In the center of the image is what I think to be a good, adult dark-morph Ross's Goose. This dark-morph of Ross's Goose is quite rare, and often birds identified as dark-morph Ross's are thought to be hybrids of Snow x Ross's. I have looked for this bird in the big flocks of Snows and Ross's that we get now, but previously hadn't seen anything really promising.
When I 1st saw this bird, the blackness of the plumage is what really struck me 1st, along with the neat white face. The white on the coverts also isn't something you normally see on Blue Snow Geese (as you can see on the surrounding Blue Geese). This bird was easily picked out of the 1500+ dark-morph Snow Geese that were in the area, just by the darkness of the plumage.
In some shots the bird looks a little large, but in direct comparison to Ross's Geese, I couldnt' tell any difference in the size of this bird, or in head or bill shape to Ross's. It didn't have a grin patch on the bill as shown by Snows, but did have the bluish patch on the base of the bill as in Ross's.
PS- These photos have since been looked at by several people familiar with this goose from CA, and agree that it is a blue Ross's. Ironically, several of them also picked up on the small, dark goose just to the left of this bird, as being an immature dark-morph Ross's! I didn't even notice this bird in the field. I don't know if this bird can be ID'd 100% from this shot, but it certainly is either a Ross's or a hybrid.
More of this goose is available here: