Found by Todd Easterla, this adult has very little head markings, but shows the slim bill with pale base and larger pale eye typical of Slaty-backed. The broad white tertial and secondary crescents and large white apical spots on the primaries are also good characters for Slaty-backed as is the pale underwing tip visible in this photo. In direct comparison with Western Gulls, the mantle appeared slightly darker. This individual seems to lack the pot-bellied appearance typical of Slaty-backed, but flight shots revealed a "string-of-pears" extending out to P8 (I think).
This location is on the county line between Marin and San Francisco.
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Digiscoped with Panasonic DMC-LZ5 | Nikon FieldScope III | 30XWA | hand-held (no adapter)