Fig. 1) the tail implies first year plumage; note the white head and breast; solid tertials; tone of gray on the scapulars; pale pink legs (paler than adjacent Westerns); bill dark w/ pinkish base; bill shape really the best thing about the bird for Western Gull. The white head exceeds even typical second winter Western in lack of markings; note the tertials and coverts are quite worn (normal for late March on most species); the shade of "adult gray" on the back is relatively close to northern "nominate occidentalis" Western Gull.

Fig. 2) note the pale barring at bases of outer retrices; this is quite unusual for Western and is more like Herring smithsonianus or some other form; unfortunately, the whole tail was not seen in flight.

Fig. 3) ah, the upper wing; note the slight pale window of the inner primaries, not as strong as Herring, and possibly within the range of Western.

The identity of this bird is not definite. An unusually white Western Gull is probably the best bet, but others have suggested an unmolted Yellow-footed Gull, Yellow-legged michahellis, and even Herring x Great Black-backed...
We would love comments! Send them to me at shampton@ospr.dfg.ca.gov