Biology Reference
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basis of modern planning strategies, especially in urban areas and protected wild
areas.
The signal-to-noise ratio is an important factor that must be considered to
understand the way organisms modify their acoustic patters to increase this factor.
6.6 Noise in Patchy Sonotopes
Ambient noise is one of the environmental factors that produces a selection pressure
on acoustic signals. Signal-to-noise ratio (SNR) and the masked auditory threshold
are the fundaments of the detectability of a signal.
The soundscape is heterogeneous, and this means that there is alternation of
quiet and noisy sonotopes. Generalist species in terms of sonic tolerance can
establish in both conditions and cope with local sonic conditions, but for specialist
species with a narrow sonic tolerance only quiet conditions can be selected.
According to a strategy to reduce the masking effect of noise, chaffinches
(
Fringilla coelebes
) living along torrents and close to waterfalls have been
observed to modify their acoustic repertoire, repeating more often the same song
type before changing to a new type (Brumm and Slater
2006
) (Fig.
6.2
). This
observation could confirm the information theory, which states that in the presence
of noise a signal must be repeated, creating a redundancy, assuring in this way the
chance that a signal may be received. These authors have observed also that song
types with faster trills were sung in shorter bouts, which opens the hypothesis that
highly redundant song is limited by a trade-off with motor fatigue.
Many studies have demonstrated that song patterns diverge between a noisy or
quiet sonic environment. However, it is reasonable to suppose that the changes
occurring in the signalers have influence also on the receivers. Mockford and
Marshall (
2009
) demonstrated, using playback experiments, a difference in spectral
aspects of the song of the great tit (
Parus major
) living in an urban and rural
context. These differences elicit the replay of the receiving males. Males living in
quiet areas showed a significantly stronger response (e.g., faster to sing over the
playback song, and approaching the speaker) as a reply to the male territory holder
song with low background noise than to the song of males living in areas with high
territory noise.
In this species the spectral differences between urban and rural song consists in a
minimum frequency that was significantly lower in individuals living in quiet areas.
Laboratory experiments carried out by Pohl et al. (
2012
) on the great tit (
Parus
major
) have demonstrated that in conditions of urban noise this species benefits
from high frequencies in detection and discrimination of intraspecific song.
If noise is important for animal communication, a study comparing ambient
noise represents an important step to better understand how local animal
communities aggregate and communicate. Slabbekoorn (
2004
) has conducted an
investigation on the environmental noise in 14 sites in Cameroon, central Africa,
8 sites in rainforest and 6 in the ecotone region of dryer savanna. The results
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