A well-known harbinger of spring, the arrival of the cuckoo in Britain is eagerly awaited each April (3). Adult males have bluish-grey upperparts and a white belly with dark barring. Females occur in two forms, one is similar to the male but the breast is buff coloured with dark barring; the other form is reddish brown, and often wholly covered with dark bars (2). Juveniles are slate-grey with touches of reddish-brown (2). The familiar call 'cuck-oo, cuck-oo' is imitated by the common name; later in the year females produce a 'bubbling' call (4).