transmission of complex sequences of information.By contrast,sound
combines a number of features that make it ideal for many forms of com-
munication.It travels fast by day and by night,it goes around obstacles,it
can be detected at long range,and it can encode complex and changing
messages.Given these features,it is not surprising that it is the medium
adopted in our own language and music as well as in the song of birds.
Birds produce sounds in a different way from ourselves.Whereas we
have a larynx high in the throat,the syrinx that birds use is much lower,
at the point where ducts from the two lungs (bronchi) join to form the
trachea.Most birds produce quite simple sounds,and their syrinx is
similarly uncomplicated.
Most complex singers belong to a group known as the songbirds (order
Passeriformes,suborder Oscines), which comprises nearly half the
known bird species.In keeping with the sounds they produce,a defining
feature of this group is that their syrinx is operated by five or more pairs
of muscles,unlike the three or fewer in most other bird groups.The syrinx
has two membranes,one on either side,and the sound produced depends
on tension in them.Because there are two membranes,each with its own
set of muscles,birds can produce two separate and harmonically unre-
lated sounds at the same time.Lips on each side of the syrinx can be
opened or closed independently,and this also means that one side can
produce a sound while the other is silent (Suthers,Goller,and Hartley
1994).A further complication is that resonances within the vocal tract,
for long ignored,are now realized to influence the sound produced
(Nowicki and Marler 1988).The exact workings of the syrinx are still a
matter of controversy (see,for example,Goller and Larsen 1997),but
there is no doubt that it is a superb musical instrument.
Birds make a variety of different sounds,the simpler of which are
referred to as call notes.The word “song”tends to be applied only to
longer and more complex vocalizations.Most of these are produced only
by males and only in the breeding season,but this is not a hard and fast
rule.Female European robins (Erithacus rubecula) sing in the winter
(Lack 1946),and in the tropics the females of many species sing (Morton
1996).The male house sparrow (Passer domesticus) has no song in the
sense of a long and complex sequence of sounds,but the “cheep cheep”
he calls out from the rooftop may well serve the same function.Some
songs are certainly very simple.Nevertheless,most of them are easily dis-
tinguished as the longest and most complex sounds of a species,and these
are commonly produced only by males in the breeding season.
The fact that song is,in many species,a preserve of breeding males pro-
vides a clue as to its function.Considerable evidence shows that its role is
in part to attract and stimulate females and in part to repel rival males
from the territory of a singing bird (see Catchpole and Slater 1995 for a
more extensive review).A number of lines of evidence point to its part in50 Peter J.B.SlaterMUS4 9/14/99 11:59 AM Page 50