Subject: Slaty-backed Gull Variation (was: Western Gull/Herring Gull query)
Date: Oct 18 00:26:57 1997
From: Michael Price - mprice at mindlink.bc.ca


Hi Tweets,

Gene Hunn writes (edited):

>but a wide range of mantle color has been described for them,
>Slaty-backed we had at the Elwha River mouth, the first state record,
>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.

Several years ago, a large group of us studied an adult Slaty-backed Gull
(SBGU) for a long time. The bird was close to the road in a field near the
Vancouver BC landfill at Burns Bog and showed much variation in apparent
mantle grey-densities from very dark grey ('blackish-grey' --Grant) to dark
medium-grey (darkest Mew or California Gull, mid-range Western X
Glaucous-winged hybrid), depending on its orientation to the light. It was
either the darkest gull in the field (which contained as well an
occidentalis-type Western) or indistinguishable from the hybrids. I asked
others in the group if they saw the same apparent variation; the unanimous
response was yes. Phew.

Besides the dark mantle, the heavy warm brown-grey blotches and short coarse
streaks on rear head, sideneck, nape and onto side-chest as well as the pale
citron eye with dark eye-crescents with post-ocular 'comma' streak have been
consistent marks on any wintering Slaty-backed here over the last few years.
Leg-color likewise has been a consistent 'bubblegum' pink.

Al Jaramillo suggested that UV radiation might bleach the mantle color into
paler shades. This effect might kick in by late winter if the winter has
been a dry sunny one.

Another factor in any variation may be hybridism. What other species may
these birds consorting with? SBGU X Siberian Gull, SBGU X Herring, SBGU X
Glaucous-winged are only three possibilities. The hybrids are gonna look a
little strange.

Michael Price We aren't flying...we're falling with style!
Vancouver BC Canada -Buzz Lightyear, Toy Story
mprice at mindlink.net