Gene makes several good points, but I feel he is overly prejudiced towards a
different, rather innovative method of birding that many people are picking
up on these days. Structure, if that's what you're used to using, is a
really good way to bird, and you don't have to be an expert. I've seen brand
new birders easily picking out Thayer's Gulls and separating them from
hybrids mostly by structure -- with 100% success, as far as I could tell.
You simply have to be open to a different approach. Dennis emphasized
variation in structure, from bird to bird and even moment to moment, and he
is supremely correct to do so (which incidentally can make single photos
impossible to analyze). This is what can make it difficult for birders who
are used to using field marks which don't usually vary temporally in the
short term, to adapt. Also why field marks should never be completely thrown
out, in my opinion. I just have to counter-emphasize that there are
structural limits, and these limits are what we focus on, just like limits
of plumage. As far as I know, there is no reason closely related gull
species should differ phenotypically only in plumage. Structure mutates and
selects naturally in precisely the same way. As for being difficult to
describe or convey to others, I would say that voice is similarly hard to
convey in words, but you can still try, and I don't think many birders hold
the position anymore that voice is invalid as an ID technique.

Gene's tried-and-true approach is certainly reasonable. It's also, to me,
rather boring, and it makes learning new things (a very large part of my
birding enjoyment) somewhat more difficult in my view. Focusing on
well-known field marks to pick out a bird is a method that birders have used
for many decades, and it works pretty well. However, in the case of
Slaty-backed Gull and many other examples, with this method you are going to
be strongly biased towards adult-plumaged, "classic" birds, and basically
you will only ever see what you expect to find. You're not looking for
anything new or out of the ordinary, you're simply looking for the
black-backed bird out there in the field. Where's the fun in that (I say
this only half-facetiously)? It so happens that due to "gestalt" traits, the
immature stages of Slaty-backed Gull are sometimes (oftentimes) very easy to
pick out from our local mongrels, but if you're focused on one or two
plumage characters found in adults, you're going to miss them. Clearly,
these more well-known characters should be carefully noted when present, but
they can only take you so far if you want to learn as much about a group of
birds' ID as you can. I've personally found around 4 Slaty-backed Gulls, and
a huge suite of structural characters were among the very first things I
used to make my identifications in all cases. As long as I documented them
with carefully written description, field notes and/or photos, I don't think
these IDs should be considered scientifically unsound, just because I didn't
bird the same way that someone else is comfortable with.

All for now, good birding!

Charlie Wright
Bonney Lake, Washington