Cuckoo-hawk mimicry? An experimental test

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.

Figure 1

Time birds spent on the feeder during 5 min exposure to mounts placed 50 cm away. Data shown for each feeder, together with means (±1 s.e.) for responses to each mount type. (a) Experiment 1 (n=11 feeders), comparing feeder attendance during exposure to sparrowhawks, common cuckoos and teal. (b) Experiment 2 (n=12 feeders), comparing feeder attendance during exposure to sparrowhawks, common cuckoos and collared doves, each with either barred (B) or unbarred (U) underparts.

Figure 2

Time birds spent on the feeder during 30 min periods of post-exposure minus pre-exposure to the mounts (therefore, negative values indicate reduced attendance post-exposure). Data shown for each feeder, together with means (±1 s.e.) for responses to each mount type. (a) Experiment 1 (n=11 feeders), comparing responses to sparrowhawks, common cuckoos and teal. (b) Experiment 2 (n=12 feeders), comparing responses to sparrowhawks, common cuckoos and collared doves, each with either barred (B) or unbarred (U) underparts.