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The easiest way to identify these birds is by the song, the Chiffchaff is named after it's call ie chiffchaff chiffchaff, the Willow Warbler has a soft descending whistling song. Another way is to check the leg colouration not always easy, the Chiffchaff has dark or black legs where the Willow warbler has pale legs.
It would be a brave man to call the id on this fella. Not being able to see the primary projections in order to determine wing-length, or have a good view of its leg colour, plus the fact that it's not singing, I wouldn't stick my neck out! It's a Chiff-willow.
yes my thinking of it been a willow warbler was a close up of the legs seemed lighter than what iv seen on chiffchaff.
I would agree that it would be unwise to give an ID based on just this one photo - once you know what you are looking for it is often much easier to identify Willow/Chiffs in the field (especially if they are vocal).
Be wary of relying on leg colour though because Chiffchaffs can sometimes have pale legs (usually with dark feet), and leg colour is sometimes hard to judge objectively anyway.
Lat/Lng: 51.2, -2.7
OS grid ref: ST5250