Here's an immature Heermann's Gull (Larus heermanni) that I photographed in La Jolla this morning. Another photographer ID'ed it first and when I passed the knowledge on, one lady replied "Oh! That's a lifer for me!" — the Audubon Society's Bird Festival is in town! :D
This is a juvenile Western Gull, for comparison with the first photo in this thread. Its plumage is more checkered and the feet are pink instead of dark grey on the juvenile Heermann's Gull.