I always feel that the spring has milestones; as you pass each milestone so summer gets closer until you suddenly realise summer has arrived! The first milestone is the first chiffchaff singing, then the first swallow over head, then comes hearing the musical notes of the willow warbler (Phylloscopus trochilus), usually in the second week in April. Bird song is not normally like human made music but the closest to it must be the song of the willow warbler as it enthusiastically sings its phrase of descending notes down its preferred scale, there is nothing quite like it elsewhere in the bird kingdom in my opinion.
Unless you have one in the hand (having been caught whilst ringing) I defy anyone to confidently tell the willow warbler from the chiffchaff by plumage alone. They are so very alike in appearance and even have similar habitat preferences. In spring, though, it is not a problem distinguishing between them because, having just arrived back from their wintering quarters in Africa, the males of both species are in good voice and establishing territories. The willow warbler's lovely, cascading song phrase is in stark contrast to the continual, monotonous repetition of two notes offered by the chiffchaff.
Once the singing ends, however, the problems with identification start and many an observation has to be put down as a 'willow-chiff' and remain undetermined. If pushed I would say the willow warbler is slightly greener in appearance than the more buff coloured chiffchaff but, as I said, you need to see them at close quarters to be really sure.