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singing birds selected the conspicuous perch. The males that before the experiment
were in the upper part of the canopy moved to the lower canopy, whereas males that
were singing below the canopy did not change their position. The perches in the
lower canopy exposes the chaffinches to a higher risk of predation but at the same
time the visibility of an oncoming predator is very good and this at the end could
prevent attacks. This is a case in which the spatial component of a song is adapted to
current conditions. The capacity of the chaffinch to modify perch position under the
presence of a sparrowhawk demonstrates the flexibility adopted to reduce the risk of
predation to a minimum. The acoustic implication of the change of perch position
that improves vigilance has to be considered carefully.
4.19 Soft Song
A low-amplitude song, more known as “soft song” or “quiet song,” is common in
male songbirds and a different significance is attributed to this in the literature. At
least two categories of soft song are located according the context. In pied
flycatchers ( Ficedula hypoleuca ) and in the great tit ( Parus major ), the soft song
is uttered only during courtship. In white-throated sparrows ( Zonotrichia albicollis )
and in redwings ( Turdus iliacus ), males were observed to emit the soft song during
aggressive contexts.
In common blackbirds ( Turdus merula ) and in dunnocks ( Prunella modularis ),
this song is heard both in courtship and in aggressive encounters.
In the song sparrow ( Melospiza melodia ), the soft song is present only during
aggressive behavior; to test if this soft song is a predictor of attack, playback
experiments were conducted by Anderson et al. ( 2007 ). Two categories of soft
song were considered during the playback experiment: a “crystallized soft song”
and a “warbled soft song.” The females responded with less courtship display to
crystallized than to warbled soft song. Males responded with aggressive behavior to
both types of playback soft song. From this experiment results showed that in the
song sparrow the soft song is a signal of aggressive intent.
4.20 Bioacoustics Interactions Between Plants and Insects
Climate changes are favoring animal displacement, invasion, and rapid pest spread-
ing. In the forest system the balance between soil, plants, and animals is disrupted
by climatic change that modifies precipitation abundance and temporal distribution,
air temperature, and wind regime. Drought stress that recently has affected many
boreal and temperate regions has favored the outbreak of deforesting insects at a
growing intensity and capacity to cover large areas.
In conclusion, higher deforestation is driven by insects and modulated by
climate (Dunn and Crutchfield 2009 ). In particular, boreal forests could face a
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