On 3/1, I had an adult gull in the large roost on A12 in Alviso showing characteristics suggestive of Slaty-backed Gull. It has been endorsed as such (with varying degrees of confidence) by most folks whom I consulted, although others have proposed Vega, Vega x Taimyr, and Slaty-backed x Vega. The principal concern is that structurally it did not appear to me quite as robust as most Slaties are. Further details and photos are on eBird.
Four of Santa Clara's five prior records of Slaty-backed Gull come from gull roosts at this location, including most recently one that Dessi Sieburth, Cedrik von Briel, and I had in March last year. Santa Clara has one record of Vega Gull (S. Rottenborn in 2014, not from Alviso).
Mike Rogers, Noah Eckman, Adam Burnett and I had a first-cycle Glaucous Gull on A12 yesterday afternoon (3/7), as well as several Glaucous hybrids and an arriving Caspian Tern.
Also on 3/1 at Alviso I noted 6 Eurasian Wigeons on A14 with about 10,000 other ducks. (The same number was noted on the CBC this year, although I had at least two new individuals for a minimum of 8 in total this season.) Most of these ducks can be scoped pretty easily from the A13/A14 levee in morning (or overcast) light. It seems highly probable that other rare waterfowl are there for the finding: e.g., there have been ~9 Tufted Ducks elsewhere in the Bay Area this winter and the setup at A14 is ideal for this species.
Birding Alviso in February and March is truly one of the best ways to maximize your chances of finding substantial rarities in the Bay Area at this season and Santa Clara birders should take full advantage! Ongoing levee construction does not impede access from Alviso Marina County Park either to the massive network of gull roosts on A12, A13, and A15, or to the incredible concentrations of ducks on A14.
Do not disclose information that identifies the location of nesting birds of any species, to minimize stress on the nesting birds and the risk of vandalism or abuse.
