A song for the South also: defining birdsong in global terms

Generally, calls are short, simple and produced by both sexes at any time, whereas songs are long, complex and produced only by males in the breeding season. Song refers primarily to the vocal displays of territorial male birds. Typically a song is defined as a relatively structured vocalization produced while attracting a mate or defending a territory. Birdsong: certain vocalizations of birds, characteristic of males during the breeding season, for the attraction of a mate and for territorial defense. Songs tend to be more complex and longer than birdcalls, used for communication within a species. Songs are the vocalizations of birds most pleasing to people. Gordon Maclean, in a text entitled ‘Ornithology for Africa’, focused on a single function: ‘Song is therefore by definition any vocalization which advertises territory, no matter what it sounds like to the human ear’ (Maclean 1990, p. 178). We believe that a more general description of song, applicable to all birds, would better guide research on the role and information content of these vocalizations. Thus we propose the following definition, to incorporate the most inclusive features of this behaviour: Song: A sequence of several vocal elements with species-specific characteristics, performed with a particular rhythmic pattern, which serves for intraspecific communication in birds. Elements of song are regarded as short sounds (a single continuous trace on a sonagram) separated by silent intervals, typically no more than 1 s in duration (cf. Houdelier et al. 2012). Songs are usually repeated many times in a song bout, with intervals generally < 10 s, often with some individual variation, and undoubtedly with interindividual variation, as all organisms are unique! The following variations on this theme are well-established: Our definition is based purely on the structure, rhythm and pattern of the vocalization, and avoids any reference to the function of birdsong. Such a definition could mean that not all passerines (or even oscines) sing, and that many non-passerines do sing. It also does not exclude the possibility that a species may have more than one song, possibly with different functions. Clearly some vocalizations currently described as ‘calls’, such as the ‘chick-a-dee’ call of the Black-capped Chickadee Poecile atricapillus (Foote et al. 2010), would be classified as ‘songs’, as they meet all criteria and serve for intraspecific communication. Functional definitions are too restrictive. A recent study of the Corn Crake Crex crex, a species which does not learn its vocalizations, stated: ‘The cracking call of the Corncrake is the functional equivalent of a songbird's song because it is used to attract mates and deter rivals’ (Budka & Osiejuk 2017). Are these its only functions? The central questions in vocal communication are always: (1) What is the message? and (2) Who is the target audience? There may be several answers to each of these questions for a given species. We suggest that a more inclusive definition of song, avoiding any listing of possible functions, will encourage researchers to explore explanations beyond the conventions of territory acquisition and maintenance, and mate attraction.