[TopOfUtah] Mystery gull summary (long)

[Ryan O'Donnell]

Earlier this week I posted photos of an unidentified gull to Utah
Birdnet and to the Frontiers in Identification listserv.  I thought it
might be helpful to post a summary of responses I received since many
were sent privately and public responses to one listserv might not have
been read by members of the other.  Below are most of the comments I
received, slightly edited and with names removed.  Thanks to Jay
Carlisle, Craig Fosdick, Alvaro Jaramillo, Nick Komar, Nick Lethaby,
Colby Neuman, Jason Pietrzak, Kris Purdy, and Larry Tripp for sharing
their opinions and helping me learn more about gull identification.
Overall, I would say that there was no consensus in the identity of this
bird, but Glaucous-winged Gull x American Herring Gull hybrid seemed to
have the most support.  In my records, it will remain as an uncertain
identification.
 
Good birding,
Ryan
 
Ryan P. O'Donnell
Department of Biology and the Ecology Center
Utah State University
5305 Old Main Hill
Logan, UT 84322-5305
 
http://200birds.blogspot.com <http://200birds.blogspot.com/> 
 
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I think your pale gull is a better match for a Thayer's; my only
reservation is that the bird looks really big in comparison to the
California in image 1. It should be bigger, but the Thayer's looks like
it dwarfs the California in that image.

The plumage features in favor of Thayer's are on the dorsal surface of
the wing as you indicated. "Striped" outer primaries, pale inner
primaries, and dark contrasting bar on the rear of the secondaries. I
think image 8 just shows the bird in very strong light and somewhat
washes out the plumage. I noticed that the left wingtip (and to a lesser
extent, the right) in that
image still shows the striped look to the primaries.

Also, I think a couple of your images show good contrast between the
darkness of the wingtips and the paleness of the body.
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(From the same person as the previous message:) 
You can't believe the gnashing of teeth I've done since I received your
latest message. Holy Lariphobia, why didn't I think of a Herring Gull? I
sure hope the gull structuralists will chime in with their opinions.

The gull you asked about has the head and bill structure that more
closely looks like a Herring to me--large head, long, sloping forehead,
and longer bill. The Thayer's head should look smaller and more round
with a
smaller bill. The Thayer's has a cuter look. Something still not quite
right about the gull, though, is how light the plumage is for a
juvenile/first winter Herring. I expect especially the darkest wing
feathers to be darker than in your bird were it a Herring Gull.

So all of this teeth gnashing puts me squarley in the undecided
category. I still feel certain, however, that the bird isn't a young
Glaucous-winged Gull.
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I do not have any references in front of me but I don't see anything
wrong with it being a GWGU.
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I'm not sure what it is, but I don't think it's a pure Thayer's (too
big) or Glaucous-winged Gull.  It seems a hybrid with 'white-winged'
gull parentage seems likely, but as to the exact parentage...who
knows...I'd be interested to know what others think, but I don't think
it's within the range for a pure Thayer's or Glaucous-winged Gull.  
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I think that it is 100% Thayer's Gull; i.e., I did not change my mind
after doing a bit of reading.  I agree with XXXX re: the secondaries and
primaries.  Not sure what to make of the comment about the Thayer's
"dwarfing" the adjacent California.  Looks like the THGU is closer;
could just be a relatively large THGU next to a small CAGU.
 
Granted I have only looked at a couple of field guides (Sibley and Nat
Geo), but I also think that the bill is slighter than would be on a
Glaucous-winged.  Glaucous-wingeds seem to have relatively massive
bills, with a "bulge" towards the end of the bill.  This bird seems to
be relatively slight, with no such bulge.  I also recall relatively
massive bills on a couple of GWGU that XXX and I had at Jack's Creek in
Idaho.  
 
I also think that your in flight pictures (I'm sure you already noticed
this) are washed out, and remove the contrast and the brown tones that
are present on the bird (visible in the pics showing the bird sitting).
The contrast between the darker secondaries vs. the "striped" (I don't
like that term, but it works) primaries is evident in pic 4 and 7.  Also
looks like most/all flight photos show the pale underwing.
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My impression is Thayer's based on size relative to the nearby RBGUs and
CAGUs as well as head/bill shape.  The bill doesn't look thick enough
for a Glaucous-winged 
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I didn't look at your photos until after I gave locating it a shot and
my gut feeling is that it's a Herring Gull in some juvenile stage, but a
lot of the arguments for hybrid make sense. 
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This certainly isn't a GWGU. It's also not a 'pale' Thayer's Gull
either. It  may well be a Thayer's Gull but it would probably be a bit
darker than most. 

