Subject: GBBG stuff
Date: Jan 26 18:24:40 2004
From: Phillip Pickering - philliplc at harborside.com


For those that aren't sick of me harping about how off the
Renton gull is structurally for GBBG, here's another ratio:

-bill width at point of end of feathering on upper mandible measured
perpendicular to cutting edge
-body width from belly between the legs vertically to top of back
(as measured in previous ratio)

I tried this in the 11 best photos of the Washington bird, including
photos from both Ruth and Denny taken on different days,
presumably a large enough sample to reduce variance in this
ratio due to posture. Range for Washington bird was .104 to 118,
with mean of .111.

Using the same method I tried this on 25 random web photos
of GBBG in relaxed postures. Range of ratio was .122 to
..160, with mean of .139

I consider the photos in Grant too small to accurately
measure this ratio (unless scanned and enlarged which
I don't have time to do). Admittedly Grant #294 seems
to likely be in the .100-.110 range. However unlike the
Washington bird, it appears to be a structurally normal
GBBG with an exceptionally small bill, which is sometimes
the case with very young juvenile large gulls. The other
photos in Grant in which I tried to measure appeared to
have a range of roughly .125-.150.

You could also try this ratio measuring the bill width at the gonys,
although I don't think the results would be significantly different.
Even allowing for some measurement error, the Washington bird
is (at best) at the extreme small end of this ratio for GBBG.

Add that to the other apparent extreme minority features noted
(bill pattern for age, mostly solid greater coverts, shallow, gradual
culmen downcurve, and probably also contrast of marking color).
In terms of being confident about this being a pure GBBG, in my
opinion these features considered together cannot be objectively
swept under the rug with a 'gulls are variable' statement. That's
the type of thing you could say if you weren't concerned with level
of purity, and it was one gull standing in a crowd of 10000 GBBG's
within their range. This is different. The odds of a unique vagrant
pure individual showing so many minority features would have to
be vanishingly small.

So, even if all of these strange extreme minority features are within
possible range of pure GBBG (and I'm not convinced at all that they
necessarily are), you could argue that it's just as likely to be a GBBG
intergrade, even if intergrades are rare. And that is without any
reliance on the very real possibility that intergrades may be
genetically more prone to wander out of range. The argument
could even be made that the odds of a pure extralimital GBBG
showing so many minority features are even longer than the odds
of this bird being some unfamilar or atypical non-GBBG
intergrade.

That is why I've suggested (hopefully stated with a little more
clarity) that, whatever anyone thinks or says the gull is, at the
very best it's not a bird that should inspire confidence as a
first state record.

Cheers,

Phil