I echo that comment. All the 3rd winter Slaty-backed photos I've
seen (sample size is small here) have much of the adult primary
pattern.

Another possibly important character is the presence or absence of
white tips to the outer primaries (apical spots). There's some
anecdotal evidence that Slaty-backs are unique in that often
2nd winter birds show these. Western Gulls, on the other hand,
seem to be at the opposite end of the spectrum; I don't think I've
ever found a 3rd winter Western with white outer primary tips.
Does this bird have white tips to the outer Ps?

And, for what it's worth, I have a photo of an adult Gl-W x Western
hybrid with a classic Slaty-backed "string of pearls", the obvious
white tongue-tips on P6,7, AND 8. In fact, that specimen is here
at UC Davis.

lurking from California
(since I'll be in Tacoma Dec 21- Jan 4),
and REALLY want to see a Slaty-backed!

Steve Hampton

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
hampton at primal.ucdavis.edu
http://www.agecon.ucdavis.edu/homepages/hampton/index.htm

Dept. of Ag. and Resource Economics
UC Davis, Davis, CA 95616 USA