Mike asked -


>I'm from Duluth, Mn. and I'am looking for some good info. on Slaty-back
>Gulls. We have a winter adult bird here in the northland and I was looking
>for some detail info on what to look for. The guides are ok but, would like
>to know more.

Same basic size/shape as Western/Glaucous/Glaucous-winged Gulls. That may not
help a mid-westerner, though. More potbellied than Herring Gull and around
the same size. Wingtips coincident with tailtip when perched. There are
several things to note for Slaty-backed:

- bill has the shallowest gonydeal angle of all the N. Pacific maritime
gulls, thus the lower mandible seems fairly straight.
- color of orbital ring (eye-ring) of adults usually a purplish-red.
- mantle color of adults lighter than Greater Black-backed Gull but
darker than Herring Gull (closer to GBGU).
- broad white trailing edge to secondaries and all but outer two primaries
in flight, most noticeable from below; this trailing edge contrasts strongly
with the dark underwing coverts.
- head streaking fairly sparse in basic winter plumage but not as
immaculate as most Westerns.
- and the clincher, the "mirrors" on the two outermost primaries of adults
are staggered such that they meet the terminal white of the rest of the
primaries at an angle; from below, this makes it look like there is a
short crescent of mirrors across the wingtip when extended.

If the bird is not in full adult basic plumage, then ID will be more difficult.
For younger birds, I suggest you take copious notes and try to get a picture.
Basically, the Slaty-backed juv. is a paler version of a Western Gull but
with same slimmer bill as adult.

good luck!

- Jack



Jack Bowling
Prince George, BC
jcbowling at mindlink.bc.ca