Sound degradation and implications for long-distance communication in song birds [abstract]

The progressive degradation of sound signals during propagation has important consequences for the information transmitted in long-range communication. The accumulating degradation interferes with the information encoded in a signal and provides additional information on the signaller's distance. Such information about distance is particularly important in birds that use their song to defend a territory or, in general, for animals that use acoustic signals to regulate their spacing. In these cases the distance and its assessment between individuals is crucial because it determines the nature of interactions. Furthermore, the ability of birds to use the accumulated degradation in songs to assess the distance of singing conspecifics (called ranging) presumably increases the efficiency of defending a territory: it allows territory holders to discriminate between distant conspecifics and more threatening rivals nearby without spending time and energy on approaching. To investigate mechanisms of ranging by Carolina wrens Thryothorus ludovicianus I conducted experiments with playback of tape-recorded songs presented to subjects in the field. Short playbacks eliminated close-range experience of subjects with the loudspeaker. Flights to the far side of the loudspeaker and strong responses beyond the loudspeaker in response to degraded songs provided unambiguous evidence for over-assessment of the distance of degraded songs. More specifically results show that Carolina wrens can use reverberation, the relative intensities of high frequencies or the intensity of conspecific song as auditory distance cues. Use of several cues could increase the accuracy of ranging by pooling information derived in different ways: such pooling of information can balance the different kinds of uncertainties associated with each of these cues. This ability also allows male birds to range conspecifics with similar efficiency under different acoustical conditions where information from each cue is not equally available. In addition, the apparently quick assessment of distance could speed up the decision how and towards whom to react. A resulting selective attention to closest rivals might speed up the processing time for additional information such as the signaller's identify and motivation.