This is by far my favorite gull this year. I am convinced it is a SBGU, but not so easy to convince others and for good reason, 1st yr SBGU's are tough to id out of range, plain and simple. That being said, they must be out there and this one comes about as close as you can with out seeing a dark 2nd yr scap yet. I can not see anything on this bird that wouldn't make it a SBGU, and several things that could make it a HERG, but several things that lead away from HERG, like short wing projection, ovaer all bulky shape and size, and most importantly the wing pattern and shape. Wing appears very broad, shortish broad hand, secondary bar is wider than greater coverts, something I think is unusual in HERG, no? The secondaries extend down on standing bird creating a skirt, something seen often in SBGU? A string of pearls appears in some of the open wing shots, the pale inner webs contrast well with the darker outer webs as seen similarly in Thayer's. In some shots there is a "ghost" pattern that shows a white wing tip on P10 and a mirror on P9, a SBGU pattern, an unusual pattern for Smith HERG. Then there is the belached out coverts and the dark centered scaps, all good for SBGU. The spotted look to the rump, another good feature for SBGU in 1st winter, unusual for a HERG in 1st winter. When the neck is up, and not relaxed, it is thick, long. Standing bird shows dark pink legs, which would be unusual, but not unheard of for HERG. But this pink does seem different than the typical. It also has that elongated look while in water, yet short rear end. I think it is good for SBGU.
Here is a link to the discussions and photos of this bird in Conn. http://www.shorebirder.com/2013/02/first-cycle-slaty-backed-gull-in-ct.html
Note last photo on this web site showing the red paint on tail as also seen on bird in my photo (256)