Originally Posted by Howard King
I recieved this message from an UAE birder on the subject
We saw 130 Marsh Warblers in the UAE today and this bird looks fairly similar. However it appears to lack the olive green upperparts and buff-yellow underparts of Marsh and perhaps has a slightly-too-prominent supercilium. I would therefore go with Reed (Eastern / fuscus presumably) but for me the light does make it impossible to say for certain. Rump colour (if noted) would be another clue (should be warm brown in Reed at this time of year but more or less concolorous with mantle in Marsh).
Blyth's Reed is another possibility but it appears to lack the 'flattened head' of that species, and also the dark tip to the lower mandible.
But your UAE birder fails to say why this bird clearly lacks the long undertail coverts of an acro. This is a much better feature to look at on a bird thats partially in shadow and in strange light.
I'd be happy to say that this could be an Eastern Olivaceous (i've been struggling to find pics of EOW in spring - best i could do was http://www.fyldebirdclub.org/images/14eoliler.jpg)
, but the apparent size ond fullness of the tail still suggests Upcher's to me. The description of the tail movement is slightly ambiguous.
In the photos on your web page, its interesting that the tail looks contrastingly darker, almost blackish in all of the photos it is visible in, especially the head on shot second from bottom where the tail does not appear to be in shadow. Its hard to see the tertial spacing, and i found this to be an unreliable feature anyway, with many EOW showing the classic Upcher's spacing. Just the overall jizz of the bird suggests Upcher's to me.
We can rule out Booted Warbler on the seven primaries beyond the tertials (i think Booted has 5-6) and the prominent super in front of the eye and lacking behind the eye. PLus the jizz would seem wrong for that species. But then i suppose you have to think about Sykes's Warbler which i've never seen...
IMO this is almost certainly an Upcher's, but can't 100% rule out Eastern Olivaceous. Its certainly one of the two however, and is clearly not an acrocephalus