Pale brownish back,buff front and distinct Stripe by eye,
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Unfortunately there is possibly insufficient in these photographs to be certain - did you hear it sing / call? If so, if it had a liquid descending tune it is likely to have been a Willow Warbler, if it sang its name repeatedly it was a Chiffchaff!
All the best
Chiffchaff or WW on these pics.
I could hear a bird and it definetly wasn't the call of the chiffchaff, I saw a willow warbler in the garden a few days before which made me think it was another willow warbler, also have once seen a juvenile willow warbler in the garden one august and emailed the photo of it to the Bird Magazine who identified it for me, dont think ispot was up and running then, so know they pass through both in the spring and then autumn but have never seen or heard the chiffchaff in the garden. many thanks for looking.
Identifying these two by sight alone is non trivial, but in future look to see if it wiggles its tail - apparently only chiffchaff does this, and not WW (and it's true for any bird I've observed so far, I think).
Also willow warbler's tend to have paler legs (chiffchaffs can look quite dark) and have longer, more pointed wings (as they are longer distance migrants), but again neither of these is that reliable without hearing the bird sing!
Undoubtedly the best indicator of species is song as Chris says. If they are not singing the most reliable field characteristic is ‘primary projection’. Simply this is the amount the primary feathers stick out from the tertiary feathers. Short for Chiff, long for Willow.
Bird books tend to make leg colour and so on sound absolute but in practice there is much individual variation; so a Willow Warbler with darker than normal legs can look very similar to a Chiffchaff with lighter than normal legs. The same applies to just about every other characteristic. Ringers who regularly catch them occasionally find individuals that are indefinable even in the hand.
Lat/Lng: 51.8578, 1.1159
OS grid ref: TM146222