A well-known harbinger of spring, the arrival of the cuckoo (Cuculus canorus) 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 (6).
The cuckoo is the only 'brood parasite' to breed in Britain (3). Individual females prefer certain foster birds, and lay eggs that closely mimic those of the foster species, 50 of which are known (3). A female will establish a territory encompassing a number of potential foster nests, and carefully observe activity, waiting until the nests are at the right stage. She then swiftly takes her chance, swooping down, ejecting an egg and laying one of her own (3). The unsuspecting host bird then incubates and feeds the impostor, who removes other eggs and young from the nest and often grows much larger than its foster parent (3). Female cuckoos usually lay fewer than 12 eggs in 12 different host nests each year (3). Cuckoos feed mainly on insects, spiders and worms (3).
The cuckoo arrives in Britain during the second part of April from Africa south of the Sahara, and leaves in September (2). It is widespread in Britain and breeds throughout Europe, reaching as far east as Japan (3).
The cuckoo is classified as Least Concern (LC) on the IUCN Red List (1). A widespread summer visitor to the UK (3). Included in the Birds of Conservation Concern Amber List (medium conservation concern) (4) and protected at all times under the Wildlife and Countryside Act 1981 (5).
The Breeding Bird Survey (BBS) has shown that the population of the cuckoo in Britain has declined steadily; this is thought to be due to a decrease in populations of key host species such as meadow pipit and dunnock (7), probably due to habitat-related factors (5).
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