Some animals that use sound to communicate have evolved adaptations to counteract the masking effects of environmental background noise. In the short-term, one such adaptation is the Lombard effect, in which a signaller increases the amplitude of its vocalisations in response to an increase of the background noise amplitude. Such a capacity has been demonstrated for zebra finches (Cynx et al. 1998: Anim. Behav. 56, 107-113), a non-territorial songbird. Territorial songbirds, on the other hand, may benefit from singing as loud as possible to defend territories and attract females. Therefore, they may maximise the amplitude of their songs rather than regulating it according to the background noise level. I addressed this issue by experimentally manipulating the background noise and measuring the sound level of the vocalisations of a territorial songbird, the Common Nightingale Luscinia megarhynchos. This species is a good model to investigate the mechanisms of vocal amplitude regulation in detail, because males typically produce their territorial songs with specific amplitude differences between the diverse song sections (Hultsch, personal communication). Subjects (n = 4) increased the mean sound level of their songs in response to increased sound levels of white noise. Within songs, such regulation was found in both the loudest and the softest notes. One bird increased its call amplitude in response to increased levels of noise, showing that nightingales may also exhibit the Lombard effect in non-territorial signals. These findings show that nightingales do not maximise song amplitude but regulate vocal intensity dependent on the level of environmental noise. Possibly, such adjustment of vocal amplitude serves to maintain a specific signal-to-noise ratio that is favourable for signal production. Concurrently, amplitude regulation may ensure maintaining a given active space for communication. Thus, vocal amplitude in a territorial songbird is be a flexible trait, which is regulated according to ecological demands from signal transmission, as recently discussed by Brumm and Hultsch (2001, Anim. Behav., 61, 747-754).
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