A Potential Metric of the Attractiveness of Bird Song to Humans Tim M

ethologyinternational journal of behavioural biology Ethology CURRENT ISSUES - PERSPECTIVES AND REVIEWS A Potential Metric of the Attractiveness of Bird Song to Humans Tim M. Blackburn*†, Shan Su* & Phill Cassey‡ * Institute of Zoology, Zoological Society of London, London, UK † Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Stellenbosch, South Africa ‡ School of Earth & Environmental Sciences, University of Adelaide, Adelaide, SA, Australia (Invited Review) Correspondence Abstract Tim M. Blackburn, Institute of Zoology, Zoological Society of London, Regent’s Park, The prevalence of passerines (mostly oscines, or songbirds) in interna- London NW1 4RY, UK. tional bird trade suggests that the possession or production of a song that E-mail: [email protected] is attractive or desirable to people may contribute to the likelihood of a species being traded. Testing this is difficult because we lack a general and Received: September 20, 2013 readily available metric that quantifies attractiveness of bird song to Initial acceptance: November 9, 2013 humans. We propose and validate such a metric, based on the number of Final acceptance: January 3, 2014 sound files lodged for a species on the Xeno-Canto website (www.xeno-- (M. Hauber) canto.org). Our hypothesis is that species with more attractive songs are likely to be recorded more often, and so be represented more often in this doi: 10.1111/eth.12211 online bird sound resource, all else being equal. Using a sample of North American and European passerines, we show that song repertoire size and geographic range size are consistently related to the number of recordings on Xeno-Canto. We use these results to derive a metric (the residuals of a model of the number of recordings in Xeno-Canto as a function of geographic range size) that may identify songs that are attractive to humans. Bird species whose songs are known to have inspired classical music, including several well known for their songs (e.g. common nightingale, European blackbird), have higher values of the metric than those that have not been referenced in classical music. The metric may help explain which bird species are present in trade, and so contribute to studies of invasion and conservation biology. ‘Lord. What music hast thou provided for the saints in annually (Inskipp 1990; cited in FAO 2011), heaven, when thou givest bad men such music on earth!’ although this number has since decreased due to the Izaak Walton, on the song of the nightingale enforcement of international regulation with respect to some threatened species, and a ban on bird imports to the European Union in response to the Introduction threat of highly pathogenic avian influenza H5N1 Human activities have introduced, and continue to (FAO 2011). Inskipp (1990) noted that most traded introduce, a wide variety of species beyond the natu- individuals relate to ornamental and singing birds, ral limits to their geographic distributions. Many of and this is borne out by figures quoted by the United these species establish viable exotic populations, and Nations (FAO 2011). They show that world markets some of those go on to become invasive. One of the are dominated by Passeriformes (passerines) and Psit- key steps in understanding this process is to identify taciformes (parrots), comprising 70% and 20% of all the characteristics of species that contributes to why trade, respectively, vs. 59% and 4% of all bird they are translocated to new regions (Blackburn species. Most passerine (81%) species belong to the et al. 2011). For birds, deliberate translocation of suborder Oscines (songbirds), as do most passerine individuals for the cage bird trade is a major such species in trade (e.g. Carrete & Tella 2008), which pathway. During the 1990s, international trade therefore suggests that song may be a significant was estimated to have involved 2–5 million birds contributor to the presence of a bird species in trade. Ethology 120 (2014) 305–312 © 2014 Blackwell Verlag GmbH 305 The Attractiveness of Bird Song to Humans T. M. Blackburn, S. Su & P. Cassey Oscine passerines are characterized by more com- attractiveness is a subjective quantity, in the ear of plex syringeal morphology and more complex songs the beholder, and culturally influenced. While there than suboscine passerines (Baptista & Kroodsma are undoubtedly some birds whose songs are gener- 2001; del Hoyo et al. 2003). Many passerine songs ally considered to be bywords for beauty (e.g. com- sound raucous or unmelodic to the human ear, but mon nightingale Luscinia megarhynchos, song thrush others possess such aesthetic value as to be described Turdus philomelos, European blackbird T. merula, as beautiful (Hartshorne 1968). Bird song may not be northern mockingbird Mimus polyglottos), the songs of music (Araya-Salas 2012), but it has inspired some most species are unknown to most people. The metric significant pieces of music and other art, from Mozart would have to sample a range of listeners from differ- divertimenti