Slaty-backed Gull (Larus schistisagus)
Fort Baker, San Francisco County CA
7 February 2012
Joseph Morlan
Photo © 7 February 2012 by Joseph Morlan. All rights reserved (High resolution).
After hearing reports that Todd Easterla had found two Slaty-backed Gulls at Fort
Baker in the Marin Headlands, Robbie Fischer and I decided to try for these birds as well as the continuing Iceland
Gull. Upon arriving around 4:00pm, we learned that the adult Slaty-backed had flown off sometime earlier. While
discussing an probable immature Slaty-backed, Todd showed me photos of the adult Slaty-backed he had seen earlier.
I was surprised by how little head streaking his bird showed.
Later we checked the beach directly below the Coast Guard pier looking for the immature Iceland Gull which had
flown in that general direction. I scanned through the mass of gulls foraging on Herring roe with my scope and
came across what I thought at first was an odd adult Western Gull. I was struck mostly by the pale base to the
bill, the narrow bill shape and the white eye. I thought this might be the adult Slaty-backed Gull seen earlier
by Todd, but lacked confidence in the identification. I asked Dan Singer to look in my scope. Honestly I didn't
quite believe that the assembled group of gull afficianados had not seen the bird as it was quite close. Dan quickly
identified it as the adult Slaty-backed and we got everybody on the bird quickly as it foraged on the beach. I
attempted photos, but the fading light made photography difficult. The image to the right is the best I could get.
Description
The following description is based on memory and on photos:
A large gull, about the size of a Western Gull but perhaps slightly smaller.
The bird was mostly white with a dark gray mantle, slightly darker than nearby Western Gulls and suffused
slightly with brownish on the wing coverts. The nape was streaked with brown and very faint brown streaking was
evident on the back of the head. The eye was larger than that of Western Gulls and the bird had a pale yellow iris.
The wing tips were black with white apical spots, noticeably larger than the spots on nearby Western Gulls. Also
the tertial and scapular crescents were broader than on most Western Gulls. The underside of the wing tips were
white with faint gray markings. The tail was entirely white. The legs and feet were pink. Overall the bird lacked
the pot-bellied appearance that I associate with Slaty-backed Gull and that lack was one reason I was afraid to
identify it with confidence when I first noticed it. However the base of the neck was thickset, typical of the
Slaty-backed shape. Todd and Dan have photos of the bird in flight which show the "string of pearls"
pattern, although I thought the pattern was not particularly evident in the field when the bird flew.
Discussion
I have more confidence in the identification of this bird than I do of the immature. For the most part this
individual seems reasonably typical except for the lack of heavy streaking on the head and around the eye, and
the lack of the pot-bellied shape. However, gulls are notoriously variable so assume that these features are variable.
This location straddles the county line between Marin and San Francisco counties. Apparently San Francisco claims
the water at mean high tide and the land is in Marin County. However, the beach this bird was on was build after
the county lines were drawn. According to Mark Eaton who researched this question, the bird was entirely in San
Francisco County the entire time.
Outside Links
Additional photos have been posted as follows: