I headed down there first thing this morning, finding Mark Dawson already on site, who had located the bird. We were joined shortly after by Tom. It was singing intermittently and somewhat quietly from the riverside shrubs and willows. It didn't show particularly well, and was obscured a lot of the time, but it did emerge on occasionally, and responded to two short bursts of Siberian Chiffchaff playback by flying towards us and over our heads.
The brevity of the views, and my desire to get some pictures meant I didn't scrutinise it as much as I would've liked, but my impression was it had greyish-brown upperparts lacking any olive component, buffish-white supercilium and underparts without any hint of yellow (with buffer flanks and upper breast and paler belly), very black bill and legs, and a faint green tinge to the fringes of the remiges. So good for a Siberian in terms of plumage.
I have no prior experience of singing Siberian Chiffchaff - it's a wacky and distinctive song. Listening to recordings on Xeno Canto, this bird sounded like a good match, albeit delivered at perhaps a slightly slower pace than some (but I'd need to listen to more recordings to be sure of this). Several phrases of song can be heard in a video I posted on twitter. However, it did occasionally throw in the odd standard Chiffchaff phrase, audible in this clip... It wasn't heard to call.
Further research suggests that this may throw a safe ID of this bird as a tristis into doubt. This article in BB indicates that 'fulvescens' needs to be considered, which may (or may not) be an integrade occurring in the overlap zone between abietinus and tristis, and that such birds can be mixed ('intermediate') singers - as in this case. The article (and the Handbook of Western Palearctic Birds) indicate that 'fulvescens' can look virtually identical to tristis... So maybe that's what we're dealing with here? Read the article though - it's complicated!
However, I do wonder if this could be a young bird (i.e. a 2cy), exhibiting song plasticity - i.e. responding to the Common Chiffchaffs it is hearing singing around it (of which there were a couple). My photos aren't really good enough, but I think a moult limit is visible in the greater coverts which would support this ageing. Note also that the BB article, on page 403 states "a certain amount of ‘mixed’ singing can result from one species adopting part of the song of another species which is present at high density in its natal area, and is not necessarily proof of hybridisation".
Anyway, whatever it is, it's come from a long way east... and this is one of the things I love about birding; there are always things to learn, even from a bird on a scruffy bit of land behind a retail park in the middle of Newark!
