Communication Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 88

In some cases males are countersinging, and this behavior grades aggressiveness
by singing the same song, by overlapping, or increasing the rate at which song types
are switched.
Song reliability may be controlled by the cost paid by the signaler or by the
behavior of the receiver. Signals can be controlled by individual performance or by
physical constraints: both are important issues to drive signal design and cost type.
Song is the result of muscular contraction that requires much energy: singing has
an energetic cost, and availability of energy affects singing performances. This cost
has been proved by supplying food to singers, and observing higher acoustic
activity during warm and calm days.
Singing is costly but singing activity is incompatible with feeding. Thus the time
subtracted by feeding produces a reduction of energy input and conversely reduces
singing performance for lack of energy budget. Direct physiological measurements
have proved that the consumption of oxygen during singing performance increases
1.05- to 1.38-fold compared with the pre-singing metabolic level. A difference in
energy consumption has been observed between loud and calm song, and species
with a higher metabolic rate have lower song outputs.
Song quality could be modified by the physical conditions to which an individual
was exposed during the first period of its life (nestling period) (Nowicki et al. 2000 ).
In some species there are syllables that represent a stimulus for the neighboring
males to challenge this individual more frequently. During singing activities males
are exposed to aerial predation if singing from exposed perches. When a singer
localizes a predator, it immediately switches the song to an inconspicuous alarm.
In all oscine passerines song is the result of a learning process: when no tutor is
present during the period between hatching and the time young males establish a
territory, the lack of tutoring reduces song complexity of the song, which appears to
be an honest age-related signal.
The theory of the honest signal has been investigated by Laiolo et al. ( 2004 )ona
population of short-toed larks ( Calandrella rufescens ). This species emits distress
calls in the presence of a predator, and the harshness of the call has been
demonstrated to be connected with body condition and T-cell-mediated immuno-
logical conditions. The harshness of the distress calls seems an honest signal of the
health status of the prey, and as a consequence a signal of ability to escape predators
and a way to inform the predator about the level of difficulty should the individual
be captured.
In a population of short-toed larks of Fuerteventura (Canary Islands), of which
55 % was infected with poxvirus, modifications were observed of the spectro-
temporal structure of the lark distress calls. Infected birds had shorter and lower-
pitched calls when compared with virus-free individuals. Call harshness was
independent of virus infection and was correlated with bird immune and body
condition, but in stressed birds the call pulse rate decreased. Infection in this case
produces two types of effects: a reduction of fitness directly by the disease and
indirectly an increase predatory pressure caused by a lower antipredatory behavior.
The rich song repertoire of male birds is an important attribute not only for male
choice but also for parental effort during the breeding period, as shown by the