Sparrow-sized but slim and with a long, deeply notched tail, the male has a black head, white collar and a drooping moustache. Females and winter males have a streaked head. In flight the tail looks black with broad, white edges.
A small olive-brown warbler which actively flits through trees and shrubs, with a distinctive tail-wagging movement. Less bright than the similar willow warbler and readily distinguished by its song, from where it gets its name. Picks insects from trees and also flies out to snap them up in flight.