I agree with whoever it was who just said that the colour of the inside of
the mouth is of no use in identifiying Australian vs. Oriental Reed
Warblers. I've seen the inside of the mouth of hundreds of Australian Reed
Warblers banded in Victoria, and the story is very consistent here - bright
orange in almost all birds, but with a strong pale yellow tinge in juveniles
(which also have age-diagnostic black spots on the tongue). Similar story
applied in the 10-15 Australian Reed Warblers I've handled in north-west
Australia. It is also true of all Oriental Reed Warblers I've seen photos,
reliable museum labels or primary literature on. I had the pleasure of
handling the Broome Oriental Reed Warbler (8 March 1998) - that was an adult
with bright orange mouth identical in colour to adult Australian Reed Warblers.
I've never seen a pink-mouthed Australian Reed Warbler, and doubt that they
occur in South-eastern Australia. There is some geographical variation in
Aussie Reed-warblers though, so I wouldn't be bold enough to say that this
can never occur in northern Australia. Mind you, I would like to see photos
of it. A problem with relying on museum label data on mouth colours is that
it can change quite rapidly post-mortem, and you can rarely be sure that
mouth colour was recorded before discoloration occurred.
I also have reservations about ID of these Reed-warblers on bill structure.
I think Orientals are slightly heavier billed, but assessing this is quite
subjective. They are definitely not shorter-billed - in fact their bill is
longer! This doesn't stand out out as you might expect, as Orientals are
bigger than Australian Reed Warblers in all dimensions - so their bill
length doesn't look proportionately different.
So how do you identify these beasts? In the field it is difficult unless the
bird calls. Their song is really distinctive - it is recognisably
reed-warblerish, but it is a loud harsh chatter of 'cucks' and 'churrs',
lower pitched and more repetitive than Aussie Reed Warblers, without the
melodious ascending notes which make Aussie Reed Warblers so pleasant to
listen to. Most but not all Oriental Reed Warblers have distinctive, short
coarse dark streaks on the foreneck, much bolder than the very faint
hair-thin streakings which can be seen on the same area of the occasional
very worn aussie Reed-warbler. It is also worth looking out for white tips
to the underside of the tail feathers - they were quite clean and broad in
the Broome Oriental; they are absent, or narrower with a murky pinkish buff
tinge, in most Aussie Reeds. Back colour might turn out to be useful in the
long run, but more needs to be known about the geographical and wear-related
variation that occurs within Australian Reed-Warblers - they certainly
average darker in WA than in SE Aust. The Broome bird had upperparts
colouration much like the local Aussie reeds.
In the hand, the larger size of Oriental Reed Warblers will usually stand
out - there is almost no overlap in the measurements of Oriental and
Australian Reed Warblers. The Broome Oriental was a large male and the size
difference was very striking - it felt a bit like handling a small thrush!
Judging this in the field without some calibrating size measure would
however be risky. Wing formula should be taken where ID is troublesome, but
I think it is over-rated and that some published literature on the wing
formula of Australian Reed Warblers is wrong - the longest primary in
Oriental Reed Warblers is primary 8, but this seems to me to be the case in
nearly all Australian Reed Warblers too. It is also worth checking primary
wear - Oriental Reed Warblers perform their primary moult near the breeding
grounds before migrating south, so in eg. Feb-April will have quite worn
primaries when those of adult Aussie Reeds should be moutling or new.
As for the reed warbler foraging in the crowns of Pandanus - that's new to
me! I have seen them quite high in trees, but only when the trees are very
close to reed-beds. I'd be interested to know if the bird was moulting.
Maybe it was in transit? It never ceases to amaze me how the most isolated
and tiny reed-beds imagineable in inland and tropical Australia often turn
out to have one or two Reed-warblers within - presumably they must at least
pass through other habitats to reach these isolated spots.
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