and serenades to poetry by Shakespeare, ent cultural backgrounds to obtain a consensus view Shelley and many others. Nostalgia for the songs of on what constitutes an attractive song, so that it is not birds from the homeland is thought to have motivated sensitive to the opinions of one or two individuals some deliberate bird introductions by European who may favour one of the elements that defines a settlers in the colonies (McDowall 1994), while In- song (e.g. complexity vs. repertoire size; see e.g. skipp (1990) claimed that informal markets for sing- Chapter 6 in Mynott 2009). Fourth, any such metric ing birds among China and other Eastern countries would ideally present variation for analysis and there- involved approximately 1–3 million birds a year. We fore quantify attractiveness to humans as a continu- expect that the possession of a song that is attractive ous trait, rather than simply presenting a few discrete or desirable to people may therefore contribute categories. significantly to the likelihood that a bird species gets Here, we propose and quantitatively assess a possi- entrained on the invasion pathway. ble metric that may give a practical solution to these To assess the contribution of song to the likelihood challenges: the number of sound files lodged for a spe- of human-mediated invasions by bird species, we cies on the Xeno-Canto website (www.xeno-canto. need a general and readily available metric that quan- org). Xeno-Canto is a non-profit website set up and tifies the attractiveness of that song to human ears chiefly maintained by Willem-Pier Vellinga and Bob and aesthetics. It is immediately clear that any such Planque, run from the Netherlands, to allow record- metric would need to overcome some significant ings of bird sounds from around the world to be problems. First, bird songs have a number of distinct shared. Its aims are to popularize bird sound record- characteristics, any or all of which may contribute to ing, and to improve accessibility to, and to increase their attractiveness. For example, some species sing knowledge of, bird sounds. As of 25 January 2013, for long periods, while others produce very brief Xeno-Canto held >115 000 separate recordings of snatches of song. Some have a broad repertoire of >8400 species (roughly 80–85% of all currently different song types, while others are more restricted known bird species), uploaded by >1000 different in their vocalizations. Some species switch between recorders. The arithmetic mean number of recordings song types more frequently than other species, for a for which the species is the target (rather than present given overall repertoire, which may make their song in the background) was 13.6 and the maximum was seem more complex. Quantifying this variation is eas- 335. The criteria that any metric must apply to a large ier in discontinuous singers, which sing discrete number of species, sampled by a wide range of listen- snatches of song, than in rambling or continuous singers, and with variation for analysis, are certainly ers. Species also vary in the pitch and amplitude of fulfilled by this database. their songs. Issues of comparability, complexity and What determines why some species have more unit are well-known problems for studies of bird song sound files loaded onto Xeno-Canto than others? We evolution and ecology (e.g. Krebs & Kroodsma 1980), can think of six reasons. First, some species are more but are no less challenging when one needs to assess common and widespread than others (these two traits how different elements of song combine to affect an are generally positively correlated in birds and other unintended receiver (a human listener). Second, the taxa: Gaston 1994; Gaston & Blackburn 2000; Black- metric would need to be applicable to a wide range of burn et al. 2006), and so are encountered more often bird species, for the purposes of comparative analysis. by recorders. Second, some species have song behav- Of the 10 000 or so extant bird species, c. 2600 have iours that make them easy to record, and so are been recorded in trade (FAO 2011), including 30% of encountered more often by recorders. Third, some all passerine species, so studying the causes of this species are present in biogeographic regions with variation would be best served by data for a signifi- larger (and richer and generally technologically better cant number of species. Third, and most difficult, equipped) human populations and therefore are 306 Ethology 120 (2014) 305–312 © 2014 Blackwell Verlag GmbH T. M. Blackburn, S. Su & P. Cassey The Attractiveness of Bird Song to Humans encountered more often by recorders. Fourth, some below. We used Read & Weary’s measures of song species or species groups are only readily identified in length (in seconds, the mean length of one individual the field by their songs and therefore are more unit in the species’ repertoire), song repertoire size targeted by recorders interested in questions of identi- (the number of song types possessed by an individual, fication and taxonomy.

Load more