Subject: Re: Western Gull/Herring Gull query
Date: Oct 18 00:40:52 1997
From: jcbowling at mindlink.bc.ca - jcbowling at mindlink.bc.ca


** Gene wrote -

> Maybe so, but a wide range of mantle color has been described for them, and
> the birds that were by general agreement Slaty-backs near Portland, Oregon a
> few winters ago were certainly no darker than our Westerns. Also, the
> Slaty-backed we had at the Elwha River mouth, the first state record, I
> forget the date, but just a year after the St. Louis bird which was the
> first south of Alaska, apparently, while darker than our Westerns, showed
> (as did the St. Louis bird) a very striking winter head pattern of heavy
> blotchy streaking on the nape and a large dark patch all around the eye. If
> this is not typical then they certainly vary all over the map.

I would not doubt that there would be a wide range of mantle colour
described for Slaty-backed. The colour does change in relation to light
angle quite a bit, being lighter in full reflected sunlight and darker
when in shade. This is typical of plumage that has both pigment and
iridescence as the basis of colouration. However, if you were to stick
a half-dozen SBGU in a row in the same light conditions, they would all
appear to have similar mantle colouration, close to nominate WEGU.

As for head streaking, I can only relate my experience. The orbital
smudging varied quite a bit from bird to bird and I wondered at the time
whether this had some sexual connection. Here is the entry from the
appendix to my field notes that I wrote up after my voyages:

" WEGU, SBGU, GWGU - these three species are so similar in form and habits
that mutual discussion seems appropriate; GWGU definitely developed
the most noticeable head streaking; adult _L. o. wymani_ and adult SBGU
similar in colouration at first glance but mantle shading and wingtip
pattern noticeably different with practice; as is evident from the
photographs (take my word for it...jb), first-winter SBGU are easily
separated from same age Herring Gull not only by sheer bulk, but by the
distinctive "zebra-stripe" barring of the undertail and underwing coverts;
most pelagic sightings of SBGU involved first-winter birds accompanied by
one or more adult birds, second-winter SBGU seemed more plentiful in
Japanese harbours, while third-winter and sub-adult SBGU seemed evenly
distributed; GWGU apparently the most pelagic of the trio."

I was out on the n. Pacific from Oct. 22, 1983-Mar. 20, 1984. It is quite
possible that SBGU just takes longer into the late winter to develop head
streaking than its congeners, so an April SBGU may have more streaking than
I saw.

- Jack


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