Upright bird, moving along the ground near hedgerow, blackbird size or slightly larger. Near marshy area at coast.
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Size can be very hard to judge, sometimes, on an individual bird. In fact, Dunnocks are barely half the size of some Blackbirds!
Was this picture the bird you are describing? I only ask as usually a Dunnock would not be described as an upright bird (rather they are sometimes said to be mouse-like and creeping), and with the size as well...just wondered if perhaps you had put the wrong photo up? :)
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No, right picture, but it does have us flummoxed. I initially thought it was a fieldfare due to size, grey on the head but after checking the books I could see this wasn't right. There were two of them both keeping near the hedgerow and near the ground. Upright when on the ground and definitely bigger than a normal size dunnock -blackbird size.
My first comment about size applies here, definitely - that is to say that the bird in the picture is almost half the size of a Blackbird, pretty much. You just do not get huge variation in size within birds like this. Bird ringers expect a Dunnock to have a wing length of 70mm...the deviation from that is only a millimetre or two either way. In contrast, Blackbird wings are above 125mm. You can see the difference in size which that suggests.
Perhaps it is the problem of seeing a familiar bird in an unfamiliar place and so having nothing to get a real sense of scale. It always strikes me how, on misty mornings, everything seems huge compared to normal. I expect something similar applies to your Dunnock!
Lat/Lng: 50.72, -1.761
OS grid ref: SZ169911