Acoustic overtones improve the discrimination of conspecific female calls by male common cuckoos from similar heterospecific calls.

Acoustic communication in obligate brood parasitic common cuckoos (Cuculus canorus) plays an important role both in social contacts within its own and with other species (including its many hosts). For example, the female cuckoo's bubbling call putatively mimics the call of the Eurasian sparrowhawk (Accipiter nisus) to serve as defence from host songbirds mobbing the parasitic female. However, several other, both raptorial and harmless, sympatric bird species also have similar vocalizations to the bubbling call (including the Eurasian kestrel, Falco tinnunculus, and the Eurasian green woodpecker, Picus viridis). Bubbling calls are also used by female cuckoos for conspecific communication with male conspecifics and so the discrimination of acoustically similar con- vs. heterospecific calls should be functionally relevant for cuckoos. We expanded upon a published playback study with all the above species' calls presented to male cuckoos. The subjects approached the speaker in 100% when conspecific females' bubbling calls were played back, but rarely (6-12%) did so when either acoustically similar natural calls of sympatric species were used. However, as structurally the bubbling call has no harmonic overtones, but the calls of the other species contain them, we also conducted playbacks with manipulated sound files where the harmonics were removed. Harmonic-free heterospecific calls attracted male cuckoos more often (40-50%) than natural heterospecific calls but still less so than the natural conspecific call. These results reveal the functional importance of the presence of harmonical overtones in heterospecific calls as they can serve to reduce perceptual auditory errors in male cuckoos.