Bird on a Wire

I think it's some kind of warbler. 

A couple of birders watching it swore to me it was a whinchat, and I nodded sagely because, well, who wants to look the birding dummy? I don't think it is, though. And now you're going to tell me it is...

Anyhoo, that was shot at Slimbridge, out on the reed beds. Whatever it is.