Nightingales produce their territorial songs with relatively low amplitudes during solo performance. As environmental noise levels vary they compensate for interference from background noise by adjusting the amplitude of their vocalizations (Brumm and Todt 2002). This mechanism of vocal amplitude regulation (aka the 'Lombard effect') has also been shown for non-territorial songbirds (Cynx et al. 1998, Kobayasi and Okanoya 2003), non-songbirds (Potash 1972, Manabe et al. 1998, Pytte et al. 2003) and primates (Lombard 1911, Sinnott et al. 1975, Brumm et al. 2004). The Lombard effect enables vocalizing animals to actively maintain the broadcast area, or active space, of their vocalizations by increasing vocal amplitude in response to an increase in the background noise level. In their natural habitats, animals that use sound to communicate have to face a variety of noises, such as abiotic noise, e.g. wind, rain or flowing water, or biotic noise, i.e. interfering sound produced by other animals. Different types of masking sounds affect the regulation of song amplitude, e.g. the sounds from heterospecific birdsong (Brumm and Todt 2004) or man-made sounds, such as traffic noise in urban habitats (Brumm 2004).
Song amplitude can also change with changes in the social context of singing (Brumm and Todt 2004). During vocal interactions with a simulated rival male, nightingales increased the sound level of their songs by more than 5 dB. This augmentation of song level cannot be completely explained by the masking effect of the rival songs, because males increased their vocal intensities to a significantly lesser degree during control experiment with heterospecific songs. By increasing vocal amplitude the birds extend the active space of their songs, helping to broadcast their songs more effectively when interacting with a rival male. In the context of signal transmission, the findings from the ontogenetic analysis of vocal amplitude in nightingales are also of interest. We found that the precursors of sound patterns destined for adult communication were produced with higher amplitudes than those vocalizations that could not be identified as imitations and thus were related to a more or less unspecified singing activity (Brumm and Hultsch 2001).
Additionally, vocal amplitude may also serve as a signal itself, as it has been shown for other song characteristics, such as repertoire size, song rate, timing of singing, and counter-singing patterns (review in Gil and Gahr 2002). More specifically, the sound level a male produces during interactions might well be related to phenotypic or genotypic quality (e.g. body size, age, muscular strength, nutritional state) or motivational states such as the willingness to escalate the interaction. Here, possible addressees may be not only the rival male, but also other males or females. These additional receivers can gather information from the dyadic information exchange between the territorial disputants and use this information for individual decisions on future actions, as has been shown for both third party male rivals (e.g. Naguib and Todt 1997, Peake et al. 2001), and females (e.g. Otter et al. 1999, Mennill et al. 2002). In turn, these decisions can affect sexual selection of songs, in terms of female choice or male-male competition. However, the nightingale studies place more emphasis on the significance to the countersinging rival male than to potential third party receivers, since the nightingales use the orientation of their bodies to direct their songs to the male rival (Brumm and Todt 2003).
The finding that nightingales sang with lower vocal amplitude during solo performance and with increased song levels during interactions points to potential costs of singing loudly. This thought seems plausible, because if the production of loud songs was not subject to some costs or constraints, one should expect that birds would always sing with high amplitudes to defend territories and attract distant females.