Chinese Birds 2013, 4(2):132–143 REVIEW DOI 10.5122/cbirds.2013.0014 The global diversification of songbirds (Oscines) and the build-up of the Sino-Himalayan diversity hotspot Jon FJELDSÅ Center of Macroecology, Evolution and Climate at the Natural History Museum of Denmark, Universitetsparken 15, DK-2100 Copenhagen, Denmark Abstract Over the last decade, molecular phylogenetic studies have provided the foundation for a comprehensive analysis of the global diversification of songbirds (Oscines), which comprise nearly half of all the birds of the world. By comparing the spatial distribution of species representing basal and terminal root-path groups, this paper provides graphical illustrations of the global pattern of di- versification for the major songbird clades. The worldwide expansion of songbirds started as an island radiation in the area where New Guinea is now located, but the mountains of southern China repre- sent a principal center for more recent diversification. The paper suggests priorities and perspectives for further research aiming to understand what determines the variation in biodiversity on different spatial scales. Keywords songbirds, Oscines, global diversification, biogeography, Sino-Himalayan biodiversity hotspot Introduction birds), distributed in all terrestrial habitats around the world, and diversified in terms of feeding adaptations. Although all organismal groups have a history, evolu- Unfortunately, earlier studies of adaptational diversity tionary history was ignored in past correlative studies had no reference to phylogenetic relationships, as past that aimed to explain the variation in biodiversity on systematists, mainly mislead by the recency of the fossil earth (e.g., Rosenzweig, 1995). Today, computer power, record, viewed this group as being the most “modern” distributional databases and molecular data for thou- and recently derived of the avian orders, too uniform sands of species allow a new class of synthetic studies. for a phylogenetic hypothesis to be developed. Because We can now explicitly document the imprint of history of the diversity of family-level taxa, the Old World (Jetz et al., 2012). By adding data for ecological traits tropics were seen as the center of origin. Thus, many and community structure at different spatial scales we odd passerines inhabiting peripheral land areas such as may finally move towards a mechanistic understanding Australasia were long regarded as aberrant members of of what determines the variation of life on earth. well-known Old World families. Only ten years ago it Songbirds (Oscines, which is the main clade of the became clear that the songbirds has a long history that order Passeriformes) represent an ideal group for such started in Australia, or East Antarctica (Ericson et al., ground-breaking studies. First of all, the global spe- 2002; Barker et al., 2004), where few suitable deposits cies diversity is well documented and the group is suf- with fossil birds are available from the early Tertiary. ficiently large (4700 extant species, or nearly half of all With a well-supported songbird phylogeny available we can now reconstruct the world-wide diversification history. This is essential for understanding how the re- Received 4 December 2012; accepted 17 February 2013 gional species pools were assembled. We can now move towards synthesis where large amounts of spatial and Author’s E-mail: [email protected] phylogenetic data can be integrated to identify different www.chinesebirds.net Jon Fjeldså. Global diversification of songbirds 133 modes and shifts in evolutionary and ecological space, relationships among them (Holt et al., 2013). The and hopefully reach a mechanistic understanding of the resulting subdivision of biogeographic realms and re- structure of avian communities at finer spatial scales. gions is somewhat more complex than the traditional With a focus on songbird distributions, this paper biogeographic scheme proposed by Wallace (1876), aims to provide a condensed overview of the biogeo- but is more meaningful in the sense that each region is graphic history of songbirds and more explicitly to defined from a clustering analysis of large amounts of place the rich songbird fauna of eastern Asia, and the data identifying areas with shared evolutionary histo- Sino-Himalayan biodiversity hotspot in particular, in a ries. In this classification, the Palearctic Realm is subdi- global context. Rather than testing specific hypotheses I vided in an Eurasian region comprising Europe and the aim to illustrate how histories and patterns vary among mountain areas of central Asia south to approximately the different songbird clades, and reflect over the pos- 37 degrees N and extending east to Manchuria and Ko- sible implications of this variation. I