Kiwis to pewees: the value of studying bird calls

Avian vocalizations are typically divided into two fundamental types: songs and calls. The dividing line between calls and songs is somewhat unclear, but researchers generally agree that songs are longer and more complex than calls, which tend to be brief and simple (Catchpole & Slater 2008). The literature on birdsong (particularly oscine passerine song) is extensive, encompassing the fields of behaviour, evolution, neuroscience, psychology and more (Slater 2003). Many fewer studies of avian vocalizations focus on the form and function of calls, but this is an area ripe for adding to our understanding of animal acoustic communication (Marler 2006). Researchers can learn much from analyses of avian calls for a number of reasons. First, calls are produced by all species. Many species of birds, particularly those in basal lineages, are not known to sing. Some nonpasserines do sing, but the majority of song research has been conducted on oscine passerine songbirds. Oscines are in part grouped together by their complex syringeal musculature and morphology that allows for the production of intricate song forms (Suthers & Zollinger 2006). However, it would be a mistake to assume that nonoscines, and even birds from basal lineages such as paleognath (ratites such as kiwis and ostriches), galliform and anseriform birds, do not exhibit complexity in the anatomy of their sound-producing structures or in their vocal repertoire. For example, ducks possess a range of fascinating syringeal morphologies (Livezey 1986, 1991); the relationship between the form of these structures and sound production remains to be studied. Basal birds can also produce a rich repertoire of vocalizations, as is seen in Wild Turkeys Meleagris gallopavo, which produce at least 28 different calls to help navigate their complex social world (Williams 1984). Both morphological and vocal complexity can hide undiscovered even in relatively well-studied species; in spite of decades of research into the vocal behavior of Greater Sage-grouse Centrocercus urophasianus, a ‘two-voiced’ system and a syringeal muscle found only in males were only recently described (Krakauer et al. 2009). Thus, while studies of song are necessarily taxonomically restricted, research into calls can take advantage of the rich diversity of morphology, ecology and behaviour found across all lineages of birds. Ornithologists are interested not only in how birds make sounds, but also why such a dizzying array of acoustic signals has arisen. Many hypotheses postulate evolutionary benefits associated with particular features of calls or songs. Inter-individual, inter-sexual and interpopulation differences in species with learned songs could result from the effects of learning. When the acoustic structure of an oscine song improves function, that outcome may be a result of cultural evolution. Calls, on the other hand, are generally thought to be innate, rather than learned (but this is not a universal rule, see Kroodsma & Baylis 1982, Zann 1985) (Catchpole & Slater 2008). Although they are innate, however, calls are not entirely fixed. The calls of many species show significant geographical and even inter-individual variation, as seen in the individual identity encoded in the harmonically rich calls of penguins (Aubin et al. 2000). Selection for communication functions has even led to adaptive variation within individuals. The calls of coucals, swans and tinamous vary with context or local environment, and the combination of vocal and visual displays in Red Junglefowl Gallus gallus has become a model system for understanding communication (Smith & Evans 2008, Geberzahn et al. 2009, Patel et al. 2010, Schuster et al. 2012). Thus, although calls are relatively fixed compared with songs, there is clearly individual variation and the potential for selection on these traits. When calls are unlearned, such selection would necessarily lead to biological evolution, allowing researchers to assess how vocal traits evolve in the absence of cultural influences. Finally, calls are particularly useful in studies of intersexual interactions because across avian species they are generally produced by both males and females (Catchpole & Slater 2008). Many studies of song-learning and song variation have been conducted in species where only the male sings. Although there is a growing interest in and recognition of female song (used in solo song or song duets), studies of song continue to be male-biased (Langmore 1998, Garamszegi et al. 2007). In contrast, studies of call form and function often focus on the calls of both sexes. By studying call form, researchers can gain insights into the power of natural selection in shaping vocal communication in both sexes. In an article published in this issue of Ibis, Digby et al. (2013) describe the calls of the Little Spotted Kiwi Apteryx owenii, providing an overview of call usage patterns and call structure. This article is one of only a *Corresponding author. Email: lauryn.benedict@unco.edu