I'm not sure if mixed pairs have ever been directly noted, but
apparent Herring x GBBG have been posted more than once
on ID-Frontiers, including a very recent gull that garnered a
clear consensus as very likely being a good example. It was also
brought up that several specimens are cited in literature. And,
it was also noted that there is some (limited) recent direct
genetic evidence for introgression between smithsonianus and
GBBG. I didn't save that reference, perhaps someone else
who subscribes did.

Given that, I think the simple odds of this gull as a vagrant being
an intergrade are substantially greater than the odds of a pure
individual showing ALL the atypical or minority features that in my
opinion it clearly shows. You could even make that argument on
the basis of features that aren't likely to be considered subjective,
such as the perfectly clean bright pink bill base on a 2nd-
winter in January, combined with the mostly solid greater
coverts. If you throw in how unusual the shallow, gradual
culmen downcurve is for GBBG (which I feel should be
discernable to anyone who takes the time to objectively
study how this feature typically looks on GBBG) you have an
individual that shows 3 features than are at best shown by only
a minority of GBBG, and that are known to vary in other
more obvious intergrades. Taken together, at the very best
they place this gull's appearance in a vanishingly small minority
of GBBG, and that is without even considering its other
structural anomalies that are clear to me but harder to
convey.

And even beyond that, consider the concept outlined in the
post below forwarded from ID-F.

Cheers,

Phil


>>Actually, I think that Wayne missed the point on this one. He stated:

"However, if-- as I suspect-- GBBG hybrids are quite rare compared with
the pure form of that species, is
it not much more likely that the Washington bird is a somewhat variant GBBG
rather than some hybrid thereof?"

The simple odds is not the argument here. I would think that the argument
is that another species (e.g., Herring Gull) would introduce genes with
different migration length, strategy, timing, etc., that might make a hybrid
MORE likely than a typical GBBG.

For instance, despite the fact that Great Black-backs breed CONSIDERABLY
closer to Colorado than does Lesser Black-backed, the latter is at least 10x
(possibly 20x) more common in Colorado than is the former. So, one can
imagine the consequences of hybridization among taxa with very different
migration strategies! It's a mess!

Another example: Though we have not ever recorded a gull in Colorado that
has Western Gull genes (either pure or otherwise), which do you think more
likely: a pure Western Gull with those fairly sedentary genes or a Western x
Glaucous-winged hybrid that has those long-distance migrant Glaucous-wing
genes? I know which I'd pick and, being on the Records Committee, I'd want
to be absolutely positive that the first CO record of Western Gull was a
pure one.

Respectfully,

Tony Leukering
Brighton, CO