Temporal patterns of duetting in African barbets
Four species of African barbets (Lybius torquatus, L. vieilloti, Trachyphonus darnaudii, and T. erythrocephalus) regularly sing in duet. Duets consist of phrases repeated at very regular intervals; in L. torquatus and L. vieilloti the standard deviation is less than a tenth of the mean interval between duet motifs. Sound spectrographic analysis of the songs indicates that an internal rhythmic behaviour pattern determines in large part the temporal features of the duet in these barbets:(1) one member of a pair of L. torquatus continues to call in rhythm even though its mate may call irregularly, (2) in the other three species the calling rhythm of each of the two mates is different, especially in L. vieilloti where the ratio of the periodicity of calling of the two birds averages 1:1.52 and where one bird continues to call at the same rate when its mate ceases to call, and (3) several non-duetting African barbets have regular calling periodicities, some with greater precision than in the duetting barbets and some with less. In the barbets the temporal pattern of duets appears to be in greater degree determined by individual calling rhythms (as in some non-duetting barbets) than by the auditory reaction time involved in a bird responding to its mate. Duetting in barbets probably functions both in maintenance of a pair bond and in territorial advertisement.