Communication Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 72

before ranging must learn the structure of each song pattern. In other word, a bird
first must be able to produce the same pattern.
Local populations try to preserve song types as true dialects. This effect degrades
with distance or because of an important barrier. Investigations carried out by
Morton ( 1987 ) in the Carolina wren ( Thryothorus ludovicianus ) have demonstrated
that individuals at the same site share 66 % of their songs (range, 49.5-89.5 %) with
a negative linear correlation between shared songs and distance, but males resident
on islands share only 20 % of their song repertoires. The author reports that 3 km
water was equivalent to 145 km in the mainland.
The ranging hypothesis follows the discovery that some cells of the forebrain in
passerines respond at the highest rate to a subject's own song (Margoliash 1983 ;
Margoliash and Konishi 1985 ).
Birds could act either in temporal or in frequency domain to evaluate with
precision the level of degradation of a song and consequently range accurately.
However, familiarity does not seem improve the ranging process.
Difficulty in interpreting correctly the ranging mechanisms is represented by the
multiplicity of explanations on lower responsiveness to degraded songs.
For instance, sensitivity to variation in amplitude is not surprising; a subject
reacts better to a high-amplitude signal than to one of low amplitude. Other types of
degradation such as frequency dependence attenuation or reverberation can lower
signal detectability.
Another important aspect is represented by the expectation from the receiver for
a signal. Familiar signals are detected more than unfamiliar ones. As a consequence
of this fact a signal degraded and unfamiliar could be detected less often, escaping
direct ranging.
A problem that arises during playback experiments is that a repeated song from a
loudspeaker can be judged by simple triangulation by an attracted subject; this can
be easily solved by broadcasting the signal only for a very short period of time.
Despite some evidence, the ranging hypothesis remains far from common accep-
tance, especially in term of accuracy by which individuals are ranging. For instance,
it is not clear if ranging accuracy is the same for the three forms of degradation:
amplitude, frequency-dependent attenuation, and reverberation. It is not clear if
ranging accuracy changes according to species and type of environment. Finally,
the capacity of ranging and competitive or cooperative interactions between terri-
torial neighbors remain open issues.
Birds immersed in a variable sonic environment receive signals from foreground
to background. Birds have the capacity to discriminate the quality of the signals and
to decide to react when a sound is emitted by a threatening intruder or to ignore the
signal from distant conspecifics. It is reasonable to expect a strong reaction from the
receiver when the signal is not degraded (Fig. 4.4 ). The assessment of the auditory
distance (ranging) is a mechanism utilized by species that have to defend territory
and habitat resources. Perceiving the level of degradation requires the receiver to
have the capacity to decodify the signal in real time. Naguib ( 1996 ) has tested the
ranging behavior in the Carolina wren ( Thryothorus ludovicianus ) by broadcasting
artificially degraded sounds (Fig. 4.5 ). In this species the capacity has been proved