These two juvenile gulls were both on the wharf at Santa Cruz, California, USA on 13 Feb 2009. Both are perched next to Western Gulls; the birds I am wondering about are the bird on the left in the first photo and the bird on the right in the second photo. Despite their differences in color - one is distinctly a colder shade of brown than the other - I think they are both Thayer's Iceland Gulls.