Spectrograms show that sounds in the rainforest have evolved to occupy different frequency bands almost like the different frequencies of broadcast radio waves. There is a surprising degree of separation both in the frequency and time domain, which, it could be speculated, is to avoid interference with each other’s ‘broadcast range’ (fig. 10.7) . Indeed, research has shown that birds may vary the pitch of their song in order to avoid them being masked by insect noise (Kirschel et al., 2009).

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Figure 10.7 The top spectrogram shows a typical rainforest at dawn with different sounds exhibiting a large degree of separation as if occupying the different frequency bands of a radio.