Hello birders,

This last monday, I observed an unidentified gull at Half Moon Bay, CA
(just south of San Francisco). Fortunately one of the local keen birders,
Ron Thorn, was there and was able to snap a photo. Here is a description and
a discussion. The mystery bird was in its first winter plumage (First Basic)
it did not show any grey on the mantle.

STRUCTURE: A large, bulky gull, slightly larger than nearby Glaucous-winged
(GWGU) and Western Gulls (WEGU). The bill was large and stout, but did not
flare out at the gonys in the way that GWGU and WEGU bills do, it was like a
Herring Gull bill, perhaps on the large side for that species. It was
long-legged and long-winged, obviously longer winged than the nearby GWGU
and WEGU. The seventh primary tip looked to end just past the end of the
tail. The outer two primaries (p10 and p9) were about even in length, so
that p10 was actually hidden by p9. Therefore, p7 was the third primary tip
visible from the tip of the folded wing.

GENERAL: What set this bird aside from the others in the flock was that it
was pale, like a 'white-winged gull' but had contrasting blackish primaries,
tail and tertials. No other bird showed this level of contrast in the flock,
except perhaps some Second Winter Herring Gulls. The paleness of the gull
did not appear to be due to extensive wear, as the primaries, tertials etc
were not heavily worn. The pale appearance may have been caused by bleaching
of the feathers coupled with the fact that the bird was pale to begin with.

BILL: The bill looked entirely black.

HEAD: Pale brown (creamy) with a contrasting black eye. There was a small
bit of darker streaking behind the eye and along the eye line.

BODY: Creamy once again (=pale brown), showing little contrast between the
head, neck and underparts. The mantle was slightly darker in colour. The
scapulars were darker centered and did contrast with the paler back and wing
coverts, however, these were not nearly as dark as the tertials.

WINGS: The coverts were pale, creamy, contrasting with the dark tertials and
primaries. They were sparsely barred with pale brown, the barring was even,
not forming any dark panels of pale panels on the coverts as a whole. The
lesser, median and greater coverts appered concolour, as in some GWGU, but
very different from first winter WEGU. The secondaries were hidden from view
while the wing was folded. The primaries were blackish, without any
noticeable pale fringe. The tertials were blackish-brown, paler than the
primaries and tail, but darker than the coverts and scapulars. Each tertial
showed a LARGE white tip, forming an obvious, wide white tertial crescent.
Each white tip was bisected by a thin shaft streak and sometimes by a
transverse pale brown bar. There was no pale scaloping along the fringe of
the tertials. The pale tertial crescent was noticeably wider than on any
nearby immature gulls.

I was able to glimpse the open wing once, but I did not see it fly. The
secondaries were dark, forming an obvious dark secondary bar and it did
appear that the inner primaries were slightly paler than the secondaries and
outer primaries, but my look was not good enough to be sure.

TAIL: Solidly blackish, except for some pale barring on the outer vane of
the outermost tail feathers. Each tail feather was thinly tipped whitish as
well. No pale 'marbling' was present on the tail.

LEGS: Pink, no different in tone from the nearby adult and immature GWGU,
perhaps a litte brighter than on immature WEGU.

DISCUSSION: Both Ron and I agreed that it was an odd, and distinctive
looking gull. Neither of us had ever noticed a bird like this before. We
wondered if it could have been a hybrid of some type, but we do not think
so. The tail pattern and darkness as well as the darkness of the primaries
lead us to believe that this could not be a hybrid of a 'white-winged' gull
to a dark gull (i.e. Herring x Glaucous). As well, the completely dark bill
makes is unlikely to be a Glaucous hybrid, however, hybrids do not always
appear intermediate between the two parental species. As well, this was not
a hybrid GWGU x WEGU, there were many in this flock to compare with. These
hybrids do tend to look intermediate between the parental species and the
darkness of the body and coverts tends to be closely correlated to the
darkness of the primaries, unlike our bird. We don't think its a hybrid, but
cannot entirely exclude that possibility.

The other options, the two we are leaning towards, is that the bird is an
odd Herring Gull or a first winter Slaty-backed Gull. This is where we need
help. My experience with Slaty-back has only involved adults, both in
Vancouver B.C.. The identification of first winter Slaty-backs is still in
the unknown stage, but I know that some of you regulars to Alaska have seen
them and perhaps you can give us some feedback. There is a photo of a
presumed first witer Slaty-back in an issue of American Birds about 3 years
ago. I looked at this photo and it certainly resembled our bird, except ours
was more massive in structure (perhaps a male?). Our bird was not as pale as
the bird in this photo, but the photo was of a January bird if I remember
correctly, which should be paler on the average than a December bird. The
National Geographic Guide pictures a first summer Slaty-back, this
illustration matches up to our bird only in terms of the body plumage and
covert pattern, the primaries, tertials and tail were much darker than is
pictured there. Grant has some photos of immature Slaty-backs, only one of
which is perching. This photo (495) is of a juvenile moulting to first
winter in November and is much darker on the body than our mystery bird. The
darkness of the primaries and tertials fits our bird as does the pattern of
white on the tertials, our perhaps showing more white than this bird. Each
tertial tip is bisected by a thin shaft streak and has a subterminal
transverse bar, no scalloping on the edge of the tertial. The flying bird
(499) on the next page more nearly approximates the general contrast of the
bird we saw, mind you this photo does look a little overexposed. The tail
pattern of our bird is a match with that of flying bird (498). Finally, I
was able to get a look at a skin of a first winter Slaty-back in the Museum
of Vertebrate Zoology at Berkeley and it was of little use. Its an old skin
that was not put up very well, however it did show the contrast of the paler
body and darker flight feathers, but it was at a more advanced state of wear
than our bird.

So can anyone give us any pointers or advice? If confirmed, which may be
unlikely given our knowledge, this would be the first or second for the
state. The records committee is voting on last years report and photo of a
second winter bird still. For us to be able to conclusively make an
identification we would have to have something discrete that separates it
from Herring Gull, any suggestions? I have been trying to relocate the bird
the last couple of days without luck, the gull flock is big and stuff is
flying in and out all the time so its difficult. I imagine that its still
around, but it may take some time to relocate the bird. I would like to be a
little better prepared next time I see this thing. So do let me know what
else I should be looking for, any Japanese Birdchatters out there that can
help?

Thanks in advance,

Alvaro Jaramillo "You are better off not knowing
Half Moon Bay, CA how sausages and laws are made"
- From fortune cookie, Vancouver
alvaro at quake.net circa 1994
http://www.quake.net/~alvaro/index.html