Noise-dependent song amplitude regulation in a territorial songbird

Some animals that use sound to communicate compensate for interference from background noise by adjusting the amplitude of their vocalizations as environmental noise levels vary. Territorial songbirds may have evolved a different strategy, since they can be expected to benefit from maximizing the amplitude of their songs to defend territories and attract females. We tested this hypothesis with calibrated measurements of the song level of male nightingales, Luscinia megarhynchos. All birds increased the sound level of their songs in response to an increase in white noise broadcast to them. A second experiment revealed that noise in the spectral region of their own songs was most effective in inducing the birds to increase vocal intensity. These findings show that nightingales do not maximize song amplitude but regulate vocal intensity dependent on the level of masking noise. The adjustment of vocal amplitude may serve to maintain a specific signal-to-noise ratio that is favourable for signal production. Concurrently, increasing the intensity of songs can maintain a given active space for communication. Thus, vocal amplitude in a territorial songbird can be interpreted as a flexible trait, which is individually regulated according to ecological demands from signal transmission. (C) 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.