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Fig. 4.12
The orientation
of a singing bird along a
steep slope makes a
difference in terms of
acoustic propagation. In
upslope orientation, the
acoustic impact on the
surroundings is more
efficient than from a
downslope orientation. The
sonic envelope around the
bird silhouette has been
generated using in an
anechoic chamber a 8-kHz
pure tone (Reproduced with
permission from Hunter
1989
)
4.16 Signaling Systems and Climatic Proxies
Signaling systems may diverge along an ecological gradient, and this has great
implications in speciation and diversification. Several studies have focused on
environmental drivers such as vegetation but little attention has been devoted, as
already stressed, on the effects of climatic changes on signal transmission (Snell-
Rood
2012
). In fact, climate is important for sound propagation because humidity,
temperature, and air turbulence largely depend on climatic conditions. It is well
known that a signal decreases in intensity because of spherical spreading, but
atmospheric absorption represents an important proxy. This sound absorption
increases with the frequency of sound but it varies also for temperature and relative
humidity. So, signaling systems are conditioned by climate. Absorption is not high,
indeed, but from a biological point of view this can represent an important element
in an evolutionary dimension. Snell-Rood (
2012
) reports that although an avian
vocalization may have an absorption of only 0.1 dB/m, this effect may be 10 dB
on a surface of 1 ha of territory. It remains unclear if the value of absorption
between species is the same within the species, and the effect of climate change on
the evolution of acoustic signals remains to be investigated. This point is of extreme
importance. We have often observed how seasonality favors or can affect signal
transmission, expressed as a useful time to sing. It is well documented that during
bad weather conditions, song activity is significantly depressed.
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