Bioacoustics Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 50

Fig. 3.7 Path analysis regarding a causal model utilized to explain the relationships between
abiotic and vocal variables. Epsilon represents the amount of variances unexplained by the model
patch coefficients, and their significance between variables is indicated (Reproduced with permis-
sion from Ziegler et al. 2011 )
this species produces trills at a slower rate and a lower minimum frequency. This
analysis has examined the white-crowed sparrow sounds recorded in 1970 and in
2005. In the studied area the vegetation showed an increase of cover and density.
The hypothesis that this species has modified their acoustic repertoire according to
the changes occurring in their habitats has been proved to be consistent. This is the
first case of evidence of acoustic plasticity of birds under the constraint of environ-
mental changes. We can expect a modification of the soundscape of entire regions
to result from vegetation changes favoring some sonic components. With increase
of the vegetation cover, the reverberation increases, and consequently the songs
will be modified to be produced more slowly and at lower frequencies.
We consider this fact to be relevant because the rapid changes of landscape
structures everywhere affected by human disturbances could be easily monitored
applying the sonics tools. Vegetation cover can affect definitively temporal and
spectral characteristics of the sonic ambience.
Experiments conducted by Naguib ( 2003 ) have demonstrated that fast trills
transmit less well than slow trills, with effects on long-range communication. In
Fig. 3.8 and in Fig. 3.9 is shown how reverberation increases with increasing
vegetation cover and in relationship with the frequency of the trill element.
Low-frequency notes slowly repeated are less affected by reverberation and can
assure long-range communication better than high-frequency notes quickly repeated,
as has been observed in birds living in open and densely vegetated habitats.
The animal phenotype is strongly affected by the environment in which a species
resides. For instance, acoustic variations in bird vocalization have been proved to be
the result of environmental constraints, which suggests an evolutionary process that
has great consequences on overall biodiversity (Slabbekoorn and den Boer-Visser
2006 ). The recent dispersion of urban settlements has created novel habitats for