It has been suspected that several factors may limit the production of loud songs in birds, including energy expenditure, social aggression and predation (review in Gil and Gahr 2002). However, whereas the vulnerability of sound-producing insects and anurans to acoustically orientating predators and parasites has been reported in a variety of studies (review in Zuk and Kolluru 1998), evidence of predation cost of singing in birds is still lacking, apart from the study of Mougeot and Bretagnolle (2000) on petrel calls. More research on this issue is needed before the predation costs of birdsong can be assessed.
With regards to social aggression, it seems that the low amplitude of solo singing in nightingales cannot be explained by the avoidance of aggression by other males. Instead I have the impression that quite the reverse is true, for as soon as the presence of a rival was perceived, territorial males increased both amplitude and transmission properties of their songs.
In contrast to anurans (Gerhardt and Huber 2002), the metabolic costs of vocalizing in songbirds are not very high, but increased vocal amplitude causes higher energy expenditure (Oberweger and Goller 2001, Ward et al. 2003). However, a study on singing activity and body weight loss in nightingales suggested rather high energetic costs of singing (Thomas 2002). It remains to be established to what degree energy expenditure limits the performance of loud songs and thus can explain the relatively low sound pressure levels during solo singing.