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Definitely a willow tit as marsh tit lacks the pale secondaries (wing panel) this bird clearly shows
Respect Ophrys opinion; however the cap looks dull to me and the light wing panel makes me think this is a Willow Tit.
Chris Brooks - www.dragonfly-images.co.uk
My Flickr site - www.flickr.com/photos/ceb1298
The pale spot on the cutting edge of the upper mandible, and two tone cheeks are two very good features that point to Marsh Tit as the correct ID. The presence/absence of a pale wing panel is rated of less importance than these two features - and I don't think that there is a genuine pale wing panel on this bird. The apparent pale wing panel is present on the primaries as well as the secondaries (it should only be the secondaries), and in my opinion is a photographic effect caused by slight over exposure.
Yes, just to confirm that I did not mention the wing panel as I do not think there is one. The key thing about the wing panel is that it is paler than the mantle on Willow. That is not the case here, as the whole of the upperparts look washed out by an effect of the light.
They are difficult, but you get a feel for them after a while. I am lucky in living near a site where both species are common still.
My Flickr photos...
Lat/Lng: 50.571, -3.76
OS grid ref: SX754760