Lesser Black-backed and Glaucous notes and photos
Here's an eBird checklist with (bad) photos and field notes on the Clifton Court Forebay LESSER BLACK-BACKED GULL, plus some slightly better photos of the GLAUCOUS GULL: https://ebird.org/view/checklist/S50482307
Note that there may have been two Glaucous, and at least one of them didn't look particularly big; it was about Herring size at most. However, it's very white and the bill is too long and sharply bicolored for it to be an Iceland; it should be easy to pick out.
Western Gulls are rare in this part of Contra Costa Co., so the Lesser Black-backed may be the only dark-backed gull on the lake and should also be pretty easy to pick out if it returns. If any Westerns do show up, they should be easily distinguished from the Lesser by their larger size (the Lesser looked no bigger than a California Gull), bulkier structure, and larger bills.
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