Slaty-backed Gull
20 November 1998
Davis Oxidation Ponds
Joseph Morlan

Much more information including larger photos, an accurate description and sketches, and a comparison with an apparent Glaucous-winged x Herring hybrid has been posted by Steve Hampten at: http://www.geocities.com/RainForest/Canopy/6181/schi8.htm. Two thumbnails are reproduced here by permission of Steve Hampton from full sized images at his site. Photo © 1998 by Marcel Holyoak

I arranged to be at the Davis Oxidation ponds at 1pm where I met Steve Hampton who had a key to the gate at the southwest corner of the ponds. Almost as soon as we entered, Steve spotted a likely candidate sitting in the water. We walked along the west levee and viewed the bird across the pond. I watched it through my scope as it flew around and landed on the levee, but I was often able to pick it out with binoculars.

My initial reaction was that the bird looked like a 1st year Glaucous-winged x Western hybrid. It was noticeably larger than the Herring Gulls, and huge compared to the many California Gulls present on the levee and in the ponds. However, unlike a typical hybrid, its bill lacked a prominent gonydeal angle and it had a somewhat paler head and breast than most such hybrids.

The following is based on memory and on brief notes taken at the time. I focused mainly on how this bird seemed to differ from possible hybrids:

The bird seemed to have a thicker neck which was broader at the base, giving it a more powerful aspect. The plumpness of the body imparted a "goose-like" appearance at times.

The head shape seemed quite odd to me, with the crown broad and flat and the large eyes located rather high on the side of the face. The crown had peculiar longitudinal streaks, but the area around the lores and bill was unmarked white. A dark smudge appeared under each eye and extending behind the eye, where it flared upward slightly, contrasting with the paler face and head.

At first the bill appeared all black, but at close range, in good light, some dull pinkish was visible at the base. The bill-shape was unique, rather thick looking with the culmen arching from about the midpoint to the tip, but there was no noticeable gonydeal angle.

The hind-neck and back were pale brown.but the wing coverts were speckled except for the greater coverts which were plain. Actually I think there was some faint speckling on the innermost of the greater coverts.

The wingtips were a shade or two darker than the rest of the bird, but later in the afternoon, depending on the angle, the wing-tips looked darker. At close range, narrow pale fringing could be seen on the primary tips. The tertials were sold brown with some pale fringing.

Leg color seemed to vary with the light. At first it seemed dark pink, but later in the afternoon they looked paler pink.

In flight the tail appeared all dark-brown, but not blackish. Most striking was the pattern of the underwing which had a dark trailing edge caused by dark primary tips but only the outer two primaries were dark (p10 and p9). The secondaries were slightly darker than the rest of the wing, although at some angles they looked slightly paler. Thus the bird looked somewhat like an oversized Thayer's Gull.

The bird had the odd habit of breaking twigs off of a small woody shrub and lying down on them. I joked that the bird was nesting.

Discussion

Joan Humphrey arrived leading Yolo field trip for the Central Valley Birding Symposium. Jon Dunn and Arnold Small arrived with Tim Manolis. At this point the bird sat down behind a bush and seemed to doze off.

Eventually the gull started to preen and fly about and Jon said that he could find nothing wrong with the bird for first-year Slaty-backed, but that he has never identified a bird in that plumage in North America before. He said that in Alaska, they normally identify only 2nd-year and older which have obvious slaty-black on the mantle. Jon remained appropriately noncommittal.

Steve Hampton felt the bird was a dead-ringer for a swimming first winter Slaty-backed filmed in Japan on Jon Dunn's "Advanced Birding - Large Gulls" video tape.

On Saturday we looked for it again, late in the afternoon, but the weather did not cooperate and we were unable to find the bird. However, there were a lot of 1st year Herring Gulls flying around and some of them seemed to have a pale Thayer's-like underwing especially when backlit and some of them were decidedly pale on the face. They differed from the putative Slaty-backed in their rounder head, speckled greater coverts, darker wing-tips, and more extensive dark primaries. Jon Dunn and others looked for the bird again on Sunday afternoon without success.

I think the bird has been documented quite well but await opinions from those with more knowledge and field experience with this plumage of Slaty-backed. From what I have seen thus far, I am uncomfortable passing it off as any kind of hybrid and am leaning toward the theory that it actually is a Slaty-backed.

There are no accepted records of Slaty-backed Gull in California, but two prior records are currently under review by the California Bird Records Committee; one near adult at the Salton Sea and a second year bird at Ventura. Images of both are at the CBRC photo gallery at http://www.californiabirds.org/photos/ .

For comparison, here is a 1st spring Slaty-backed Gull photographed by Peter LaTourrette in Siberia, 27 May 1998 which I think compares favorably with the Davis bird. Note particularly the thick stubby bill, large eye and broad flat head. Photo © 1998 by Peter LaTourrette	Here is another 1st spring Slaty-backed Gull photographed in Honshu, Japan in February 1998. This is a somewhat paler individual than the Davis bird with more heavily marked, paler greater coverts. Click on the image to view full size. Photo © 1998 by Arnold Small