It's interesting to see how educational one rarity can prove as a result of the concentrated attention it receives, in a way that - for most observers - never seems to apply to common species. I am referring, of course, to the London Dusky Warbler, which I noted as showing an olive tone to the secondaries, and about which Roy Woodward has commented on the condition of the tail.
Anyone who thought the Dusky Warblers had brownish upperparts should now be aware that - apparently - it's not as simple as that! Having checked a few references this morning, it's interesting to find that the upperparts of Dusky Warbler are variously described as being from "essentially dull brownish-olive" (Baker 1997) to lacking "the olive tinge above which is shown by many Radde's ..." (Lewington et al 1991). Perhaps more accurately, Svensson (1992) describes them as "(greyish-) brown, with a very slight olive tinge in many birds (1Y only?)", adding that there is "some individual variation in the coloration of the upperparts of this species" (though the latest edition of the Collins Bird Guide - Svensson again - summarises it instead as "rusty-tinged dark grey-brown upperparts"). Clearly, for a Phylloscopus warbler it can be surprisingly variable; the same might also be said about its bill and leg colour (again variously, and occasionally wrongly, described in some of the above references).
Roy was correct to point out the anomalies with the tail, which should have 12 feathers but which appears to have probably only nine - see the new images which I've just uploaded to my Flickr site. The bird fanned its tail often while moving about and feeding, revealing that it had shed some feathers. From Svensson 1992 again, it seems that the moult in this species is variable and not fully understood, with some adults undergoing a complete moult in winter as well as in summer. But with first-years also partially moulting in winter, and in view of the slight olive tone on the secondaries - as well as the fact that most vagrant 'Sibe' passerines tend to be youngsters - I suspect this individual is probably in its first year.
Now, who would honestly have given as much time and consideration to an individual Carrion Crow, or House Sparrow? Even if lack of field experience is partly the reason, it's so often more the rarity 'value' of the species, rather than the bird itself, that piques our curiosity.