Chorus-call classification in the southern pied babbler: multiple call types given in overlapping contexts

Individuals of many species communicate with one another using a range of vocalisations and there is often variation in the use and structure of these calls depending on sex, status and context. In social species, two or more group members may also combine their vocalisations to produce duets or choruses. While the function of duets and the different contributions of males and females have received considerable research attention, less is known about the different calls used by group members in choruses. Southern pied babblers (Turdoides bicolor) are cooperatively breeding birds of Southern Africa that live in permanent stable groups. In addition to a variety of calls given by individuals, group members frequently combine to produce raucous choruses which include several different call types. Here we describe these different call types for the first time and explore their usage, with respect to the sex and dominance status of callers, production of the call alone or as part of a chorus, and the social context. Eight out of nine possible calls used in choruses on some occasions were found to be statistically distinct. As expected from the variation shown in individual calls, some of those call types included in choruses were sex-specific and some were used more by dominants than subordinates. Moreover, there was variation in the use of different call types as solos and within choruses, as well as their occurrence in different contexts. We discuss what might be concluded about the functions of the different call types from their patterns of usage.