Comparative analysis of female bubbling calls: Within- and between-species variation among the four species of Cuculus cuckoos

In brood-parasitic Cuculus cuckoos, male vocalizations are species-specific and easily distinguishable, whereas female calls are remarkably similar across species, making species identification challenging. In this study, we examined the structural characteristics and variability of female bubbling calls among four Cuculus species (Common Cuckoo C. canorus, Oriental Cuckoo C. optatus, Indian Cuckoo C. micropterus, and Lesser Cuckoo C. poliocephalus) breeding in South Korea. Comprehensive acoustic analyses of seven call parameters, using recordings from 2021 to 2023, were conducted to quantify the characteristics of their calls and compare within- and between-individual variability across species. Significant differences were found across all call parameters, with the Common Cuckoo producing the highest number of notes and the Oriental Cuckoo the lowest-frequency calls. Despite these differences, the overall structure of the calls remained acoustically similar, with overlapping characteristics across species. Furthermore, female Common Cuckoos exhibited greater within-individual variability in their calls, while the other species showed higher between-individual variability, which may further complicate species identification based vocalization alone. These findings highlight the complexities of female vocalizations in Cuculus cuckoos and suggest that ecological, social, and evolutionary factors may contribute to this vocal variability.