Bird with rusty brown head - not sure if this is a tree sparrow? About that size.
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...or possibly a juvenile of either sex. Once they undergo their post-juvenile moult, in the next month, they develop the appropriate colour on their cap. Can't see whether it is an adult or juvenile from the pictures.
My Flickr photos...
... for the ID and comments. I have not noticed Blackcaps in the area before.
Looks like a youngster to me. Don't you think the rufous-brown cap gives it away? - a bit richer than an adult.
Probably. Difficult to trust the colour in a photo, though.
Yes, I'm sure you're right about that. Problem with me is that I arrive at a conclusion based on instinct/intuition, unlike your own approach which is, correctly, more scientific.
Lat/Lng: 54.5, -6.0
OS grid ref: NW4526