Hi Andrew,
First off, well done on getting so many images of this bird that, while not of photographic field guide quality, are still mostly quite sharp and allow a surprising amount to be seen. It's more the fault of the ID issue at hand than the quality of the images that things aren't clear-cut!
I might thank Mark for drawing this bird to my attention. First off, I might point out that I have no immovable opinions here, set in rock. I will admit that my first impressions were that the bird looked to show appeal for Marsh Warbler, then I began to think that it maybe was just a 1cy Reed Warbler whose bill had yet to fully grow, and now, the more I look, I see features that do suggest that the rarer option of Marsh Warbler may yet be correct. I think it's safe to say that nobody really thinks that this could be a Blyth's Reed?
Anyhow, as I see it, first things first, the bird needs to be aged. I think that the generally fresh appearance strongly points to this being a 1cy bird. Therefore, while that could support a hypothetical young Reed Warbler whose bill was still growing, 1cy nominate Reed tends to be much warmer-toned above than the subject bird, with rustier flanks also. Therefore, the fact that this bird is more sandy brown with olive elements (especially on the mantle) would suggest either fuscus Reed Warbler (so-called 'Caspian Reed Warbler') or a Marsh Warbler.
The leg colour is sort of pale, and not what one typically sees on a Reed Warbler, but variation makes its usefulness somewhat questionable, to say the least. All the same, this doesn't take anything away from this bird's prospects of being a Marsh Warbler. Claw colour is more useful, and the uppersides of this bird's claws appear pale, another pro-Marsh feature but some Reed Warblers can also show pale uppersides to their claws.
The primary projection is long, but not too long, and seems comfortably within the range of variation shown by both species. I have strained looking for the exact position of the emargination on the third primary, and while it does seem like it may have laid further back on the wing than I'd like for a textbook Marsh, the fact that this ambiguity, or difficulty, exists either means that the feature isn't perhaps as useful here anyway or else I need to get a new laptop!
Bill shape and length do seem quite good for Marsh, though I suppose, as surmised, a 1cy Reed could still have its bill growing, just to make life harder. The supraloral doesn't stand out as much as on some Marsh, which almost rival Blyth's Reed in that respect, but nor does the eyering appear to dominate the facial expression in the way that it does on many Reeds. In some images, and I appreciate this is just a soft feature, the combination of bill shape, general colour and head shape give the bird a vague passing resemblance to a Common Whitethroat or something, something that I and others have noted on Marsh but it's by no means absolute, just a supporting feature at best.
Regarding calls, I think a lot of what has been written about vocalisations among unstreaked Acrocephalus warblers has been somewhat confusing and/or simplified. There is a common misconception that Reed Warblers just give a rolling 'tr'r'r'r'r' type call, and that is commonly heard from the species indeed, and then, when faced with any Acro that tacks or chacks there can be the assumption that this must be something rarer. I'm not suggesting you're saying that, Andrew, just to make that clear because online fora are a great place to be misunderstood! Just that the misconception exists, despite the fact that the modern literature points to the true situation.
Basically, Reed Warbler does give a 'chak' type call, and not infrequently either. To my ears, it is somewhat reminiscent of the 'chak' of a European Stonechat. Marsh, and I must stress that I have very limited experience of calling birds of this species, can often give a 'tack' call, reminiscent of a Blackcap or something, and very similar to the typical calls of Blyth's Reed Warbler. I don't hear either often enough to be able to seperate them by ear, and I don't have the means (as yet) to go and convert recordings of both from xeno canto into sonograms. The point is that, and I realise that this is somewhat subjective, all these species give tacking-type calls, at times (just as all of them give a more rolling rattle as well, but, again, I am sure that these all look different on sonograms). Reference to 'tacking' rather than 'chacking' here arguably favours a Marsh/Blyth's Reed type call, but one must always be careful of reading intent into another person's words that may never have been there in the first place...
So, in short, I'd hate to have to make a firm conclusion either way, but suffice it to say that I'd not be quick to dismiss it as 'just' a typical Reed Warbler, at the very least, and many features do seem suggestive, some strongly so, of Marsh Warbler.
Regards,
Harry
