Juvenile Surfbird (Aphriza virgata) on Bodega Head, 18 August 2012
To identify it as a juvenile, note the very neat feathering, the upper feathers with distinct white tips and dark bars just inside the white tips, and the fine gray and white mottling on the head and neck.
Adults differ in either having a more uniform gray appearance in non-breeding plumage — see photo from 25 February 2012 below.
Or in having a strong overall mottled appearance in breeding plumage, with rufous feathers above and heart-shaped markings below — see next photo taken on 8 April 2011.
Surfbirds breed in the mountains of Alaska and Yukon. They are now migrating south, with peak numbers in California in mid-August. They'll spend the winter anywhere from Alaska to Chile.
The genus, Aphriza, has an interesting origin. According to American Bird Names, "aphros" means "sea foam" and "zoa" means "to live." So one possible interpretation is "to live near sea foam," an apt description for this sandpiper during most of the year.