[texbirds] Slaty-backed Gull Discussion Summary
- From: Erik Breden <erik.breden@xxxxxxxxxxxxx>
- To: TexBirds <texbirds@xxxxxxxxxxxxx>
- Date: Fri, 21 Feb 2014 01:01:46 -0600
TexBirders,
NOTE: This is a long email about Gull identification. If this is not
your cup of tea, remember that the DELETE key is your friend.
First I would like to thank the people who responded to my original
email about the mantle color of the Lake Casa Blanca SP Slaty-backed Gull...
Clay Taylor, who provided insight into the dreaded "hybrid"
possibility which I raised (mostly to spur discussion). The physical
characteristics of the bird (what is often referred to as "jizz" or
"giss" - General Impression of Size an Shape) match those of
Slaty-backed Gull, and not Herring Gull.
Dan Jones, who, in his typical "succinct" way, referred us to photos
from the SurfBirds website. (I had also done a similar Google search
on the web.)
Chuck Sexton, who addressed the issues of "expectations", and the
effect of varying lighting on perceived color. More about this below...
Tony Leukering, who provided comparative information for Slaty-backed
Gull and "Vega" Herring Gull about the "string of pearls" field mark
seen on the underwing (this is a series of small white circles which
curves in from the trailing edge of the wing on the outer 5
primaries). Tony also provided reference links for a Japan Gull Site
for Slaty-backed and "Vega" Herring Gulls (see also the same website
link provided by Wes Fritz below):
- http://www23.tok2.com/home/jgull/slayty.html
- http://www23.tok2.com/home/jgull/vega.html
Martin Reid, who provided a detailed discussion of plumage variation,
especially the variation in the appearance and orientation of the
"string of pearls".
All of these folks responded on TexBirds, so their full emails are
available at:
- //www.freelists.org/archive/texbirds/
In addition, the following folks contacted me directly...
Wes Fritz, a California birder/tour leader/photographer who is
familiar with vagrant Slaty-backed Gulls on the West Coast, provided
(in a private email) the following references and forum discussions:
-
http://www.birdskorea.org/Birds/Identification/ID_Notes/BK-ID-Slaty-backed-Gull.shtml
-
http://books.google.com/books?id=TMtB8CG767EC&pg=PA163&dq=Slaty-backed+Gull&hl=en&sa=X&ei=ywD3UqyYDoneoAS_3IHwBQ&ved=0CDMQ6AEwAA
- http://www23.tok2.com/home/jgull/gullidentifi_.htm
Mary Gustafson, who spoke to me by telephone and in person, for being
a sounding board for identification and general discussion.
Thank you all for your input and support of this discussion.
Now on to specific topics...
THE DREADED "HYBRID"!
There are classic cases of hybridization in Gulls. The best known (at
least for North American birders) is Glaucous-winged X Western Gull.
There are plenty of discussions about this on the web and in field
guides, so I will not go into it here.
It has been suggested that Slaty-backed Gull may hybridize with both
Herring Gull and Glaucous-winged Gull. One of the forums above has a
lively discussion of this, with the general feeling being that it has
yet to be proven.
My email about a hybrid Slaty-backed X "Vega" Herring Gull was more
to spur discussion than suggesting that is really was. I think hybrid
is called far too often when a "funny" bird shows up.
"GREAT EXPECTATIONS"
Chuck Sexton hit the nail on the head when he mentioned that people
expecting a "black-backed" Gull might overlook this bird. This was
certainly my situation. In this case my limited personal experience
with this species led me to "expect" a black-backed Gull. The
following two photos illustrate this:
- an adult bird in Winter (January 1996) on Hokaido, Japan:
- http://www.otterside.com/slidejpg/gullsb-l.jpg
- a dead adult bird (shot by the village children!) in Spring (May
1989) at Gambell, St. Lawrence Island, Alaska (a "young" David Sibley
is holding the bird):
- http://www.otterside.com/temp/gull_sibley_slaty-backed-1.jpg
THE BIRD!
The discussions and photos from SurfBirds and the web show that there
is considerable variation in the mantle color of Slaty-backed Gull.
This can be exacerbated by light conditions.
One of the key field marks, the so-called "string of pearls" on the
underwing primaries, is apparently highly variable. In some birds it
is "perfect" and in others much reduced. In some birds it curves
inward as the field guides describe, and in others it remains closer
to the "mirrors" on the tips of the outer primaries.
The photo below (the underwing of the dead Gambell bird held by David
Sibley above) shows the "string of pearls" gently curving in from the
trailing edge of the underwing as the field guides describe:
- http://www.otterside.com/temp/gull_sibley_slaty-backed-4.jpg
I think the following photos show the salient field marks of the Lake
Casa Blanca Gull.
- Standing:
- "pot-bellied" appearance
- large bill
- streaking on head and neck
- smudge around eye
- short primary projection
- http://www.otterside.com/winter2014/gull_slaty-backed-1017.jpg
- Flying:
- broad white trailing edge of wings
- "string of pearls" on underwing
- not very well defined
- parallel to the trailing edge of wing
- http://www.otterside.com/winter2014/gull_slaty-backed-1007.jpg
- http://www.otterside.com/winter2014/gull_slaty-backed-1015.jpg
So...where does this leave us? After all the discussion, input, and
research, we have learned a lot more about this Asian Gull than we
knew going in. I hope that nobody felt that I was questioning their
birding skills or qualifications. This discussion was certainly
educational for me.
My feeling is that this is, indeed, a Slaty-backed Gull.
Thank you all again.
Erik Breden
Harlingen, Texas
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