Given the lack of any response to the heartfelt plea on the GOS sightings page, I feel it's only fair to scribble a brief discussion of the Magor 'Marsh Warbler(s)',...
First off, I dunno what happened on the Tuesday evening, as far as I know nobody but the original observer was present and it seems all the fun the following day occurred at some distance from the location of the initial sighting,... gawd knows!
Wednesday morning, several observers visited the site, including myself, prior to the claim of the bird being present, no Marsh Warbler was heard or seen. However, a Blackcap with an abnormal song was present in the wet woodland behind the hide and, nearby, Reed Warblers were dotting about on the edge of the reedbeds. A short recording of the Blackcap is available here during which the bird begins with a few
more-or-less normal phrases followed by a long series of barely
separated phrases which take on a very non-Blackcap rhythm. By the way, there is no doubt this song emanated from a Blackcap, I watched the bird opening and closing its bill during the recording.
Anyhoo, I left at around 11:00 without even considering the fact somebody might misidentify the Blackcap for a Marsh Warbler. Within 45 minutes I started receiving tweets saying the Marsh Warbler had been relocated, I queried whether it had been seen or only heard but, before I could send a message along the lines of 'I had a funny Blackcap there this morning', I was told it had been seen. I was slightly miffed at having failed to find it and proceeded to turn the air a wonderful shade of blue. Then, over the following hours, news dribbled through the various channels that something might be amiss and the consensus of those present was that a, now skulky, Blackcap [a skullcap p'raps?] was the bird singing behind the hide. Presumably, the observer who had claimed the sighting had heard the Blackcap and seen a Reed Warbler.
Strangely, nobody on site appeared to inform the rare bird news services so, when I arrived back at Magor in the evening, to try for more recordings of the Blackcap, people were still appearing thinking a Marsh Warbler had been present, largely on the basis of the "showing well" report,... and that is how I think it happened, largely based on guesswork, half-truths, reading between the lines and half hearing distant jungle drums, feel free to leave a comment if you know any better.
PS. One last thing, whilst I need Marsh Warbler for my Gwent list, and would hate to miss out on a suppressed individual, they are a Schedule 1 breeding species, in the first instance it might be best to inform the county recorder before releasing the news. Birds showing behaviour indicative of breeding may well have to be kept quiet, at least until the chicks hatch.