Subject: Re: hybrid gulls
Date: Dec 22 10:13:26 1994
From: Dennis Paulson - dpaulson at ups.edu


I'm impressed that Chris Hill took the time to look that closely at a flock
of over 100 "Larus vulgaris," and he touched on a subject that probably
puzzles many.

About this particular hybrid pair; my rule of thumb (or of alula) has been
that the mantle color varies with the wingtip color, so that westerns, with
their essentially black primaries, have a dark gray mantle (but in this
region not so dark as many field guides show for western), and
glaucous-wings, with their gray primaries, have a much paler gray mantle.
This is pretty straightforward, but there is a caveat: in neither species
is the mantle and primary color the same. It is rare to find a
glaucous-wing with wingtips as light as its mantle, contrary to the
illustration in the NGS field guide. I think this is most likely in summer,
when the wingtips have faded more than the mantle and approach it in
coloration, but even in a bunch of wings from AK (where there should be no
western influence) from summer, there are obvious slightly darker gray
spots at the wingtips corresponding to the black tips in other gull
species. The more proximal parts of the primaries are about the same color
as the mantle, whereas in westerns the entire wingtip, including the
outermost primaries, is always darker than the mantle--*not* the same
color.

If one accepts the assumption that mantle color varies with wingtip color,
then there shouldn't be hybrids with primaries much darker than the mantle.
A typical hybrid should be rather inbetween--dark gray wingtips, mantle
about intermediate between western and glaucous-wing. The birds that Neil
Fergusson described as having "jet black wing-tips, but sporting a mantle
of the same color as a glaucous-winged" by my definition could not be
hybrid GWGU x WEGU. Mantle and wingtips do vary somewhat independently in
hybrids, but not enough, in my opinion, to produce birds colored like this.
There are no such birds among the many, many specimens of this group in
the Slater and Burke museums. By my criteria these should be herring,
Thayer's, or possibly herring x glaucous-winged hybrids. This last type is
apparently not all that rare in this area and may be manifested by a bird
that looks like a herring but has brown eyes! At least that's what we're
calling such birds; if they're not HEGU x GWGU hybrids, something else is
going on.

An important thing to remember about Thayer's is that its eye color varies
from fairly dark brown to yellow, perhaps a variation that comes from
intergrading with Kumlien's Iceland gulls in the eastern part of its range.
And it's not correct that Thayer's breeds "not far to the north" of us,
Janet; they're way over in the central Canadian Arctic. But their primary
winter range is on this coast, so this is the place to look at them. They
are locally common and were in amazing numbers on the Tacoma waterfront
where the slaty-backed spent last winter. They are also common around
Victoria (I've seen hundreds there) and are, I would say, the third most
common gull on salt water here in winter after glaucous-winged and mew.

Herring gulls seem to be characteristic of fresh water here in winter, and,
as Neil wrote, Green Lake is a fine place to look at them. On the diving
platform at the northwest corner of the lake you can see as many as 6-8
gull species in late fall, most of them into the winter. It is mostly
glaucous-wings and ring-bills that hang around the shore, but sometimes
Thayer's, especially immatures, perch right on the concrete wall and allow
close approach. Both of these species are at gull roosts anywhere on Lake
Washington, too. Magnuson Park usually has a few of each, along with
glaucous-wings, ring-bills, Californias, and mews, even pure westerns, and
sometimes they are in a parking lot where you can drive right up to them.
Log booms in the southeast corner of the lake are often covered with gulls.
Bring some bread to any of these places, and you'll be swamped by them,
although the other species often stay back, behind the aggressive
glaucous-wings and ring-bills.

Like Thayer's, glaucous-wings and westerns vary in eye color from dark
brown to light brown, even yellowish. Some of this is probably due to
hybridization (westerns tend to have paler eyes than glaucous-wings). A
good mark for "typical" western is a white head retained through the
winter. I don't know if the fact that so many of our birds that look as
dark as typical westerns have smudgy heads in winter indicates more
hybridization than we realize or just variation in this population. It
really seems as if there is a cline from glaucous-wings (which get darker
to the south) to westerns (which also get darker to the south), but there
*is* a break between paler and darker populations in WA, and hybrids are
still a minority on the outer coast where the two come together, so there
isn't complete interbreeding by any means.

You are right, Neil. Those small-headed, small-billed birds that look
about like the hybrids being discussed probably are Thayer's. That's about
the only way you can tell them apart; some hybrids look amazingly like
Thayer's, and *all* of these gulls have the underside of the primaries at
least a little paler than the upperside, hybrids perhaps more so, but
Thayer's exhibits the extreme of this attribute.

Glaucous gulls are rare anywhere in the Northwest, although a few are
around every winter. Most are immatures, classic white or pale buffy-white
birds with bubblegum-pink bill base and black tip. Beware glaucous x
glaucous-winged hybrids, which are also around. They can look pretty
darned white but usually have mostly blackish bill and maybe aren't as big
as a typical glaucous (but some glaucous are as small as glaucous-winged).
Also beware of immature glaucous-wings in late spring; they can be so faded
as to look essentially white, but they have entirely black bills. It's a
jungle out there when you commit yourself to gull identification!

I would study gulls in late fall and winter, just after they have completed
their wing molt. They all get paler in spring and especially in summer
before they molt into their next year's plumage, and they can look
sufficiently scruffy to defy categorization. And we're talking only about
adults here; the immatures are another topic altogether.

As Russell wrote, if anyone is interested in looking at gulls in the hand
(including lots of spread wings), you are welcome to come to the Slater
Museum to do so, by appointment. In the field, Bob Sundstrom often does
gull workshops at Green Lake in fall, advertised in the Seattle Audubon
newsletter.

Gull identification is easy on the Atlantic coast when compared to this
area. I have watched gulls closely, photographed them by the dozens, and
examined a lot of specimens, fresh and stuffed, and there are still
individual gulls I hesitate to name. This is of course true primarily with
the large pink-legged species, but even in the smaller size range of
California, ring-billed, and mew, the occasional bird seems puzzling.

Well, only one more comment. I wish I could add these essays to my list of
publications! I think a lot of people are doing a lot of their writing on
e-mail now, at least some of it of lasting value, but destined to remain in
the memory banks of main frames rather than be out on the printed page and
accessible to everyone. I guess it's just another one of our many class
systems. There are ways to search for old e-mail messages, and most
bulletin boards are archiving them, but I wonder how much will actually be
stored over the years. And just think how much we write that *isn't* worth
storing, yet it'll be there too.

Dennis Paulson phone: (206) 756-3798
Slater Museum of Natural History fax: (206) 756-3352
University of Puget Sound e-mail: dpaulson at ups.edu
Tacoma, WA 98416