[mou] Slaty-backed Gull (?), Superior Landfill

[Joshua Christian]

 While birding the Duluth area yesterday, I made a stop at the Superior Landfill.  Since I am not great at gull ID, I took plenty of pictures to look over later.  There were very few gulls moving around, but at one point an eagle passed over and many of the gulls that were just over the ridge flew up briefly.   When this happened, I started taking pictures of the flock without really looking at the birds closely.  This morning as I was going over the pictures, I noticed an odd dark-backed gull in two of the photos, which I believe may be a Slaty-backed Gull.  The pictures can be found at the links below.
 
http://s194.photobucket.com/albums/z43/GreatGray/?action=view&current=DarkGull2.jpg
 
http://s194.photobucket.com/albums/z43/GreatGray/?action=view&current=DarkGull1.jpg
 
I am using the following reasoning for the ID.
 
The solid color of the mantle and the pure white tail indicate an adult bird.  In terms of size, it is roughly equivalent to a Herring Gull.  The gray of the mantle is too dark to be a Herring Gull, but not dark enough to be a Kelp or Great Black-backed.  The black on the wingtips is not extensive enough be a Lesser Black-backed.  This essentially leaves Western Gull and Slaty-backed Gull.  The most compelling evidence for Slaty-backed Gull is the white subterminal spot that can be seen on the third primary.  
 
Please feel free to comment on or critique my reasoning.  Especially if there are any rare hybrids or color abnormalities that appear similar to the bird in question.
 
Good birding,
 
Josh Christian
Hutchinson, MN
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