Communication Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 80

When the receiver (a tape recorder) was changed in position from lower to highest
positions, the resultant sound degradation was also reduced. This observation
explains the position chosen on the highest perches by a singing male, which favors
the propagation all around of the song that has a definitive meaning of territorial
demarcation.
4.7.3 Some Caveats on Playback Methodology
The playback methodology to assess the potential ranging in animals today represents
a standard method and is intensively used, but this methodology can introduce
biases by simplification of the sonic condition during the experimentation.
For instance, Penna et al. ( 2012 ) warn that the use of broadcast signals for a
transmission experiment may introduce a simplification of the conditions in which
animals communicate in nature, as recently tested on the Iberian midwife toad
( Alytes cisternasii ).
A recent study conducted by Thomas et al. ( 2003 ) on the response of robins
( Erithacus rubecula ) during nighttime playback of conspecific songs shows a quite
interesting pattern: two populations of robins (Irish and Welsh) responded differ-
ently to nocturnal playback experiments. The Irish population rarely reacted to
playback, but the Irish birds were reactive to playback stimulus. The unexpected
difference between these two populations is not found during the day. The
differences in nocturnal playback response must be explained excluding the inten-
sity of illumination or air temperature. An explanation can be found in the different
predatory pressures of these two populations in Ireland and Wales. In Wales, the
long-eared owl ( Asio otus ) is common and diffuse but in Ireland it is rare, and we
know that this predator localizes, as does every nocturnal predator, their prey by
ear. This study demonstrates how prudence must be used to plan playback
experiments because of unexpected differences in interpopulation behavior.
4.8 Distress Calls
Among several behavioral traits used by animals to signal the presence of predators,
functioning as public information, distress calls are used by many birds to warn
other species about the menace and simultaneously to confound predators. In fact,
distress calls are extremely difficult to locate aurally. The distress calls have a basic
structure: a carrier frequency with numerous harmonics, slowly modulated in
frequency.
Distress calls have a complex structure and suffer in their propagation when
uttered in dense vegetation. To test the hypothesis that distress calls of birds living
in vegetated habitat are better adapted than calls of species living in an open field,