Abstract and Summary
Antiphonal duets between male and female of a mated pair have been described for many bird species. Most authors admit that duetting may have different functions in different species, but they usually treat it as a uniform phenomenon within one species, although the same pair may duet with a variety of different songtypes. This paper presents evidence that in Laniarius funebris different duettypes indeed serve different functions.
In the study population at Lake Baringo (Kenya) males duet with four songtypes, females with only one. By means of multivariate analyses it was found that the occurrence of the four male songtypes cannot be explained by a single, uniform tendency for duetting, but only by 3–4 factors representing different functions. These functions were analysed by studying the frequency of songs initiated by males and females, the frequency of answers in different situations, and the effect of songs and answers on other behaviours. The following result emerged:
One male songtype is used to reach a breeding synchrony in mates. Two other types both have territorial functions but their relative frequencies seem to change with the extent of threatening by other pairs and cooperation between mates.
For the fourth type a mate-guarding hypothesis is proposed: in this case the duet is not performed in the common interest of two mates, each informing the other about its intentions; rather, each answers the mate's song to prevent solo-singing, which could attract other birds as competitors.
The analysed functions are discussed in the light of sociobiological reasoning and with respect to the higher number of duetting species in the tropics than in temperate zones.