In most bird species, Acoustic Communication Signals (ACSs) fulfil various functions and form the major part of the intra-specific communication system. The first function to be fulfilled by any animal communication system is the Species-specific Recognition (SSR). In the few animal groups that developed ACSS, these signals generally carry that information. This is particularly the case of birds where, except or a few species that replaced sound signalling with a visual one, a given ACS permits instant SSR. Such ACS should be called the "functional song'', whose definition becomes: ACS carrying the information for SSR. Identification of the "functional song'' is more objective than any previous definition of song, but is not always evident. Our definition implies firstly that ACSs associated with territorial defence and mating, even when similar to the "functional song'', need to be properly distinguished for they fulfil subsequent and fully distinct functions. It implies also that such "functional song'' provides the code message necessary for efficient SSR: this is clear in species with a genetically determined song, but becomes more complicated when learning is involved, reaching a challenging situation in the case of species with versatile or imitative song. This exposition will be illustrated by examples taken from our researches on Brazilian avifauna.