Chiffchaff

Chiffchaff - Andreas Trepte. Licensed underCC BY-SA 2.5 via Wikimedia Commons

Our Gilbert (White) separated this bird from the almost identical looking willow warbler and the slightly less similar wood warbler, back in 1789. He did so by realising their songs were very different.
They still are and chiffchaff, Phylloscopus collybita, is one of the easiest bird songs to recognise (up there with cuckoo).
They are migratory, spending winter on the Med and North Africa, but in the last couple of decades a number of birds stop their southward journey in southern Britain. I see them around Lewes all year round. It is always great to hear the first male singing its onomatopoeic song.