Communication Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 74

The same effect has been found in the winter wren ( Troglodytes troglodytes )
when experimentally exposed to playback of song degraded by long-range propa-
gation and undegraded song. The wren is sensitive to degradation, with a less
intense territorial response to the stimulus, but with a change in song post that
enables the bird to better improve assessment of long-range propagation and the
receiver's ability to hear the opponent's song (Mathevon and Aubin 1997 ).
Several playback experiments have been carried out in recent years, and in
particular the ranging behavior of the chaffinch ( Fringilla coelebs ) has been studied
by Naguib et al. ( 2000 ). Playback songs degraded for emulating a distance of 0, 20,
40, 80, and 120 m were reproduced and the distance of territorial males from the
loudspeaker was noted. The results confirmed previous studies that birds use
acoustic cues to evaluate the distance of singing conspecifics. The experiments
demonstrated that chaffinch males are able to evaluate the distance of the singing
conspecific and to regulate their behavior accordingly.
Location of conspecifics, especially when individuals are hidden in dense
vegetation or when the light is not sufficient to use visual cues, can be determined
using acoustic cues. There is evidence that birds are able to evaluate the distance of
a conspecific competitor (male) according to the quality of the sound signal. The
acoustic adaptation hypothesis (AAH) emphasizes that birds are adapted to mini-
mize signal degradation by adapting their acoustic cues to the character of the
environment. Acoustic adaptation to the habitat offers at least the advantage to
signal to competitors from a great distance, and the emission of a sound that copies
the structure of habitat results in an acoustic message that could be interpreted as
being closer. The AAH has been proved in many conditions and by experiments
such as those designed to compare the magnitude of degradation of song propagated
in native versus foreign habitats. For example, Fotheringham et al. ( 1997 ) have
used playback experiments on four species of wood warblers (Parulinae). Males
were found to respond to near songs in the same way as a conspecific intruder, but
with less intensity to distant songs. These results suggest that males are able to
range the distance at which another male is singing, and their reaction is based on
the level of degradation with which an individual perceives the conspecific song.
These authors suggest that selection should favor individual males that structure
songs to reduce degradation and thus create an advantage because the rivals are
discouraged or intimidated by an undegraded signal.
At least two mechanisms of ranging are contemporarily in action: the signaler
tries to have a nondegraded signal to be transmitted and to intimidate the rivals. A
second mechanism consists in the capacity by a receiver to estimate the precise
distance of a conspecific competitor.
The ability to perceive song degradation may be useful either for signaler or for
receiver by reducing the uncertainty in behavior that could affect individual fitness.
There are different ideas about the use of cooperative behavior. In fact it is more
reasonable to admit that selection favors the acoustic cues that manipulate or
misinform the receiver.
Environmental and social conditions shape communication in songbirds and
humans as well creating a continued challenge between signalers and receivers.
In natural systems the unpredictability of events often creates a permanent evolu-
tionary tension in communicative life traits.