Sonic Patterns II: The Animal Choruses - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 192

Fig. 7.1 Distribution of choruses at dawn and dusk during 1 year of sound recording in a
Mediterranean maqui. The three-dimensional (3D) model was obtained by interpolating the values
obtained by the acoustic complexity index (ACI) (Reproduced from Farina et al. 2011 )
7.2 The Chorus Hypotheses
Several hypotheses have been proposed during these past years to explain the
origins and causes of choruses, which seems an acoustic paradox in terms of energy
cost against an apparently modest individual benefit. In fact, choruses create a
considerable background and foreground noise from the multiplicity of individuals
or species that are vocalizing at the same time.
In birds, the beginning of singing activity varies according to the species. In
general larger species with large eyes (and greater pupil diameter) are the first
species to initiate the chorusing: they start to sing when the light level is low, as
confirmed by the work of Thomas et al. ( 2002 ). For instance, in a Mediterranean
maqui the common blackbird ( Turdus merula ) is the first songbird to sing, followed
immediately by the European robin ( Erithacus rubecula ) (Fig. 7.2 ).
Several hypotheses have been reviewed by Staicer et al. ( 1996 ) to explain the
choruses, and these hypotheses can be grouped in three broad categories of proxies:
intrinsic, environmental, and social factors.
The intrinsic factors are driven by the circadian cycles of testosterone.
Environmental factors invoke a low predation risk, good acoustic transmission
in quiet air, inefficient foraging at low ambient light, and unpredictable nighttime
conditions that produce a probable excess of resources in the early morning.
The social factors are based on mate attraction, territory defense, and resolution
of social dynamics. Females can receive important information on the quality of the
male considering the song as an honest signal.
Among the environmental factors, the acoustic transmission hypothesis (ATH)
claims that birds sing most intensively in the quieter time of the day when song is
not degraded by air turbulence or anthropogenic noise. During the day the local