Chiffchaff or Willow Warbler….? The perennial question but it's not hard to tell the difference with a bit of experience. The more you see them the easier they are to separate. They just look different and usually they will be singing and then there is absolutely no likelihood of confusion. In my opinion, Willow Warblers are a more delicate little bird and usually they have flesh coloured legs whereas the Chiffchaff as you can see, has dark legs. But beware because this is not always the case and it is not a certain way of making sure.  The stripe above the eye is called the supercillium, a word often used by ornithologists and serious birders.  Both Chiffchaffs and Willows have a supercillium.  

Heres' a Willow Warbler that I phoographed in Gambia earlier in the month. It was a nice surprise to see this bird in Africa, (where they over winter),   when I know the species so well in the UK.  You can see how easy it would be to confuse them but look carefully, the suppercillium is more pronounced and the legs are fleshy coloured and the bird as a feel of being longer whereas the Chiffchaff is a little bit stockier.  Beware though, light conditions play a massive part in the colour of a bird when they are photographed.