The key properties of acoustic signals are frequency, duration, and amplitude. The frequency and temporal characteristics of animal vocalizations have been well studied in a variety of taxa, but much less is known about the role of vocal amplitude (Bradbury and Vehrencamp 1998). This is surprising, since the amplitude of an acoustic signal is a key factor for the exchange of information, determining the broadcast area, or active space, of the signal (Marten and Marler 1977). However, it is important to be clear that the communication range is not only determined by the absolute signal amplitude. The level and spectral characteristics of background noise also contribute considerably to the transmission distance, because detection and recognition of signals substantially depend on the signal-to-noise ratio (Klump 1996).
In addition, the intensity of animal vocalizations can also encode information that is used by a receiver for decisions relevant to sexual selection. For instance, the sound level of male vocalizations and stridulations affects female mating preferences in several anuran and insect species (review in Gerhardt and Huber 2002), as well as in Red-winged Blackbirds Agelaius phoeniceus (Searcy 1996). In addition to mate choice, another mechanism of sexual selection is male-male competition. Vocal amplitude can also be important to repel rival males, as it has been shown for anurans. In birds, similar evidence has been obtained for Eurasian Blackbirds Turdus merula. The strength of territorial male responses increased with increasing song amplitude of simulated rival males, suggesting that the intensity of territorial songs is used by receiving males to regulate distance (Dabelsteen 1981, Todt 1981).
Despite its significance, the use and regulation of vocal amplitude has been neglected. This is partly due to methodological difficulties and, in the sphere of birdsong, to the fallacy that the amplitude of territorial songs is a trait that does not vary much.
To help filling this gap, my co-workers and I examined whether and how a territorial songbird, the Common Nightingale Luscinia megarhynchos (hereafter called nightingale), is able to adjust the intensity of its song. Nightingales have a discontinuous singing style and individual song repertoires can comprise about 200 song types (Hultsch and Todt 1982). There has been extensive research on song development and learning (review in Todt and Hultsch 1996), and the use of songs in male-male interactions (review in Todt and Naguib 2000) in this species.
I have shown that males did not simply maximize the sound pressure levels of their territorial songs. Instead the issue of vocal intensity in territorial songbirds proved to be more complicated than previously supposed. The amplitude of nightingale songs appeared to be individually adjusted according to a variety of factors, especially in relation to the actual environment of the bird and the memory of the songster (Fig. 1). With regards to environmental, or ''Umwelt'', factors, the level and spectral characteristics of background noise and the social context of singing appear to affect the amplitude of songs. Our results indicate a noise-dependent regulation of vocal amplitude (Brumm and Todt 2002, Brumm 2004) and an increase in song intensity during male-male vocal interactions (Brumm and Todt 2004). Additionally, during vocal development, the intensity of songs was related to memory and learning. Embedded in an age-dependent increase in overall sound level, imitations of acquired model songs were produced with a higher amplitude than the remaining vocal patterns that could not be identified as imitations (Brumm and Hultsch 2001). This plasticity in signal amplitude was based on active variation of vocal sound levels. Directional sound radiation of songs due to sound shadow effects of the birds' head and body also leads to vocal amplitude variations (Brumm 2002). In contrast to active adjustments of song level, this variation in song intensity is merely related to the position of the receiver in relation to the sound source, the singing bird. The directional sound radiation pattern of songs turned out to affect behavior. Territorial males changed their singing direction more often and showed more lateral head movements within songs during solo singing than during vocal interactions with a simulated rival. These results suggest that nightingales either counteract or exploit the directionality of their songs depending on the perceived position of intended receivers (Brumm and Todt 2003).
In the following, I will discuss song amplitude in relation to vocal interactions in birds, ecological and physiological limitations of singing loudly, and the mechanisms of song production.