There are several things that look a bit odd for a Thayer's:

1. Appears rather large compared to the nearby CAGUs
2. The bill is fairly heavy and already seems to have quite a bit of
pale coming in at the base, even though it's only mid-October 
3. Although the photos don't show it well, the primary projection
appears a bit  short. 
4. The pale crescents on the primary tips are not very bold.

I'm not sure the bird isn't some type of HEGU hybrid, although it may
just be a  large male Thayer's. 
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The time of year, so early on, makes it a little more difficult in my
opinion, for example molt timing is not an issue yet as a way to
separate Thayer's from some others. I must say that my reaction to this
gull is that it is a Herring x pale-winged gull. The structure along
with the finely streaked pattern on head, tendency for white
background of head (look at those flying shots where the white head
stands out), and the rather pale area at the base of the bill and in
front of the eye area all classic smithsonianus traits and not expected
in Thayer's or
Glaucous-winged which are much more evenly colored on the head
particularly early in the season. The bird also looks too big and bulky
compared to California for a classic Thayer's. The wing coverts are
rather uniform, while Thayer's is much more well speckled, checkered
looking greater coverts are the norm. Structurally this bird looks quite
unlike a Glaucous-winged to me due to the thin bill, and the pattern of
pattern of the head and neck are unlike Glacous-winged as well. I would
vote for a Herring x Glaucous-winged hybrid, and while this may seem
like a letdown in some ways, it could be a rarer find than either GWGU
or THGU! 
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Given the apparent plasticity among individuals belonging to described
species of the pink-legged gulls, I would hesitate to identify this gull
with absolute confidence to species. That said, I think that the large, 
all-dark bill with med-large gonidial angle; and overall dark plumage,
small look to the eye, shallow forehead, and flat crown, as well as
short wing-tip extension beyond the tail tip argue towards
GLAUCOUS-WINGED parentage. That said, the tertials are more patterned at
the tips than many Glaucous-winged gulls of this age and the head looks
less massive and more HERRING GULL-like than Glaucous-winged. I would
call this a *possible* Glaucous-winged x Herring Gull hybrid.
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I see many traits of Slaty-backed Gull (SBGU) in this large, juvenile
gull.  First, if you view the silhouette of the standing bird you would
see that structurally this bird smacks of Slaty-backed Gull with the
vertical head 
and neck posture, the heavy dumpy chest and the short projection of the
wings beyond the tail. Second, the whiter head with a dark smudge around
the eye is consistent with juv SBGU whereas juv American Herring Gull
(AHGU) has brown head until later in the winter (December??). Third the
darkish bill blacker near the tip reminds me of other young SBGUs.
Fourth, bright pink legs is a feature of older SBGU (not sure about
younger birds) and those legs are very bright. Fifth, the pale inner
webs of the flight feathers extend well into the secondaries, a feature
of first cycle SBGU. Sixth, the unpatterned greater coverts is a feature
of first cycle SBGU. Seventh, the tail pattern featuring a very thick
darker band (but not totally dark as in AHGU or Glaucous-winged Gull,
GWGU) is typical of SBGU and should rule out AHGUxGWGU hybrid. Eighth,
pail edges to the primaries when folded is consistent with an identity
of SBGU. Ninth, appearance of darker feathers among the median coverts
in the mantle (photo gull8) suggests that some darker mantle feathers
may be molting in (although this may be photographic artifact). The
overall paleness seems odd but I would not be surprised if it were
within natural variation for SBGU or this could be a genetic abnormality
(leucism) or indication of hybrid heritage involving a "white-winged"
gull such as Glaucous Gull. Size seems appropriate for SBGU also.
Alignment of 8 or 9 characteristics for a specific identity is strongly
suggestive for that identity. OK, now that I have shared my opinions
about this interesting individual, I'll sign off with the caveat that I
am not an expert on SBGU and have very little direct experience with the
species.
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