will also point out rea, and an Arctico-Siberian region comprising Siberia, groups of Chinese songbirds that hold particular prom- areas around the Bering Strait and the entire circum- ise for resolving questions of great generality, and also polar arctic. Between these vast northern regions and point out where a broader geographical sampling and the (tropical) Oriental realm a distinctive Sino-Japanese international collaboration will be needed. realm is recognized (encompassing distinct Chinese and Tibetan regions, the latter including the Yunnan- Materials and methods Szechuan mountains). The western Eurasian region is separated from the Afrotropical realm by an arid Saha- This review will be based on knowledge that is currently ro-Arabian realm, that includes the Iranian desert. available, but will be underpinned by original graphical presentations that illustrate global patterns of diversi- Results fication for six main clades of songbirds. I combined information from a global phylogenetic supertree and A concise outline of the songbird radiation a distributional database in a one-degree geographical grid (Holt et al., 2013). The deeper branching of this The traditional classification of songbirds (Wetmore, supertree is close to that presented by Jetz et al. (2012) 1957) was based on the recognition of three major but further details concerning the terminal branches are complexes of (1) Old World insect-eaters and their rela- added as new phylogenies are published. tives, (2) sparrows, finches and New World nine-prima- This phylogenetic tree is not time calibrated (as a ried oscines and (3) crows, birds of paradise and their chronogram), and in order to illustrate the different allies. These were regarded as representing “three broad distributions of species representing early and more levels of evolution”, from “lower” to “higher”. This view recent radiation I classify the species according to the was complicated by the fact that crows and some other length of its “root path”, which is the number of phy- groups with superior cognitive abilities had a primitive logenetic nodes from the root of the phylogeny to the modification of the humerus. species in question. For a simple visual illustration of With the advancement of molecular systematics the diversification history and geographical imbalance since the 1980s it became apparent that the (expanded) in the distribution of phylogenetically basal and termi- group of crow-like birds (parvorder Corvida; Sibley nal species I superimpose the distributions of species and Ahlquist, 1990) had its origin in Australasia and representing the 1st root-path quartile (the 25% of spe- had later spread to the north. With development of cies with the shortest root-paths) and the remaining DNA sequence data and better analytical approaches (75%) of more terminal species with different colors, as it became evident that the entire Passeriformes had an explained in the legend to Fig. 2. In order to interpret austral origin (Ericson et al., 2002; Barker et al., 2004). these color patterns I took into account all published The suboscines radiated from West Antarctica-South biogeographic analyses that identified likely areas of America, and all the true songbirds (Oscines) origi- origin at phylogenetic nodes. nated in East Antarctica-Australia and expanded during Names of biogeographic regions refer to a new global the upper Tertiary to other continents. This expansion analysis where distributions of 21000 land vertebrate appears to have started 30–40 million years ago, when species were analyzed in relation to the phylogenetic the Australian plate moved north (Fig. 1), causing an © 2013 Beijing Forestry University and China Ornithological Society 134 Chinese Birds 2013, 4(2):132–143 uplift of coral islands in the shallow epicontinental seas eas. The first of these (the “core corvoids” of Barker et where New Guinea and the southern Moluccan islands al., 2004) is characterized by a number of small clades are now. The molecular phylogenies suggest rapid ra- that mainly inhabit the forest habitats of New Guinea. diations centered in this area, followed by multidirec- Embedded among these is a number of subclades that tional dispersal and island-hopping in the mobile arc of expanded more or less successfully over the surround- emerging oceanic islands along the borderline between ing archipelagos (and even back to Australia), with the Australian, Pacific and Asiatic plates (Hall, 2011). some cases of long-distance expansion to the Old World Thus, islands apparently provided the physical setting tropics. The second (provisionally called “transitory os- that triggered the most remarkable increase in diversity cines”) is a smaller group of berrypeckers Melanochari- of birds (Jønsson et al.,