Bioacoustics Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 52

many species (from plants to animals) that may find resources more abundant than
in natural conditions. The management of urban space to also preserve biodiversity
is not an extravagant idea but an important issue for the long-term vision of
biodiversity on this planet. In many species of birds, males sing to attract females
and to defend their territory. If such communication is masked by background
noise, as is very common in urban areas, the males must fight directly with potential
intruders, vesting energy without the biosemiotic support of a sign process, and
females may be not attracted or attracted with greater difficulty.
In the great tit ( Parus major ), one of the most common birds adapted to live in
urban areas, differences in song performance have been observed in different cities
and habitats. Slabbekoon and den Boer-Visser ( 2006 ) have investigated the song
repertoire of this species in ten large cities across Europe, comparing their songs
with those of adjacent forested areas. Interesting modifications were found in
strophes, which are a repeated set of phrases where each phrase is composed by
two, three, or four notes that finally produce a song type. In the urban environment,
this species produced only 80.7 % of song types compared with 90.8 % in forested
areas and sang more rare song types.
A comparison between urban versus forest areas show that the song repertoire is
different and that in urban areas the average minimum frequency is significantly
higher than in forested areas.
It could be a dimension effect on the frequency shift if this species should have a
change in body size with a lower body size in urban area compared to forest areas,
but this is not been found up to now, so effects resulting from different body size
cannot explain the differences in frequencies.
Change in song performance has been observed not only in the frequency
domain but also in the temporal domain. Songs of “city birds” are shorter in
duration with shorter inter-song intervals.
The first note of the phrase is shorter in city singers, but not the other notes, and
this is in agreement with the fact that urban areas are not only noisier than forest
habitat but are open areas when compared with forest. To assure better long-
distance communication the signals should be of lower frequency, shorter, and
produced at a fast rate. These authors have argued that the modification of the first
note creates an advantage because the first note is critical for detection and
recognition. The phenotypic plasticity found in the great tit could accelerate
genotypic divergence in the future, with potential urban speciation.
The changes in great tit song performance have been observed in many countries
and in many noisy environments.
The habitat conditions outside tropical areas are quite variable, similar to
deciduous habitats where, according to the season, there are different acoustic
conditions for the presence or absence of foliage cover and for the status of leaf
maturation which alter the acoustic conditions. These differences produce effects
on the acoustic impedance and on reflection and diffraction of the acoustic signal on
the leaf surfaces. In temperate biomes the seasonal growth of plants such as ferns
(e.g., Pteridium aquilinum ) or other annual tall forbs modifies the sonic environ-
ment with an increase of sound degradation of the high frequencies. Turbulence