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Cooktown Gull

To: "Jon King" <>, "Kingfisher Park Birdwatchers Lodge" <>, "richard baxter" <>, <>
Subject: Cooktown Gull
From: "Geoff Carey" <>
Date: Thu, 21 Feb 2008 16:27:03 +0800
Dear All,

I was invited by Jon and others to offer my opinion on the large gull at 
Cooktown, photos of which I was subsequently sent from a number of sources, for 
which many thanks. I have attached 8 of the 10 as reference to what I have been 
looking at, as the other two were not so helpful and were of a large file size. 
I have the following thoughts:

My first impression was that this was a Slaty-backed Gull, similar to the kind 
that we see here in HK. In particular, picture 4 of the attached set shows the 
paler head, dark bill, dark underwing and belly and dark tail that I have come 
to associate with this species. On further examination, I still feel it to be 
this species.

SBG does not generally have an all-dark tail as far as I'm aware, and the 
pattern shown on this bird, with paler bases to the outer rectrices, appears to 
fit comfortably. Although the rump and uppertail coverts are contrastingly pale 
compared to the tail, there is still fairly dark and broad dark barring in this 
area, and the contrast is not startling. Mongolicus, on the other hand, does 
not seem to show such dark or broad barring this late in the winter, and the 
contrast with the dark tail band is heightened.

As Jon has indicated, the large scapular feathers of mongolicus show a large 
black basal diamond on a very pale background. However, to my eyes this pattern 
is not of the type shown by this bird, which has a rather more complex, even 
pattern. This kind of pattern is shown in Plate 30.8 of Howell and Dunn (Gulls 
of the Americas).

I find it difficult to make out exactly what the pattern of the wing coverts 
is, but it doesn't seem to me rule out SBG. They do not seem pale enough for 
mongolicus.

A pale head contrasting with a darker body and wings, as shown in Photos 4 and 
5, seems just right for SBG to my eyes. Certainly mongolicus also have a paler 
head, but there is far less contrast with the body, in general. The pattern of 
nape and mantle also seem, to my eyes, to be fairly typical for SBG.

Certainly the most troubling feature for SBG is the paler inner primary window. 
I have been unable to trace a photo of a bird showing a similarly pale window. 
Certainly the subterminal pattern of these primaries is fine for SBG (e.g., see 
Plate 30.24 of Howell and Dunn, and Plate 504 of Olsson and Larsson: Gulls of 
Europe, Asia and North America). I believe the inner primaries of mongolicus 
are paler still, and would show significantly more contrast.

An important feature in favour of SBG is the dark underwing primary and 
secondary coverts and tertials. Mongolicus tends to be much paler in this 
region, and the axillaries show rather fine, darker barring only. Although this 
bird appears to be rather soilded on the belly, it does appear to be genuinely 
dark in this area, which would fit SBG. The legs also appear rather a dark pink 
in Photo 5, which favours SBG over mongolicus.

Regarding structure, Jon described it thus: thick full tail, relatively short 
broad based wings, a shortish neck, quite a chunky body, and a proportionately 
stubby (short and thickish) bill. An accurate summary, but one that I feel fits 
SBG! With regard to the bill, it is certainly rather more petite than many 
mongolicus, but, again, I don't think it is out of range. It would suggest this 
bird is a female. In addition, mongolicus at this time of year usually shows 
rather more pale at the base of the bill than this bird. Similarly the head, 
while small, it appears within the range, especially for a female.

Regarding vagrancy potential, although, as Jon points out, mongolicus is the 
commonest 'Herring' Gull in south China, it appears to have little track record 
of vagrancy. It appears to have occurred in Thailand, though it is probably 
regular there, rather than a true vagrant. SBG has also occurred in Thailand 
(based on a photo I saw earlier this year), and, more significantly, I believe 
it has also occurred in Mexico and Hawaii. It does appear to wander more than 
other large gulls of the north Pacific.

I don't believe it is vegae, due to the broad tail band, relatively 
poorly-marked pale inner primaries, stocky, broad wing-based structure, dark 
underwings and belly, and, possibly, the rather liver-toned pink legs.

Whatever species it is, a great record. I presume this is the first record of a 
northern large gull in Australia.

Regards,

Geoff Carey
_________________________________________
Director
Asia Ecological Consultants Ltd
127 Commercial Centre
Palm Springs, Yuen Long
Hong Kong
Mobile (+852) 94399250
Tel. 24862885
Fax 24718389
www.asiaecol.com.hk
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