Communication Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 81

Mathevon et al. ( 1997 ) compared the distress call of the common blackbird ( Turdus
merula ) and the starling ( Sturnus vulgaris ). The distress calls of these two species
have the same characteristic structure: a carrier frequency with harmonics slowly
modulated in frequency, but in the starling the spectrum is broader than that of the
blackbird. Playback experiments conducted in dense vegetation at a distance of
40 m have indicated a significant difference between the two species. As predicted
by the hypothesis, the distress call of starling was strongly affected by vegetation
with a modification of the frequencies above 4 kHz which are strongly attenuated.
The distress calls of blackbirds conserve their structure, confirming the hypothesis
of a better adaptation to a long-range transmission in dense vegetation. Despite the
less efficient transmission of the starling distress call, the frequency modulation and
message decoding are preserved, although at lower reliability (Fig. 4.8 ).
The difference in propagation across vegetation could be discussed also from an
etho-ecological aspect. The two species have a different social structure. Starlings
live in flocks and probably use also other antipredatory signals, whereas blackbirds
live in a very scattered population and require more acoustic signals.
4.9 Soundscape Orientation and Spatial Source
Segregation
Most animals use conspecific signals to find a partner or to avoid a competitor,
although heterospecific semetic relationships are also extensively used. For
instance, bats use frog choruses to locate rich areas of aquatic insects, and some
birds recognize the alarm calls of other birds when a predator is nearby. Sound can
be used to orient a species in a landscape (Slabbekoorn and Bouton 2008 ). In this
way, birds, pelagic fish larvae, and newts extensively use sounds to find sites for
breeding or for foraging (Fay 2009 ).
In people who have visual restriction, acoustic cues are used to move around
indoors and in the outdoors. For blind people, soundmarks such as the noise of a
road, the sound of a bell, etc., are fundamental to self-orienting. Acoustic cues are
used by animals to move in the dark, in dense vegetation, or in turbid waters. Frogs
use conspecific choruses to find a common pond. Nocturnal migratory birds can find
landmarks in the call of other species of birds or frog choruses.
Bee ( 2007 ) has proved experimentally that the female grey treefrog ( Hyla
chrysoscelis ), which has a phonotaxis for the male chorus, is able to segregate
sound sources in space.
In newts, a phonotactic produced by the chorus of frogs has been proved
experimentally, although a definitive confirmation is still required.