the ones i saw in san fran looked slightly darker to me but that's 16 winters ago and i wasn't looking at them with particular interest. actually i knew nothing more than my peterson guide and my (not so sharpened) eye told me. i even didn't know about the frequency of hybrids with GWG. but they looked lighter mantled than the LBBGs i was used to see (of intermedius/graellsii mostly dutch intergrades i think). well, there's no doubt about it being am occidentalis. aren't wymani sometimes darker than your 1st wingpic?
A common misconception but generally no. Wymani are about the same shade as
graellsii for the most part. There is a cline within Western Gulls as well. The step is said to be in central California but many birds in the San Francisco bay area, show a range of mantel shades with many matching size and shade of
wymani even though it's not supposed to occur north of Monterey, which I think is just their northern most breeding area and not necessarily the stop of it's range.
Not that this does much good here but I'll mention it anyway; size and jizz also help with distinguishing these two subspecies. Occidentalis are big 'ol birds, being the same size or even slightly larger than Herring Gulls. Wymani averages smaller with some small females matching LBBG in overall appearance, although still maintaining Westerns primary projection and of course the pink legs. With experience, voice is also a clue. Occidentalis' long call is slightly slower in tempo but still with short notes and usually lacks the slight dry undertone. A few
wymani have deep gruff voices almost matching Yellow-Footed Gulls.
Ok I'll stop now, sorry lol.