Chiffchaff singing in September

Anne, Derek and Andy have all heard Chiffchaff singing at various locations around the village during the last week or two.  Anne has seen this Chiffchaff in her garden for the past three days, her pictures nicely showing the dark legs and short 'primary projection' which help distinguish this Chiffchaff from the very similar Willow Warbler.  The entry for Chiffchaff in Anne's bird book indicates that "Bright autumn days may cause migrants to burst into unseasonal song".