Great Black-backed

Phillip Pickering philliplc at harborside.com
Tue Jan 20 11:22:38 PST 2004


Washington Birders,

Just wanted to summarize in this forum the concerns that many
of you already know I have about the Renton gull.

While the Renton bird is very much like a GBBG in plumage
patterning, I feel very strongly that there is something wrong with
its structure for that species. Structural aspects are every bit
as vital as plumage for supporting such an extralimital large gull
as this, and although Washington birders have done an excellent
job with plumage analysis, I respectfully suggest that the bird's
structure isn't being scrutinized at the necessary level of detail.
Yes it's a large bird, easily within the range of bulk of GBBG,
but that in itself is not enough. Different species of gulls vary not
only in size, but also consistently in certain aspects of shape, so
shape is absolutely critical to ID support in a case such as this.

These shape aspects can be subtle (although some can be fairly
obvious), and might require a good deal of experience with a
species to get a feel for. Due to my relative inexperience with
that species, when the first series of photos were posted I
thought the Renton bird appeared to be shaped reasonably
similar to what is typical for GBBG (other than the bill), but as I've
studied numerous GBBG photos it has become more and more
clear to me that this bird has multiple specific structural features
that seem at best borderline for that species, and as a whole
they seem likely to be completely outside GBBG's apparent
normal range of variation. These structural anomalies are
consistently evident in every photo that has been posted, and
they are largely independant of changes in appearance due to
posture. In most cases they are also independant of male/female
variation -

- head shape is strange for GBBG. Although some females
can have fairly rounded heads, the vast majority of GBBG
show a flatter forehead lacking such a pronounced angle out
in front of the eye, and show a shallower forehead/bill angle.
- perhaps due to the atypical head shape, the eye placement is
somewhat centralized in the face, while on a typical GBBG the
eye appears slightly higher and/or closer to the forehead.
- body shape is very strange for GBBG. The vast majority
including both male and female do not show such a rounded
look, having flatter bellies and a more attenuated rear end in
all postures. On average they also show a bit longer primary
projection beyond the tail, although that feature is somewhat
more subtle and more subject to photo angle.
- proportions are radically off. The Renton gull's head, eye, and
bill are actually proportionately quite small compared to its body
size for any species that I can think of, and exceptionally so for
GBBG, which is probably proportionately the largest headed of
all North American gulls. This takes practice to see, but after
looking at it in many photos it's fairly obvious to me, so obvious
in fact that I suspect it's something that could be measured and
compared in photos. Since the URL to photos of the accepted
Idaho GBBG is below, look at the proportionate head and
bill size of that bird to its body compared to the Renton bird.
The two birds are about as radically different in head/body
proportion as any two large gulls can be. In my experience,
misproportioned head and body is a relatively common
feature of more obvious large gull hybrids.
- as far as bill shapes vary, the Renton gull's bill is shaped quite
different than the vast majority of GBBG. GBBG have thicker
bills, typically with a more concave edge to the lower mandible
and more gonydeal angle. The Renton gull's bill is remarkably
straight for that species. Also, I am unable to find any photos
of GBBG with such a long, gentle downslope to the culmen
end. GBBG typically show a much steeper, sharper downslope,
and a blunter look to the bill.

Given how well the plumage patterning seems to match 2nd-
winter GBBG, I must concede that there is a very good
chance it is part, perhaps largest part that species. However,
my opinion based on a lot of my life wasted over the last
several years studying gull structure and how it varies among
individuals of a given species, and hybrids, is that this bird
is very unlikley to be a pure GBBG. I'd also suggest that
this bird's size alone doesn't necessarily preclude crosses
not involving GBBG. Occasionally more obvious large gull
crosses will exhibit features seemingly not matching either
parent or what a mix of the two would be logically thought
to produce. And, although this gull's body is huge, it's head
and bill in comparison to the other species in the photos don't
appear to be outside the normal range of Glaucous (can't
necessarily be assumed it would be a barrovianus cross).
It of course could be argued that the size combined with
plumage patterning makes non-GBBG crosses unlikely,
but that's a separate issue.

Perhaps the bird will stick around long enough to molt further
into 3rd-summer plumage, which should be quite telling.

Cheers and good gulling,

Phil Pickering
Lincoln City, Oregon
philliplc at harborside.com


>Seriously though, I was able to view it at ca. 100 feet this morning
(Monday,
Jan 19) between 11:30 AM and noon standing on a log boom at the north end of
Gene Coulon Park in nice sun. I can see no reason to think it's anything but
a
perfectly good example of a 2nd winter Great Black-backed Gull, with a bill
that is a bit advanced, cf. 2nd summer. Nothing unusual about that. It's not
huge but well within the range of a Great Black-backed in size. In
particular,
every plumage detail seems to match the photo in Grant of a 2nd winter bird
with the greater coverts a bit more worn (figure 301, pg. 277, a November
bird)
with the bill transitional to the photos of 2nd summer birds (figures
302,303,304). The white spot near the tip of the outer primary is shown in
Grant's figure 304 for a 2nd summer bird.


Sibley's illustration of a 2nd winter bird standing is a dead ringer,
including
the bill coloring and the P10 spot, though perhaps too clean gray on the
scapular ground color.


The Idaho record from last December (2003) is clearly of a different bird,
in
1st winter (cf. www.octoberweb.com/birds/gbbg).


Gene Hunn.



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