Communication Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 89

investigation of Buchanan and Catchpole ( 2000 ) in the sedge warbler
( Acrocephalus schoenobaenus ). They found that chick weight, when controlling
for chick age, was correlated with the repertoire complexity (size) of the male.
Repertoire size is recognized by the female of this species as an important factor of
mating selection. Thus, the selection of a male with a complex sound repertoire
assures indirect and direct benefits such as increase of parental effort.
The honest signal hypothesis has found a confirmation in a population of song
sparrow ( Melospiza melodia ) living on Mandarte Island, British Columbia, Canada.
Reid et al. ( 2005 ) have observed that males with a larger song repertoire contributed
more independent and recruited offspring and independent and recruited grand-
offspring. This result was possible because these males were living longer and
reared a greater portion of hatched chicks. Independent offspring of males with a
larger song repertoire had a higher probability to recruit and then to have more
grand-offspring than males with a lesser song repertoire.
After a male acoustic performance, the females decide whether to mate or to
ignore the male. The choice may depend on several factors. Food seems one of the
most important factors acting as an evolutionary force, but recently Ritschard and
Brumm ( 2011 ) tested this hypothesis on the zebra finch ( Taeniopygia guttata )by
manipulating food availability in males. The results did not demonstrate a relation-
ship between food availability and syllable repertoire, proportion of sound versus
silence within a song, and mean song frequency. Treated birds showed a reduced
song rate, an increased latency to sing, and a lowered song amplitude and funda-
mental frequency.
4.13 Dialects as a Divergent Communication
Dialects are important variations in communication sets among birds (and also
among humans). Dialects represents variation of the acoustic theme performed by a
local population of a species under specific environmental constraints. Acoustic
divergence seems the first step for a more pronounced and definitive divergence for
speciation. Recently, Slabbekoorn and Smith ( 2002 ) discussed the role and the
signification of dialects in birds, arguing that many studies on dialects have not
considered the role of ecology; in other words, the consequences on fitness for
female mate preferences have not been considered, or only divergences in realized
acoustic variation without a consideration of the potential acoustic variation have
been considered. These authors stress the importance of the learning characteristics
of an individual in song variation at the population level.
Acoustic mechanisms contribute to species differentiation and isolation. The
subspecies represents the first degree of speciation, and acoustic cues can contribute
to this process. Experiments conducted in Emberiza schoeniclus and in the Medi-
terranean subspecies ( E. s. intermedia ) have not produced definitive results in terms
of acoustic differentiation (Matessi et al. 2000 , 2001 ), but this work represents an
attempt to investigate the role of acoustic performance as initiator of an isolating
process that in the long term should produce speciation.