The Western Gull, Larus occidentalis, is a large white-headed gull that lives on the western coast of North America. It was previously considered conspecific, the same species, with the Yellow-footed Gull (Larus livens) of the Gulf of California. The Western Gull ranges from British Columbia, Canada to Baja California, Mexico, and because of its convenient colonies on the coast of California it is well studied. Despite being a well-known bird species on the West Coast of the US, it is of some slight conservation concern given its restricted range (for a gull).
San Mateo Coastline
This gull appears to be carrying nesting material.