Heermann's Gull, Seaside, Oregon, 6 August 2011 by Greg Gillson.
Over 90% of the world's population of Heermann's Gulls nests on Isla Riza in the Gulf of California, Mexico. After the nesting season concludes, Heermann's Gulls join Brown Pelicans moving north into the productive waters of the California Current, as far north as British Columbia. They are often seen feeding together, the gulls stealing food from the pelicans.
The eBird frequency chart (below, click for larger view) shows that a major movement of Heermann's Gulls on the Oregon and Washington coasts begin the first week in July. They are most widespread the first week of August and remain common through October. By the end of November, most are gone.
While Grant in his 1986 book "Gulls" says that Heermann's attains adult plumage in the 3rd year, Sibley indicates that Heermann's may take 4 years to become an adult. Indeed, I believe the bird above is in third summer plumage. The primaries and tail are worn but apparently not in adult plumage, the body plumage does have an adult-like aspect. This bird will likely undergo a full molt into adult non-breeding plumage before it heads back to Baja in November.