Looks to me like a wren but sat at the top of this dead tree for ages. Perhaps it was just getting warm in the early morning sun!
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It's actually considerably larger than a Wren, with a white rump, long wings and a more upright habit. Wrens are very small, dumpy birds, all cryptically brown coloured and with a very short, erect tail.
As you say, the behaviour was probably because it was warming up. Normally, they are actively hopping around on the ground, standing erect on top of stones and so on.
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Thanks very much. You've made my day!
Lat/Lng: 54.1676, -2.8008
OS grid ref: SD478749
RSPB Leighton Moss