Bioacoustics Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 44

Fig. 3.1 The four
bioacoustics hypotheses and
their major constraints

birds body size is correlated with frequency. This correlation is explained in birds
by the capacity to vibrate the syringeal membrane, which can produce high-
frequency sound only in a small apparatus.
The relationship between body mass and frequencies utilized in vocal
performances has been proved by Wallschlager ( 1980 ) in 90 European passerine
birds and by Fletcher across different species of birds and mammals (Fletcher 2004 )
with a relationship between frequencies and body mass expressed by a power law
f M 0.33 (f

body mass) (Fig. 3.2 ).
Similar results have been found by Boeckle et al. ( 2009 ) from 76 species of frogs
where 25 % of the variation in dominant frequency has been explained by
body size.
In birds, besides body size the dimension of the bill is a further important
element to modulate sounds, especially high-frequency sounds. In fact, the change
in bill size and its morphology are not only correlated with foraging habits and the
structure of the habitat, but the suprasyringeal tract that functions as a resonating
space is directly related to bill dimension and shape. Small bills can produce more
modulations than large bills, and this morphological trait is negatively related with
the trill rate. Smaller bills produce trills at a higher rate and definitively larger bills
are less versatile and less able to produce sound than smaller bills. In particular, the
bill is gaped more during the production of high-frequency notes, and the speed
with which the bill is opened or closed depends on the dimension of the bill. Podos
( 1997 ), investigating 34 species of birds belonging the family Emberizidae, has
found that the increase of trill rate corresponds to a reduction of frequency band-
width utilized.

frequency, M

/ ¼ ¼

3.3 The Acoustic Adaptation Hypothesis (AAH)

A community of acoustic animals is arranged according to the acoustic signals that
individuals emit to fulfill different functions. Long-range and short-range
communications occur intensively during specific active times. Density, receiver
psychology, signal used, and environmental conditions are the major factors that