AUDITORY DISTANCE ESTIMATION IN SONG BIRDS - IMPLICATIONS, METHODOLOGIES AND PERSPECTIVES

Acoustic signals such as bird song degrade progressively during atmosp heric propagation and consequently provide information about the dista nce of the signaler. Information on the signaler's distance is particu larly important for animals that use acoustic signals to defend a terr itory or, in general, to regulate their spacing. Male territorial song birds can use this information to assess the distance of a conspecifi c singer (called 'ranging'). This ability presumably increases the eff iciency 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 influe nce the outcome of 'ranging experiments' of which some are discussed h ere. So far, playback experiments in the field that impeded close-rang e experience of subjects with the loudspeaker yielded the dearest evid ence for ranging. Flights of subjects to positions beyond the loudspea ker in response to playback of degraded songs provide unambiguous evid ence for over-estimation of distance of degraded songs and thus reduce problems of interpretation encountered in experiments that allow subj ects' close-range experience with the loudspeaker. Furthermore, the ac curacy 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 b e taken into account in future experiments.