Spectral analysis of vocalisations of the Moluccan megapode Eulipoa wallacei (Aves: Megapodiidae) during egg-laying activities

Vocalisations of the Moluccan megapode Eulipoa wallacei during nightly egg-laying activities on the communal nesting-grounds of Tanjung Maleo near Kailolo village on Haruku Island (Moluccas, Indonesia) were tape recorded. The recorded vocalisations are associated with aggressive interactions between two or more birds. Spectral analysis of these so called ‘kek’ calls show that spectrograms consist of a harmonic pattern with increased intensity at frequencies higher than the fundamental frequency. From that phenomenon it can be concluded that the spectrogram does not give a full representation of the bird’s vocalisation, but only shows the harmonic pattern of the ‘pulse repetition frequency’ (PRF = number of pulses produced per second). It appears that the ‘kek’ call of Eulipoa wallacei consists of a series of pulses, of which each pulse has a fundamental frequency between 2 and 2.5 kHz and a pulse duration between 2.1 and 2.3 milliseconds. The duration of the pulse series itself (one ‘kek’) is highly variable between 40 and 210 ms and the pulse repetition rate within a ‘kek’ varies between 300 and 650 pulses/second. In general the pulse repetition pattern is rather symmetrical in time, but irregular patterns are also possible. The repetition rate of the individual ‘kek’s’ might reflect the state of aggression: the higher the rate the higher the state of aggression. Three types of ‘kek’ calls can be distinguished (1) short lasting kek’s (< 100 ms), produced in bursts with short interval times and a rather symmetrical spectrum; (2) long lasting kek’s (≥ 200 ms), individually produced and consisting of a rather steady frequency; (3) a mixture of both with usually a rather symmetrical frequency spectrum and a duration between 100 and 200 ms. The fact that ‘kek’ vocalisations of Eulipoa consist of pulse series, instead of (continuous) sinusoidal signals, is an interesting phenomenon that has not yet been described earlier in studies on avian vocalisations.