Having had the chance to review some excellent open wing and flight
photos sent to me by Richard Baxter and Keith Fisher, I am now confident
that the Cooktown gull is a first winter Mongolian Gull (mongolicus),
formerly part of the "herring gull" group, and often - probably
incorrectly - placed with the Vega Gull (vegae) group.
It is important to understand the detail of the tail pattern, as the
specific pattern shown is actually very helpful in the identification of
this bird. The Cooktown bird shows dark outer webs at the bases of the
tail feathers. On the closed tail, and even on the partially open tail,
this can give the impression of an all dark tail. However, close
examination of the superb flight shots by Richard and Keith shows that
the inner webs of the outer three pairs of tail feathers are actually
extensively whitish, and the outer web of the outermost tail feather
shows quite extensive white spotting. When fully fanned, the tail is
consequently banded - in fact it is quite a narrow band on the outer
part of the tail. This pattern is typical for mongolicus, scarce in
vegae, and unusual in Slaty-backed.
Also important in the tail is the strong contrast between the dark tail
band and the very white rump and uppertail coverts. In taxa such as
vegae, Slaty-backed, and Thayer's, the uppertail coverts are generally
much more heavily marked with brown at this time of year, not leaving
the impression of a very black and white tail area which is typical of
mongolicus. (This effect is usually even more striking in mongolicus
usually, but as mentioned earlier this bird has faded somewhat having
been exposed to stronger sunlight than these northern gulls typically
encounter.)
The pattern of the newly moulted upperparts feathers is extremely
helpful, as I alluded to in a previous message. The large scapular
feathers show a large blackish basal diamond (at least that is how it
appears sticking out from under the feathers above it) and then there is
a narrow shaft streak with a thin subterminal bar we often refer to as
an anchor pattern. This very dark diamond and anchor pattern on a very
pale background colour is classic for mongolicus. On the Cooktown bird
it is a little less clear than on birds in Korea and Japan in winter,
but again this is due to the heavy "wear" (actually photodegradation)
evident throughout this bird. A barred scapular pattern like this
eliminates Slaty-backed, which shows unpatterned second generation
scapulars. The pattern of the original juvenile wing coverts is classic
for "herring" gulls and effectively eliminates Slaty-backed, which shows
much plainer coverts in general. The effect of black-and-white lines in
the coverts (again, dulled in the Cooktown bird) is actually classic for
mongolicus.
The newest (tiny) mantle feathers on the Cooktown bird show large
blackish feather centres, almost giving the impression of black dotting
on the mantle, again classic for mongolicus. The body moult sequence of
mongolicus classically results in a contrastingly white head as shown by
the Cooktown bird. One of the flight shots by Keith shows this to
perfection as it results in the appearance of a greyish collar, another
great feature for mongolicus in this plumage. The same photo also shows
the mantle feathers referred to above, which in combination almost
appear to form a speckled grey saddle, again good for mongolicus.
The flight shots show a dark-light-dark pattern of very dark secondaries
and outer primaries with a strong pale inner primary window typical of
"herring" gulls, and especially good for mongolicus. The secondaries are
too contrastingly dark for thayeri, and the inner primary window likely
too pale for Slaty-backed. The inner primary window is even too pale for
most vegae, and the subterminal dark spots or diamonds on the inner
primaries are very typical of mongolicus. The overall wing effect is
dulled due to the browner faded tones of the Cooktown bird, but the
pattern is distinctively mongolicus.
A combination of the flight and perched photos show the structure well
now. For a "herring" gull, the Cooktown bird has a thick full tail,
relatively short broad based wings, a shortish neck, quite a chunky
body, and a proportionately stubby (short and thickish) bill, all
excellent features for mongolicus. Although these features are of course
somewhat subjective, with experience with all the taxa, these structural
features can be very helpful with large gulls, as the plumage patterns
can be so variable. Some aspects of this structure could have led to an
initial suspicion of Thayer's - which often appears to have a "cute"
bill and tubby body - but the Cooktown bird is too short- and
thick-winged for that species (and almost every plumage feature is also
wrong for Thayer's too!).
The bill structure is a little too "cute" even for the smallest female
Slaty-backed, but fits mongolicus very well. On mongolicus the
extensively blackish bill adds to the effect of a short stubby bill, as
in the Cooktown bird, while similar aged vegae can often show more
extensive pink at the base by February. First year thayeri also have
largely dark bills, but unlike mongolicus and the Cooktown bird, when
the bill starts to pale at the base, it is a darker purplish pink on
thayeri, not very pale pink.
