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effect of urban noise and that males shifted more energy in the higher frequencies
(4-9 kHz).
Although animals have evolved an acoustic signaling system very adaptive to
background noise that minimizes the masking effect, the recent unprecedented
diffusion of human-altered landscapes has modified the sonic environment, and
animals have to face this relevant novelty.
In fact, anthropogenic noise often is louder and more persistent than natural
sounds, and often the major ambient novelty.
The effect of anthropogenic noise was investigated by Francis et al. (
2011
)in
two species of suboscine birds: ash-throated flycatcher (
Myiarchus cinerascens
)
and grey flycatcher (
Empidonax wrightii
). At increasing noise exposure, grey
flycatcher (
Empidonax wrightii
) occupancy declined but no frequency changes
were observed. In contrast, the ash-throated flycatcher (
Myiarchus cinerascens
)
does not show modification in density with the increase of noise dose but an
increase of frequency of approximately 200 Hz was observed. This increase
resulted in improving communication only slowly and could be the effect of an
increase in vocal amplitude. The different behavior between these two species
toward the same environmental changes opens a great debate on the effects of
noise exposure for biodiversity.
The nightingale (
Luscinia megarhynchos
) has been proved by Brumm (
2004
)to
react to urban noise by increasing song intensity (measured using the sound
pressure levels parameter), demonstrating for the first time the noise-dependent
vocal amplitude regulation (Fig.
6.6
). This observation points out that birds try to
reduce the masking effects produced by urban noise. However, this increases the
energy cost of singing, which could create negative effects in the behavioral
ecology of singing males.
The European robin (
Erithacus rubecula
), a highly territorial songbird, uses
intensively vocal communication to defend territories. In the urban habitat, this
species is also singing during the nighttime. This effect has been attributed first to
light pollution (presence of artificial light all night), but recently Fuller et al. (
2007
)
hypothesized that the nocturnal song may be a way to avoid noisy and masking
daytime noise.
Noise playback experiments on the greater sage-grouse (
Centrocercus
urophasianus
) conducted by Blickley et al. (
2012
) have demonstrated the sensitiv-
ity of this bird to noise, with a decrease of birds to leks, although this effect showed
no “memory” in the second year after the noise playback experiment. These authors
confirmed the threat that anthropogenic noise has on population viability on this
sensitive species.
In fishes especially, hearing specialists are limited by the noise regime of their
environment, as demonstrated by Wysochi and Ladich (
2005
) using audiograms
obtained by two hearing specialist otophysine fishes (
Carassius auratus
and
Platydoras costatus
) and an hearing generalist (
Lepomis gibbosus
).
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