Information Content in Chorus Songs of the Group-Living Australian Magpie (Cracticus tibicen dorsalis) in Western Australia

Songs of birds encode several kinds of information that may be used during interactions with conspecifics. Some bird species live in social groups and perform territorial advertisement song as a group. Group singing raises questions concerning its function in the social life of the group. Australian magpies (Cracticus tibicen = Gymnorhina tibicen) are group-living birds that sing chorus songs known as carols. Each carol is composed of a series of individual syllables. Previous spectrographic descriptions of carols in eastern Australian populations found individual-specific and sex-specific acoustic characteristics in carol syllables and group-specific characteristics in carol songs. These findings suggest that carols might reveal not only the identity of the group, but also other properties, such as its size and sex composition, which could influence outcomes in assessment/management interactions between groups. I tape-recorded and analyzed carols of seven magpie groups in Western Australia, a location and subspecies (C. t. dorsalis) differing from the previous studies of singing by magpies. I analyzed carols of each group to determine acoustic features of syllables, syllable syntax within carols and the way carols are delivered as a repertoire. Visual classification of individual song syllables as seen on sound spectrograms, automated quantification of spectral features of song syllables, examination for carol repertoires and call-answer syntax within carols all revealed little evidence that magpies have individual-specific or group-specific vocal characteristics. Thus, chorus songs do not appear sufficiently distinctive to communicate identity at the level of the group nor reveal group size and composition. Instead, caroling may signal only territory occupancy and willingness to participate in territorial defense. A new hypothesis is advanced suggesting that chorus songs of group-living birds are an unselected consequence of the coordinated duet songs of the breeding male and female whose duets are joined adventitiously by other group members.