This rarity was found yesterday by Teresa Ely and Ben Dudek at the end of the point. It was hidden behind a rock when we arrived, but patience was eventually rewarded. This bird is in formative plumage. The wing coverts and tertials are juvenile showing conspicuous white fringes. Otherwise this bird resembles a basic plumage adult. This plumage is tricky to distinguish from Purple Sandpiper (C. maritima), an accidental visitor to California. Note rusty fringes to a small section of upper scapulars in the top photo. I believe these are retained juvenile feathers which are rusty on Rock Sandpiper, but not on Purple. Rock Sandpiper has four subspecies. The nominate Pribilof race is larger and paler in all plumages. The others are distinguishable only in breeding plumage, but the Northern Bearing Sea race C. p. tschuktschorum is the only one known to migrate regularly to the West Coast. This appears to be the first record for the city of San Francisco although there are several SF county records from Southeast Farallon Island.
Reference:
Chandler, R. (2009). Shorebirds of North America, Europe, and Asia: A photographic guide. Princeton Univ. Press.