It's not just distance, it's general migration strategy including
direction, and I believe the point was that a genetic mix
of strategies could potentially produce (logically) unexpected
and unpredictable results. That is certainly the case with
appearance in more obvious genetic mixes.

I included the Idaho record of GBBG in my photo study,
and I see no reason to question it. However, the Washington
bird is radically different in structure, and (at least in photos)
appears to have much paler plumage marking.

Haven't seen photos of the other far western records, but
I don't think a few records constitute enough data to form any
conclusions on pattern of vagrancy supporting the notion of
the Washington bird necessarily being more likely to be pure.

I should restate that the reason I'm being so obnoxious about this
is hopefully to simply convey that this the Wash. gull is atypical
enough to where it can't *confidently* be called pure, or even pure
"enough" to confidently be accepted as a first state record of
GBBG, at least at the criteria level typically used by BRCs.
Call it whatever you want.

Cheers,

Phil