Biomedical Engineering Reference
In-Depth Information
partially adducted membranes are set in motion by Bernoulli forces. This
motion affects the airflow, which further affects the dynamics of the mem-
branes modulating the airflow. Song will be produced as long as the motion
emerging out of these interactions is of oscillatory nature, since it is capable
of modulating the airflow in a way that gives rise to sound.
In agreement with this basic idea, there is a wide diversity of anatomical
devices that birds use to produce song. What are the membranes that oscil-
late? Where are they located? How can the vocal organ be controlled? What
are the muscles in charge of this control? In order to describe the physical
mechanisms involved in the generation of birdsong, we have to review part
of what is known about these issues.
3.1.2 Morphological Diversity
The taxonomic classification of songbirds and nonsongbirds was originally ba-
sed on syringeal morphology, which varies considerably [Elemans et al. 2003,
Suthers 2001]. Most bird species have a tracheobronchial syrinx (which is
particularly common in the suborder Oscines, order Passeriformes), in which
the syringeal membranes are located between tracheal and bronchial carti-
laginous rings at the tracheobronchial junction. Two other types of syrinx
are found: the tracheal syrinx, involving only tracheal cartilaginous rings, oc-
curs mainly in the Furnariidae (ovenbirds) and in some Ciconiidae (storks);
and the bronchial syrinx, involving only bronchial cartilaginous rings, oc-
curs mainly in Caprimulgiformes (goatsuckers), Cuculiformes (cuckoos) and
in some Strigidae (owls) [Casey and Gaunt 1985].
The syringeal anatomy of some birds (such as the ones with tracheal sy-
rinxes mentioned above, and also the parrots, Psittacidae) resembles that
of the larynx in humans in that there is only one vocal valve. However,
Oscines and other taxa have two sound sources, one in each bronchus below
its junction with the trachea. Some birds use mostly one source (such as the
waterslager canary), but some use both (such as the brown thrasher, Toxos-
toma rufum , with both sides of the syrinx contributing equally to phonation).
Some birds can use the two sources independently, producing either the same
or a different frequency, and some can even make them interact, as we shall
see in Chap. 6. Some syrinxes are displayed in Fig. 3.1.
3.1.3 The Richness of Birdsong
The details of the configuration of the syringeal structure are controlled by
the bird by means of a set of muscles, in order to create a wide range of
sounds. Remarkably, some muscles at each side of the double structure receive
instructions in an independent way. This allows the bird to produce sounds
using both sources either in a simultaneous or in a successive way, providing
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