The results of psychoacoustic experiments on nightingales indicate that the Lombard effect is induced most effectively by noise in the spectral region of their songs (Brumm and Todt 2002). The same applies to heterospecific songs which acoustically mask this frequency band (Brumm and Todt 2004). Thus it is reasonable to assume that nightingales assess the signal-to-noise ratio between their songs and the background noise and adjust their vocal intensity accordingly. This finding provides further evidence for a neuronal feedback loop between auditory perception and song production (Leonardo and Konishi 1999). Like humans (Lee 1950, Belmore et al. 1973) adult birds obviously monitor their own vocalizations and the adjustment of vocal amplitude may serve to maintain a specific signal-to-noise ratio that is favorable for signal production.
Perceptual mechanisms are also involved in vocal learning in juvenile birds. Here, birds also monitor their singing and consolidate their vocal memories by auditory feedback (Konishi 1965, Marler 1967). Such feedback mechanism may relate to the production by young nightingales of imitations of acquired model songs with higher amplitude than unidentified patterns (Brumm and Hultsch 2001). In their attempts to map the acoustic configuration of their own vocalizations on to acquired instructions, songbirds may progressively increase the amplitude of imitations of acquired model songs through a feedback mechanism. Whether auditory feedback can account for both the long-term ontogenetic changes, and also for short-term differences in vocal intensity needs further examination. To sum up, I conclude that song intensity is a significant component in the development of a dynamically unfolding vocal signaling system. Like other major ontogenetic trajectories such as the timing, phonetics and syntax of vocalizations, sound level is probably highly interrelated with overall developmental progression, and thus should be viewed as an integrative part of the vocal learning process.