Identifying the Thayer's Gull is much more difficult than the available
literature suggests. The problem is that the ID literature has been too
typological in trying to pigeonhole a bird that is actually part of a cline
with the eastern Iceland Gull. What does this mean, well it means that
Thayer's gull is extremely variable, especially in the younger ages. As
far as plumage characteristics go I would predict that young Thayer's can
show most patterns that are found in young Glaucous-winged Gull. Now the
Glaucous-wing is as variable as Thayer's because of the rampant hybridization
with the Western Gull. In short, identifying gulls in the west is a mess.
Contrary to what one would expect it gets more difficult as one observes
more carefully! The more you see, the more confusing it becomes.
The main way I identify young Thayer's is by some of the points you
mentioned, small bill, rounded head, smaller size, relatively large eye,
the typical "jizz" field marks. The pitfall is that all the large Larus gulls
show a great deal of sexual dimorphism in size and shape. I imagine that a
small female Glaucous-wing may be similar in size to a large male Thayer's.
I have become really confused looking at some local gulls here, it happens
particularly when they are just a few meters away in perfect light. The better
the look, the harder it is for some reason. They are much easier when seen
through a scope as they fly half a kilometer away. Well one mark I have
observed is the length and shape of the primary extension. Thayer's have
longer wings relative to their overall size, which can be seen as a longer
primary extension (the lenght of the primaries that extend past the
folded tertials, when perched). I recall that when I have counted the
number of primaries past the tertials Thayer's has at least one more than
Glaucous-wing. Off the top of my head, I think that Thayer's shows 5-6 while
Glaucous-wing shows 4. The shape of the primaries extending past the tertials
is also different. The triangle of primaries seen on a Glaucous-wing is much
wider relative to that on the Thayer's, this gives the Thayer's a sleeker,
more gradually tapered rear end, while the Glaucous-wing looks blunter in
this area.

I hope this helps. BTW, I haven't field tested these points as much as I
should have, so any input would be great. Does it work, or doesn't it?

Alvaro Jaramillo
jaramill at sfu.ca
Vancouver, B.C.