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Fig. 7.2 A dawn chorus sequence of spectrograms at 4:00 a.m. ( a ), 4:22 a.m. ( b ), and 5:06
a.m. ( c ) on 6 June 2011 in a Mediterranean maqui. The common blackbird ( Turdus merula ) is the
first species to sing at dawn, immediately followed by the European robin ( Erithacus rubecula )
winds increase in intensity, and the air temperature increases as well, and other
biophonies from insects become dominant. More in general, the background noise
increases consistently, reducing the signal-to-noise ratio (SNR), producing attenu-
ation of spherical spreading (6 dB for doubling distance) and excess attenuation
(EA) from absorption and multiple scattering. Other effects are produced by
filtering, atmospheric turbulence, and reverberation. Definitively, during the day
the sonic ambience becomes acoustically more complex with a higher risk of sound
attenuation by wind turbulence, vegetation rustling, air temperature, and humidity
increase. The background noise increases, and this contributes to reducing the
signal-to-noise ratio (SNR).
The acoustic transmission hypothesis has been experimentally tested by Brown
and Handford ( 2003 ) by transmitting the song of the white-throated sparrow
( Zonotrichia albicollis ) and swamp sparrow ( Melospiza georgiana ) both at dawn
and at midday and re-recording at distance of 25-100 m in open and closed habitats.
No significant differences of mean overall absolute transmission were found at
dawn or at midday for both habitats, but the signal transmission quality was higher
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