Sex-specific responses to simulated territorial intrusions in the common cuckoo: a dual function of female acoustic signaling

The two-note call of the male common cuckoo (Cuculus canorus), the so-called cu-coo, is well known to people as a natural and cultural signal. However, the so-called bubbling call of the female cuckoo is almost unknown to most, and its function in the social organization of cuckoos remains understudied. We carried out a study of a possible intraspecific communication function of female bubbling calls, using playbacks to female cuckoos in their natural environment. Regarding vocal responses, both female and male cuckoos paid attention to the bubbling calls as they consistently responded acoustically by calling but did not so during control playbacks of collared dove (Streptopelia decaocto) calls. Accordingly, in about 63% of trials, females approached the loudspeaker closely and 81% uttered bubbling calls themselves during the experiment. These results are consistent with a function that the bubbling call plays a role in territorial signaling and defense among females. Male cuckoos also showed strong responses to playbacks of bubbling calls, as they approached the speaker and themselves called in 94% of playbacks; this is consistent with a scenario that they are interested in unfamiliar, new females in the area. Specifically, males approached the speaker repeatedly by flight, often flew around it and then perched on a tree, and uttered different call types beside the general cu-coo (e.g., quick cu-cu-coo, gowk call, and guo call). Our results represent an illustrative example that a simple female call may have multiple functions, as the cuckoo bubbling call advertises territory need for female cuckoos and attracts males.Significance statement Avian brood parasites lay their eggs in nests of other bird species, causing hosts to incubate, feed, and rear the parasitic offspring. Parasitic adult common cuckoos maintain a complex acoustic communication system, but female cuckoo calls are only beginning to be studied. The basic intraspecific functions of females' sparrowhawk-like bubbling calls have not yet been characterized, whereas interspecifically, they use it for reducing antiparasitic attacks by their hosts. Our playback experiments with bubbling calls revealed that both female and male cuckoos responded acoustically to unfamiliar bubbling calls and more males than females approached the speaker, relative to control playbacks. We conclude that bubbling call has dual basic intraspecific functions: mate attraction, and territorial spacing.