Communication Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 77

(receiver) was placed high in the vegetation in respect to the speaker. This experi-
ment has demonstrated how the blackcap benefits by staying in a higher perch when
it functions as the receiver rather than as the sender.
The conspecific recognition of the acoustic signal (song or call) is fundamental
to assure a channel of communication is always active. All the information present
in an acoustic signal (song or call) could appear necessary for communication. In
reality, the background noise, the masking effect, and interference with other
subjects can reduce the efficiency of this acoustic message. Mathevon and Aubin
( 2001 ) have tested in the blackcap ( Sylvia atricapilla ) the hypothesis that in some
songs the meaning of the message is not lost when the song structure is degraded. In
this species, the male responds even to a highly manipulated model of conspecific
song. The reply is suspended when frequency modulation is suppressed or the
number of syllables is reduced drastically. In terms of code biology, a receiver
male when a manipulated song is administrated is able to intercept the fundamental
code that persists. In other words, the capacity showed by the blackcap male to
decode the conspecific song even when manipulated could represent an adaptation
to transmitting acoustic signals in a forest environment where these signals are
degraded by vegetation.
Ranging seems present in monkeys such as the pygmy marmoset ( Cebuella
pygmea ) that, according to the distance separating sender and receiver, utilizes
three different vocalizations (trill, J calls, and long calls), adapting also to the
characteristics of the habitat where they are calling (Fig. 4.6 ). The first two calls
have a higher frequency and degrade rapidly, so they are used in short-distance
communication; the long-distance call that is a less pulsatile vocalization is utilized
to communicate over longer distances (Fig. 4.7 ) (De La Torre and Snowdon 2002 ).
4.7.1 Amplitude Fluctuation and Reverberation
and Ranging Efficiency
Amplitude fluctuations and reverberations are two important variables affecting the
identification and localization of acoustic signals uttered by birds. Amplitude
fluctuation is the result of the scattering from air turbulence that varies with weather
conditions (wind, temperature) and hour of day. This factor produces difficulties in
birds in accurate ranging; the distance at which a species detects a sound and
consequently the intelligibility of the signal are falsified.
Reverberation represents a further distortion of the signal in a forest habitat. This
factor is easily observed in a spectrogram. Reverberation produces a more confused
design in the spectrogram, and we expect identical interference in the process of
identification of the signal by the receiver is likely. Amplitude fluctuation and
reverberation are operating in the forest landscape, reducing the efficiency of the
communication network, but on the other hand this represents an acoustic buffer to