Although I cannot profess to be an expert on gulls. I have seen tens of
thousands of Great Black-backed Gulls and banded a couple of thousand
birds. My first impression of the photos are that the size and shape
fits a great black-backed gull - the variation on the species in
northern Scotland can be quite marked with a few (but not many) as
small as a herring gull.

My main concern is that the mantle and wing converts seem to be too
brown. Most of the birds I have seen show greater contrast between the
light and dark markings on the feathers making the bird appear cleaner
in appearance. The colouration in Scottish birds can sometimes be
almost black and white. However the brown colouration could be an
artefact of the photos - some shots do give the bird a cleaner
appearance.

I would say that the idaho bird

http://octoberweb.com/birds/gbbg/

is a classic example. While the Renton bird is similar I would expect
it to show the same plumage markings. Superficially it does but it
does not seem quite right. I would be less worried about size however
if the relative size in this photo

http://osprey.bardill.net/gallery/album10/P1170086a

is correct then I would have to say that the bird is a bit on the small
side.

As for hybridization I am not sure the status of the birds from the
northeast of North America but in Scotland hybrids are in my experience
very rare and I cannot recall seeing any while I birded over there.
Although I realize that the Larus family is considered a complex rather
than a series of species I think that the experience with Pacific
Northwest gulls is leading some people to jump to conclusions on how
'pure-bred' this bird might be.

I am not sure what the alternative species might be but I don't think
they can be ruled out.

my $0.02,

Stuart
--
Stuart MacKay, Seattle, WA, USA