Sonic Patterns I: The Noise - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 148

masking effect of background noise, some species have modified their vocalizations,
shifting the vocalizations to ultrasonic frequency bands. It is the case of the concave-
eared torrent frog ( Amolops tormotus ), a species of arboreal frog of Central China
that has ultrasonic harmonics used to communicate in a noisy environment
(Narins et al. 2004 ). Ultrasonic components have been found also in the song of
the black-faced warbler ( Abroscopus albogularis ), an oscine songbird that lives close
to the torrents in a constant noisy environment (Narins et al. 2004 ).
To adjust to masking noise, some species increase the amplitude of their
vocalization. As documented by Slabbekoorn and Peet ( 2003 ), in great tit ( Parus
major ) populations a shift toward higher frequency bins has been observed for
urban populations living in noisy conditions. The same effect has been observed
from experiments on the zebra finch ( Taeniopygia guttata ). Birds subjected by
Cynx et al. ( 1998 ) to various levels of white noise increased significantly the
amplitude level of their vocalizations.
A background noise of 60-65 dB, such as that during the nights in tropical areas,
can render the sound signal ambiguous, as studied by Hartbauer et al. ( 2012 ), who
experimentally masked the acoustic mating signal of Mecopoda elongate , a tropical
katydid. Synchronous entrainment was lost at a signal-to-noise ratio of

3 dB when

a native noise was used.
Pytte et al. ( 2003 ) have observed that the male of the blue-throated hummingbird
( Lampornis clemencia ) has the capacity to modify the amplitude of their
vocalizations when a natural noise occurs or when an experimenter-controlled
change in ambient noise is created. This effect is known as the Lombard effect
and has been found in many other animals. This species uses a serial chip territorial
advertisement call that modulates in amplitude according to the level of environ-
mental noise. With experiments of playback of a creek noise, these authors have
observed a change in the amplitude of serial chip production. In this species this
stereotyped simple call is emitted from a high perch for a long-distance transmis-
sion, and consequently the amplitude modulation seems an efficient mechanism to
assure the long-distance broadcast.
The cacophony of sound perceived in urban contexts is considered a necessary
price to be paid for living in the highly structured and social-service dispenser that
is the urban environment.
Such sounds that are loud and intrusive become noise when perceived as
indistinct and are accepted as a necessary compromise for logistic services offered
by the urban environment. In natural areas, however, people consider every non-
natural sound very disturbing and try to select remote areas with a quiet sonic
ambience.
The effect of urban noise on animals (and especially on birds) can be distin-
guished at least in two major groups: amplitude shift and frequency shift (Fig. 6.4 ).
The amplitude shift or Lombard effect is the tendency for a speaker to increase
the intensity of its vocal level to improve the intelligibility of a speech signal when
in the presence of a loud background noise. This effect has been named after the
French ear specialist, Etienne Lombard, who discovered this phenomenon in human
speech (Lombard 1911 ).