Communication Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 75

In some conditions such as the tropical forest the dense vegetation is a charac-
teristic that probably has persisted for thousands of years, and this environmental
characteristic probably has shaped communication in several acoustic species. In
such an environment Mathevon et al. ( 2008 ) have studied the effect of vegetation on
the singing pattern of the white-browed warbler ( Basileuterus leucoblepharus )
(Aves: Passeriformes: Fringillidae, Parulini). This species, which lives in the
undergrowth of Atlantic forests where there are dense vegetational barriers, has
an acoustic repertoire composed of two major signals: a high-pitched contact call
and a territorial song. The song is used to attract females and to maintain territorial
interactions. Despite the simplicity of the acoustic repertoire, this species has been
proved to ensure with its vocal signals species-specific recognition, individual
identity, and the location of the sender. Species-specific information is performed
for a wide audience, but the individual identity is a signal more susceptible to
propagation and is reserved to neighbors. The receiver can locate the signaler
according to the degradation of the acoustic signal. The signal emitted by this
species is degraded in the higher frequencies just at 100 m distant from the
broadcaster. The low-frequency modulation allows species to maintain information
for species identification and seems tolerant to sound simplification resulting from
vegetational constraint. Individual recognition is largely the result of the distribu-
tion of gaps in frequencies between notes. A playback experiment conducted by
these authors has demonstrated that these subtle changes are sufficient to identify
familiar from unfamiliar neighbors. By considering that this part of the song
repertoire disappears with distance, this type of recognition is reserved to
individuals that are close others and which have created a private sonic environ-
ment. In other playback experiments using two loudspeakers, one to attract a male
and a second used to emulate another distant male, Mathevon et al. ( 2008 )
have observed that the investigated male is able to range the presence of a second
male according to the level of an ad hoc degraded song. Decrease in frequency of
the succession notes may enable males to range the distance with good precision.
Thus, considering the regular structure of the tropical undergrowth forest, the
amount of remaining notes is directly correlated with the distance from which a
signal is emitted. An azimuth assessment is necessary to localize the direction from
which the signal is emitted. Experimental variation of amplitude modulation has
demonstrated that birds made many errors when tested with a signal without
amplitude modulation. Thus, ranging and direction are based on degradation cues
and on amplitude modulation characteristics.
In a coniferous forest the small leaves are more efficient in scattering higher
frequency sounds than those of broadleaf species. In the pied flycatcher ( Ficedula
hypoleuca ), the female spends a shorter time inside the nesting cavity when
compared with other nesting cavity birds. This fact was explained by experimenta-
tion by Lampe et al. ( 2007 ) showing that from the inside the nest box the degrada-
tion of the male song is high. In this species the male is polyterritorial, although
monogamous males increase the breeding efficiency of the female. Song degrada-
tion with distance has been proved to be greater in broadleaved and coniferous
forests.