a real Slaty-backed to have a broader white trailing edge to the wing, more white spotting in the outer primaries in second or third winter plumage, paler underwing tips, and a more obviously slaty-black back. The apparent small size could be explained by it being a female. Male Slaty-backs are evidently quite massive and powerful looking.

I am not completely sure why this bird is not the Siberian race of the Herring Gull, the Vega Gull (L. a. vegae) which is similar to Herring, but with a darker back. That might well explain the dark underwing tips and the pale wedge on the underside of the inner primaries. I suspect that Vega Gull is not quite as dark on the mantle as the mystery bird but I am not sure of the extent of variation. The closely related L. a. taimyrensis is darker, but supposedly has more obviously yellow legs.

Another possibility is a Lesser Black-backed Gull, but I would expect the legs to be more obviously yellow and the primary pattern to be somewhat different.

Another candidate which could easily explain most if not all the discrepancies would be Lesser Black-backed x Herring gull hybrid.

I think the mantle is definitely too pale for Kelp Gull, but the odd leg color might be explained by Kelp x Herring gull hybrid.

Without more information I would have considerable difficulty accepting this as a Slaty-backed at this time. I believe we need input from people with more experience with Slaty-backed and various hybrid combinations to help resolve this identification.

Further details and photos of this bird are now available at Angus Wilson's web site and at Gary Rosenberg's site.