Mongolicus winters from Honshu, Japan (scarce) getting commoner in
Kyushu, through Korea (common, especially on the west coast) and down
the Chinese coast (common). It is by far the commonest "herring" gull in
Hong Kong after the dark-backed and generally yellowish-legged
"taimyrensis". Incidentally, as Nikolaas pointed out,
taimyrensis/heuglini shows characters similar to the Lesser Black-backed
Gull group in all ages, and hence nothing like the Cooktown bird
(although it is otherwise definitely a candidate to reach Aus). The
wintering range of mongolicus in the south-east Asian archipelagos is
very poorly known at best, so its occurrence in Australia is not a
complete surprise.
Many thanks again to Richard and Keith for their excellent photos which
really augment the earlier perched shots and allow a more comprehensive
identification analysis. Once I get back home from my business travels
later today, I will circulate a few photos of mongolicus illustrating
some of the features I have highlighted. I won't go into great deal on
the taxonomy of mongolicus at this point, suffice to say that it is
controversial. The taxon has been variously allied with "herring gull"
(back in the halcyon days of Peter Grants seminal gull work), then more
recently with almost all of the other Asian gull groups at one time or
another, e.g., as part of the Vegae Gull (vegae), the Yellow-legged Gull
group (cachinnans), and the Baraba Gull (barabensis), plus of course
consideration as a species in its own right!
Best wishes, Jon.
>>> "Jon King" <> 02/16/08 8:17 AM >>>
Hi Aus birders,
Repeated below is the text of a message I sent to Mike Carter yesterday
after he sent me a couple of the initial photos of the Cooktown gull.
I have had the good fortune (!) to study gulls in throughout the North
Pacific rim very extensively over the past decade while resident in the
western US and travelling widely throughout East Asia, much of the time
specificially surveying for gulls.
The relatively small head and bill, and the patterning of the scapulars
quickly eliminates Slaty-backed Gull (their new second year scaps are
largely unmarked and clean darker grey, and they are generally much
heavier in the head and bill). The relatively white head, scapular and
covert patterns, and much darker primaries, secondaries and tail largely
eliminate Thayer's Gull, and immediately strongly indicate this bird is
in the "herring gull complex", the taxonomy of which depends very much
on which book you read and personal opinion.
Among the likely candidates (ie. those "herring" gulls occuring in
western North America or East Asia, vegae, mongolicus, smithsonianus and
"taimyrensis"), the latter two can be eliminated on structure and
plumage traits, which pretty much leaves the two common "herring" gulls
of East Asia, vegae, Vega Gull, and mongolicus, Mongolian Gull.
In my note to Mike my initial look indicated that this bird fitted Vega
Gull well, but in my early morning haze I hadn't given adequate thought
to mongolicus. The Cooktown bird is a little browner in the remiges and
tail than is typical for either of those taxa, but that is to be
expected for a temperate bird that has been worn and faded in tropical
sun recently. Pending some more photos, especially flight shots, it
should be possible to distinguish between those two options, as the tail
patterns especially are rather different. What is currently visible of
the tail in perched shots suggests vegae, but some aspects of the
pattern of the new scapular and mantle feathers may lean towards
mongolicus.
I look forward to the ID discussion in due course, and hope to provide
some comparative photos and more commentary when time permits.
Best wishes, Jon.
Jon King
California, USA and West Sussex, UK.
+++++++++++++++++++++++++++++++
Hi Mike,
A quick initial look suggests that this bird shows many of the
characters of the herring gull taxon vegae, referred to under most
modern gull taxonomies as Vega Gull. That was my suspicion when you
suggested the possibility of thayeri, as some worn female vegae can have
a surprisingly similar look to that species. The pattern of the coverts
is typical of vegae, and especially helpful is the pattern of the new
scapulars, which looks classic for that taxon. The exposed primaries and
tail look rather browner than usual, but then it has also seen a lot
more potent sun than usual in its recent travels than a typical vegae
wintering in Japan or Korea!
Some flight and open wing/tail shots would be very helpful, and probably
decisive.
Without checking all my photos from East Asia - I am travelling on
business until Sunday - I would prefer not to be definitive but that is
my very strong suspicion. Great record (although not entirely
unexpected).
Will be in touch again with more detail shortly, including comparative
shots of similar birds from Japan and Korea.
Cheers, Jon.
>>> richard baxter <> 02/15/08 8:40 PM >>>
Hello all,
I'm on my way back from twitching the vagrant gull which is
currently in Cooktown FNQ. For those intending to head up that way, the
bird is fairly easy to find and was, this morning approachable to within
2m. One of the locals was feeding it his left over bait and chicken..
The bird readily ate everything thrown near it.
To find it, drive into the main street of Cooktown and down to the boat
ramp/jetty area. It has been seen daily in this 300m stretch for 3 days
now. The road from Cairns is all tarred and fine but be very careful of
wandering cattle at night.
I've taken over 100+ pics, so now its time to hit the books and try to
ID it. So far its been narrowed down to Thayer's, Herring,
Slatey-backed, Vega, the list goes on!!
Cheers
Richard Baxter
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