Call rate in Common Cuckoos does not predict body size and responses to conspecific playbacks

The brood parasitic Common Cuckoo Cuculus canorus is best known for its two-note “cu-coo” call which is almost continuously uttered by male during the breeding season and can be heard across long distances in the field. Although the informative value of the cuckoo call was intensively investigated recently, it is still not clear whether call characteristic(s) indicate any of the phenotypic traits of the respective vocalising individuals. To fill this gap, we studied whether the call rate of male cuckoos (i.e., the number of calls uttered per unit of time) provides information on their body size, which might be a relevant trait during intrasexual territorial conflicts. We captured free-living male cuckoos and measured their body size parameters (mass, wing, tail and tarsus lengths). Each subject was then radio-tagged, released, and its individual “cu-coo” calls were recorded soon after that in the field. The results showed that none of the body size parameters covaried statistically with the call rates of individual male Common Cuckoos. In addition, we experimentally tested whether the “cu-coo” call rates affect behavioural responses of cuckoos using playbacks of either a quicker or a slower paced call than the calls with natural rates. Cuckoos responded similarly to both types of experimental playback treatments by approaching the speaker with statistically similar levels of responses as when presented with calls at the natural rate. We conclude that male Common Cuckoos do not advertise reliable information acoustically regarding their body size, and so, cuckoo calls are neither useful to characterize cuckoos’ phenotypic traits directly nor to indicate environmental quality indirectly.