The structural function of the bubbling call of the female common cuckoo (Cuculus canorus)

Abstract

The vocalizations of male Cuculus cuckoos exhibit clear interspecific differentiation, whereas female vocalizations, characterized by the distinctive bubbling calls, exhibit a high degree of similarity across species. Although structural differences in female bubbling calls among Cuculus cuckoos have been reported, their functional role and practical use by males for species delimitation have rarely been established. In this study, we conducted playback experiments with manipulated female calls to identify call parameters that elicit male responses in Common Cuckoos Cuculus canorus. To this end, we modified three parameters (number of notes, delta time and highest frequency) of female calls recorded in the field and recorded the response of males to these alterations. We found that male cuckoos exhibited varying approach rates in response to subtle changes in female calls, with responses differing according to the specific parameters: a linear relationship for the number of notes and non-linear relationships for others. Among these parameters, the highest frequency of the bubbling call appeared to be the primary criterion for delineating species boundaries. However, the overall results suggested that multiple parameters of the bubbling call, rather than a single feature, collectively contribute to species delimitation in Common Cuckoos. Further playback studies incorporating multiple manipulations of bubbling calls simultaneously would provide deeper insights into the evolution and functional significance of female calls in cuckoos.