Calling song (Fig. 29). To the ear, the song of M. riverina appears to contain monotonously repeated long echemes, each increasing slightly in amplitude throughout and each separated by short sequences of macrosyllables. Detailed examination of three available recordings reveals that the echemes are organised collectively into phrases of quite variable duration. Indeed each phrase contains 2 to 60 (or more) echemes (1.0–1.5 s duration), each separated by gaps (0.17–0.38 s duration). The gaps are typically not silent, instead containing 3–7 macrosyllables (0.02–0.06 s duration, 2–4 syllables, with the syllables themselves not coalesced). The end of the phrase is signified by a short gap of approximately 0.04 s duration followed by a long macrosyllable (0.07–0.09 s duration) and a longer gap (0.10–0.12 s duration) (all statistics, n= 17 recordings). Following the stereotypical structure of male-female communication in this group, it is anticipated that the female would respond with a wing- flick during the long gap at the end of each phrase.
This species has been observed calling during the day and it is not known whether it also sings at dusk. The calling song maintains an even frequency distribution throughout, with a high amplitude plateau of 12.2–18.5 kHz and a dominant frequency of 14.2–14.7 kHz. [1]