The Willow Warbler is very similar to a Chiffchaff, but their legs are paler and they tend to have more pale yellow in their plumage, plus a more pronounced supercilium (eye stripe). However, this is not always reliable or clearly visible. They do have a very different song - a sweet lilting, descending whistle. For me, it's the descending of notes that make this bird easy to recognise, as it is so different from the repeating "chiff chaff" song. They are quite elusive birds, hiding in the foliage. They can be found in the Spring through to Autumn, in the trees, especially willow (as this one was).