Acoustic signals such as bird song degrade progressively during
atmospheric propagation and consequently provide information about the
distance of the signaler. Information on the signaler's distance is
particularly important for animals that use acoustic signals to defend a
territory or, in general, to regulate their spacing.
Male territorial song birds can use this information to assess the
distance of a conspecific singer (called 'ranging'). This ability
presumably increases the efficiency of defending a territory because it
enables a territory holder to discriminate among threatening intruders and
distant conspecifics without interrupting current behavior to, for
instance, spend time and energy in approaching.
There are a variety of factors that can influence the outcome of 'ranging
experiments' of which some are discussed here. So far, playback
experiments in the field that impeded close-range experience of subjects
with the loudspeaker yielded the clearest evidence for ranging.
Flights of subjects to positions beyond the loudspeaker in response to
playback of degraded songs provide unambiguous evidence for
over-estimation of distance of degraded songs and thus reduce problems of
interpretation encountered in experiments that allow subjects' close-range
experience with the loudspeaker.
Furthermore, the accuracy of ranging can be influenced by the kind of
degradation and the availability of song features that facilitate its
assessment so that these factors in addition to an appropriate playback
design should be taken into account in future experiments.