Slaty-backed Gull (Larus schistisagus)
Half Moon Bay, San Mateo County, California
13 January 2005
Joseph Morlan

Photo © 13 January 2005 by Joseph Morlan. All rights reserved.
About 3:30 pm, we received a call from Al Jaramillo that he had a probable 2nd cycle Slaty-backed Gull at Venice State Beach in Half Moon Bay. Robbie Fischer and I made a few quick telephone calls and then drove down in hopes of seeing the bird. We arrived about 4pm to find Al Jaramillo there and he soon refound the bird near the mouth of the confluence of Frenchman's Creek and Pilarcitos Creek. The bird was not particularly cooperative and was mostly hidden behind other gulls (mostly Western and Glaucous-winged). It spent much time sitting with its belly on the ground. I attempted to digiscope the bird with the result you see to the left.

By about 4:30pm the gulls all flushed towards the south, but we were unable to find the bird again. I also looked for the bird the next morning, but was unsuccessful.

Description

It was a large gull, about the size of a Western Gull. The head and breast were mostly white, but there was strong dark streaking around the eye which accented the pale yellow iris. There was also distinctive blackish streaking on the crown extending back down the nape. The belly was darker than the breast and shaded dusky. The mantle had a mixture of pale gray feathers with frosty-white fringes and blackish feathers. The wing coverts were brown with some mottling on the median coverts, but very little mottling on the rather plain greater coverts. The primary tips were blackish with white fringing. The bill was quite long looking, appearing black, but showing some pale on the basal two thirds when viewed closely through a scope. The legs and feet were a dull, dark pink with grayish shading.

The neck had a thick look to it, compared to adjacent Western and Glaucous-winged Gulls. The bird seemed pot-bellied and relatively short winged.

My views were not completely satisfying. Much better photos along with more thorough analysis as been posted by Al Jaramillo here.

Discussion

There are currently no accepted records of Slaty-backed Gull for California, although there are at least four other plausible claims which have been documented by photographs as follows:

  1. A second cycle bird photographed at the Ventura Marina VEN Feb 1995. Photos here. CBRC decision here.
  2. An adult at Folsom Lake PLA Jan 1998. Photos here. (Still under CBRC review).
  3. A near adult photographed at the Salton Sea RIV Feb-March 1998. Photos here and here. CBRC decision here. (page 189)
  4. A first cycle bird photographed at Davis YOL Nov 1998. Photos here. CBRC decision here (page 25)

The reasons for the lack of acceptance of the above four published birds varies, but mostly center on the difficulty of eliminating all possible hybrid combinations. The California Bird Records Committee tends to be very conservative when evaluating potential first state records. A particular problem with Slaty-backed Gull is that they may hybridize with Glaucous-winged Gull in Asia. Such hybrids might be difficult to separate with certainty from pure Slaty-backed Gulls. In particular, some of the type specimens appear to be rather pale on the mantle and do not resemble what most birders now regard as pure Slaty-backed. Yet those birds must be regarded as Slaty-backed because they are types. However if those pale-mantled types are really hybrids, nomenclatural and taxonomic revision may be needed. Further details on such pale Slaty-backed Gulls, along with photos of the specimens in question can be found at:

Gustafson, M. E., and B. G. Peterjohn. 1994. Adult Slaty-backed Gulls: variability in mantle color and comments identification. Birding 26: 243-249.

In addition, a 2nd-3rd cycle bird photographed at the Salton Sea Dec-Mar 1998-1999 was originally thought to be a Slaty-backed. It was eventually submitted as a Lesser Black-backed Gull, but was not accepted as that or any other species. Details of my observations of that bird are here. CBRC publication here (page 23). Other unpublished, but possibly correct, unsubmitted reports include:

  1. A sighting of an adult at Crescent City DN 18 Jan 1999
  2. A 2nd cycle bird photographed at Half Moon Bay SM winter 2000-2001
  3. A sighting of a possible adult at Half Moon Bay SM 5 Jan 2003 (hybrid with Glaucous-winged not ruled out)

My only experience with Slaty-backed Gull is of good numbers well seen and studied at St. Paul Island, Alaska in June 1999. However, those birds were mostly adults and did not present much identification challenge. Thus I do not feel completely qualified to render judgment on the bird I saw and photographed here, although I think Slaty-backed Gull is a reasonable explanation for the combination of characters shown.