Rhythmic coordination in rapid duets of little crakes: cooperative signalling without shared experience

Coordinated vocal displays are common in animals with long-term social bonds, reinforcing alliances and aiding territorial defence. However, they also occur in species with brief social ties and limited learning, raising questions about their strategic function. Little crakes (Zapornia parva) provide a useful model to explore this, as territorial pairs produce antiphonal duets of rapid trills despite short-term pair bonds. We investigated the structure, formation and function of little crake duets. Pairs produced rapid trills with strict note alternation and high isochrony, eliminating note overlap after a brief synchronization phase to maintain a stable rhythm. Playback of solo trills showed that, despite the high tempo, unpaired males spontaneously synchronized their notes with the playback, demonstrating that note alternation does not require prior interaction with a specific partner. A second experiment showed that temporally precise duets elicited stronger responses than irregular, overlapping ones, indicating that rhythmic regularity enhances signal efficacy. We conclude that coordination in little crake duets results from spontaneous rhythmic alignment rather than learned note-by-note adjustments. We suggest that precise alternation of notes, strictly enforced by the close proximity of the male and female, may, independently of shared experience, serve as an index of pairing, indirectly supporting the cooperative territorial defence.