A rarity in North America, California has 32 previous Black-headed Gull records. Immatures and adults in winter plumage lack the dark brown hood. They are distinguished from the similar Bonaparte's Gull by their larger size, thicker orange based bill, and darker underwings as seen here. Note also the dark "comma" on the outer primary coverts, unique to this species. This first cycle bird was found by David Yee on 04 December where it has been associating with hundreds of Bonaparte's Gulls at the water treatment plant.
Canon SX70 HS.