Abstract
The similarity between many Old World parasitic cuckoos (Cuculinae) and Accipiter hawks, in size, shape and plumage, has been noted since ancient times. In particular, hawk-like underpart barring is more prevalent in parasitic than in non-parasitic cuckoos. Cuckoo-hawk resemblance may reflect convergent evolution of cryptic plumage that reduces detection by hosts and prey, or evolved mimicry of hawks by parasitic cuckoos, either for protection against hawk attacks or to facilitate brood parasitism by influencing host behaviour. Here, we provide the first evidence that some small birds respond to common cuckoos Cuculus canorus as if they were sparrowhawks Accipiter nisus. Great tits and blue tits were equally alarmed and reduced attendance at feeders during and after the presentation of mounted specimens of common cuckoos and sparrowhawks, but not in response to control presentations of collared doves or teal. Plumage manipulations revealed that the strong alarm response to cuckoos depended on their resemblance to hawks; cuckoos with barred underparts were treated like hawks, while those with unbarred underparts were treated like doves. However, barring was not the only feature inducing alarm because tits showed similarly strong alarm to barred and unbarred hawks, and little alarm to barred doves. These responses of tits, unsuitable as hosts and hence with no history of cuckoo parasitism, suggest that naive small birds can mistake cuckoos for hawks. Thus, any cuckoo-hawk discrimination by host species is likely to be an evolved response to brood parasitism.
Highlights
To human observers, many parasitic cuckoos of the Old World (Cuculinae) look remarkably similar to birds of prey, Accipiter hawks ( Wallace 1889)
Plumage manipulations revealed that the strong alarm response to cuckoos depended on their resemblance to hawks; cuckoos with barred underparts were treated like hawks, while those with unbarred underparts were treated like doves
Reed warblers (Acrocephalus scirpaceus) often mob and sometimes attack a stuffed cuckoo, whereas they are much more wary of a stuffed sparrowhawk (Duckworth 1991; Welbergen & Davies in press). This certainly shows that reed warblers distinguish cuckoos from hawks, but again we cannot conclude that this is an evolved response to brood parasitism; perhaps non-hosts have this ability too. That it should first be tested whether naive small birds, which have no history of cuckoo parasitism, regard parasitic cuckoos as hawk-like
Summary
Many parasitic cuckoos of the Old World (Cuculinae) look remarkably similar to birds of prey, Accipiter hawks ( Wallace 1889) They share a close resemblance in size and shape (an elongated body, with long wings and a long tail), in plumage (greyish or brownish upperparts, with paler barred underparts) and in their swift direct flight. Our experiments involve great tits (Parus major) and blue tits (Parus caeruleus), unsuitable as hosts because they nest in small tree holes that are inaccessible to laying female common cuckoos They feed in woodlands and hedgerows, where they are vulnerable to attack from sparrowhawks, and form a major component of the sparrowhawk’s diet throughout the year ( Newton 1986). In a parallel study ( Welbergen & Davies in press), the responses of reed warblers to these caged cuckoos (intense mobbing) and caged sparrowhawks
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