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Hi,
The "slightly paler olive-brown lower back and
rump" that Chris Hassell and I saw on an Oriental Reed Warbler mist-netted in
the Crab Creek mangroves was a subtle marking, and we only noted it because we
had the bird in the hand and did a full plumage description. It certainly didn't
make the bird look pale-bummed in the field (we watched it fly away on
release). The lower back and rump of Australian Reed Warblers is also
subtly paler than the mantle. In short I don't think this is something that
helps in ID.
I've also got reservations about using mouth colour
in identification (cf Anthea Felming's posting). In Australian Reed
Warblers, adults have vivid orange-pink mouths, quite often visible when the
bird is singing. The same applies to Oriental Reed Warbler. Australian Reed
Warblers have yellow to yellowish orange mouths for the first few months of
their life, and they initially also have black spots on the tongue. I suspect
the same thing applies to Oriental Reed Warblers (and other Arocephalus) too,
and wouldn't trust it as an ID feature.
In the field, the two features that I'd consider
really objective for picking out Oriental Reed Warblers are voice and
throat-streaking. The song is distinctive - obviously a reed warrbler of sorts
(If you are familiar with the Australian call) but it sounds harsh and crackly,
without the sweet whistly phrases than make our Australian Reed Warblers so
pleasant to listen too. Most Oriental Reed Warblers, especially males, have
distinct dark streaks on the lower throat; all Australian Reed Warblers lack
this. Some Oriental Reed Warblers (especially females) lack throat streaking,
and if you do see throat streaking in an unidentified reed warbler, you have to
see it well enough to be confident that you aren't looking at dirt, or some
strange shadow effect. Still, throat streaking is the most objective plumage
character I know of for identifying this pair. There are more subtle characters
but it would take a very experienced and confident observer to make use of them;
I certainly don't claim to be able to identify all reed warblers I see in the
field. If you want to get into the nitty-gritty, a good place to start is
Shirihai, H. (1995. "Identification and taxonomy of large Acrocephalus
warblers." Dutch Birding 17: 229-239.)
Cheers, Danny Rogers
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