Communication Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 76

In the whitethroat ( Sylvia communis ), Balsby et al. ( 2003 ) have found that the
five types of vocalization were constrained differently according to habitat consid-
ered and behavior. Sound degradation was found to degrade with distance along a
hedgerow but to be negligible in open meadows. Song flights seem not different in
terms of efficiency when compared with the other types of vocalizations. These
authors have argued that a song flight may have a different purpose, such as being a
visual location by potential receivers or surveillance by the territory owner. Thus, a
different level of degradation for each sonic type is expected.
Birds that defend territories generally are more reactive toward songs that are
played in the middle of their territory than those at the periphery or outside the
territory. This effect has been observed in different species such as the great tit
( Parus major ), gray catbird ( Dumetella carolinensis ), or white-throated sparrow
( Zonotrichia albicollis ). This fact is interpreted as a capacity of ranging from birds
that recognize, using sound degradation, the distance at which another individual is
singing. This ability enables an individual to react accordingly when the intruder is
close or inside the territorial limit or to ignore such individuals considered to be
outside the territory and not a threat.
Amplitude and degradation are the two cues probably implicated in the ranging
process, but amplitude is under control of the singer whereas degradation depends
on distance and seems a more effective cue for correct ranging. In particular, it
seems that ranging between two singers that use the same song type (McGregor
et al. 1983 ) is more accurate and that this occurs also in the presence of undegraded
song rather than degraded song. This observation confirms the hypothesis of
Richards ( 1981 ) as tested on the Carolina wren ( Thryothorus ludovicianus ) that
degradation cues are used for ranging and that ranging is more accurate when the
song sung is of the same type.
Richards ( 1981 ) found that this species uses distance information of sound to
react differently: attacking when the singer is close and countersinging when the
singer is far. Degraded sound means that a bird is outside the territory and this
produces a singing reply. When the sound arrives undegraded this means that the
conspecific singer is close and the reaction is a direct attack. This capacity allows
wrens to conserve time and energy to be used in territorial defense.
The position of a singer in dense vegetation and the position of a receiver are
important because sound degrades according to the amount of vegetation that
interferes with it. To bypass this problem some birds select a high perch for singing,
such as the blackcap ( Sylvia atricapilla ), a common warbler in Europe dwelling in
densely vegetated habitats. Mathevon et al. ( 2005 ), using playback techniques,
investigated the effect of the selection of high perches on the receiver and
conducted an experiment in which the height of speaker and microphones were
changed. Four acoustics parameters that are sensible to sound degradation were
measured: signal-to-noise ratio, excess attenuation, distortion within the sound
produced as a blur ratio, and tail-to-signal ratio. These measures indicated that
song degradation was decreasing with the increase of height at which speaker and
receiver microphone were placed. Decrease was quite great when the microphone