Does audience affect the structure of warble song in budgerigars (Melopsittacus undulatus)?

In many bird species, male song functions both to defend a territory against other males and to attract a female mate. Male budgerigars (Melopsittacus undulatus) produce a song-like vocal signal, the warble, that can be directed at either females or other males. Warble is a long, complex, low amplitude, and variable vocalization composed of different element types. While there is some evidence that warble can induce reproduction, the function of this signal is largely uncertain and it is unclear whether male- and female-directed warble differ in either function or structure. We recorded male budgerigars in the presence of either their mate or a familiar male to identify whether the warbles produced with different audiences differed in structure. We dissected each warble into specific element units, classified units into a limited number of types by rule-based visual classification and calculated the proportion of each element type, element diversity, and total duration for the male- and female-directed warbles of each male. We also examined the sequential organization of warble element types (syntax) using time-window lagged sequential analysis. We found no differences in the proportions of different elements used, element diversity or duration of warbles between male- and female-directed warble. The syntax of warbles is similar when directed at males or females. However, we found greater between-individual similarity in the sequential organization of warbles directed towards females than in those directed towards males. The greater syntactical consistency in female-directed warble suggests that females may prefer either specific types of element sequences, or consistency itself, and thus shape the organization of warbles.