looks like a Siberian Chiffchaff it is one (amazing that) there seems to be more regular sightings. At least three Siberian Chiffchaffs were at Burton Marsh as well as the Buff-bellied Pipit.
Note: since writing the above I have discovered that at least ten Siberian Chiffchaff (tristis) look a likes have called like Chiffchaffs (collybita). So it's back to listening for the call I'm afraid, as no one yet knows what the ones that look like Sibe Chiffie but sing like ordinary Chiffchaff are.
Two were really showy, but the constant procession of cyclists tried my patience, as the birds were trying to feed at the side of the track and were disturbed by the bikes. Even so I was pleased with the selection of photos I got.
They looked even paler grey as I watched them through my bins and the Bonelli's Warbler like green on the edge of the wing was quite distinctive.
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