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Fig. 3.5
Long syllables,
such as whistles (
a
), can
cross dense layers of
vegetation with minimal
interference, whereas short
syllables (
b
), such as trills,
are more likely to be
transmitted in open
environments
subspecies lives in a denser understory with greater vertical heterogeneity.
According to the AAH, the song of
M. m. heermanni
has lower pitched and more
widely spaced elements than that of
M. m. fallax.
As an adaptation to the conditions of dense rainforests, birds use low-pitched
whistle-like songs.
Narrow-frequency bandwidth notes (NFB) are pure tones that do not have great
changes in frequency; these tones are emitted by birds that live in dense forests and
differ from frequency-modulated notes. These notes benefit from reverberation,
allowing the transmission of a longer and louder signal as demonstrated by play-
back experiments conducted by Slabbekoorn et al. (
2002
) on the green hylia (
Hylia
prasina
), a bush warbler passerine (family Cettiidae) living in dense forests of
tropical Africa. Its song is quite simple, composed of two successive NFB.
Longer notes produced a stronger behavioral response, demonstrating that
reverberation can improve signal transmission when the signal design is adapted
as in NFBs. The positive effect of reverberation has been tested on the white-necked
thrush (
Turdus albicollis
) and on the sympatric species cocoa thrush (
Turdus
fumigatus orinocensis
) by Nemeth et al. (
2006
). The first species has song elements
that are double those of the cocoa thrush, and an experiment of long-distance
transmission showed that the white-necked thrush, which has a longer sound, has
a less-attenuated signal (Fig.
3.6
).
The authors suggest that the positive effect of reverberation favors longer songs.
A more complex scenario, depicted by Ziegler et al. (
2011
), showed that the
call attributes of the frog
Hypsiboas pulchellus
are strongly affected by habitat
structure in terms of temporal parameters of the call and that temperature affects the
size of individuals and consequently the dominant call frequency. These authors
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