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
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