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Hi Andrew, your observation seems to have ended up in the in the invertebrates section - if you click on "Edit", above, you should be able to re-assign it to the Birds group.
Entomologist and biological recorder
I can't see any reason why this isn't a Willow Warbler. You can listen to brief clips of the song on the RSPB website to try and confirm (is you remember what it sounded like!). Willow Warbler has a cascading trill for a song. http://www.rspb.org.uk/wildlife/birdguide/name/w/willowwarbler/index.aspx
If you add the ID I'll agree with it!
I would go with willow warbler as it has brown legs (chiffchaff has black) also a distinctly yellow hue to the chest (more prominent than chiffchaff) and finally the distinctly light lower bill (again not so prominent in chiffchaff) but the main point is the legs.
The features visible in this photo, incuding those mentioned above + the prominent face pattern, all point towards Willow Warbler but none are really diagnostic. The fact that several features point more towards WW than Chiffchaff supports the observers opinion, which was based on song, and does seem to make WW the most likely ID.
It's remarkable just how similar these two birds are and from this photo it could be either. I would go with Willow Warbler on the strength that the observer heard its song. Chiff Chaff are unmistakable and if it wasn't a Chiff Chaff then Willow Warbler is most likely.
Just go out there and do it!!!
Lat/Lng: 54.5, -0.9
OS grid ref: NZ7512