Quantitative analysis of vocalisation types in male common cuckoos' "gowk" call complex.

Acoustic communication of animals often contains two types of vocalizations: loud sounds for long-range and soft sounds for short-range signalling. Brood parasitic common cuckoos (Cuculus canorus) are suitable study objects for research on acoustic signalling as they have a simple acoustic repertoire, of which the loud territorial advertisement calls of males, the "cu-coo", is the most famous type. Although a distinct group of soft calls has also been reported in early naturalists' works, no systematic studies compared them acoustically. Even the classification of these soft calls is lacking. Here we surveyed these neglected types of calls, and evaluated them through bioacustic analyses. Specifically, we compared the following soft calls: single gowk, guo, and the series of gowk calls. The advertisement call ("cu-coo") was used as the referent for these comparisons. As the male's gowk series call most typically contains 3-5 elements with decreasing volume (disappearing at the end), we compared the first two elements of this call. Our analyses revealed great acoustic similarities between three call types (gowk series call 1st note, gowk series call 2nd note, and single gowk call), and the distinctiveness of the other call type (guo) from this group. Structurally the gowk cannot be regarded as a separate call from a male's gowk series call, and the only difference is that the series call contains a set of the gowk calls. Our classification of cuckoo calls offers the possibility for further research into their functional importance and communicative role in male-male or female-male social contexts.