Per Alstrom. Photo: Shashank Dalv
Per Alstrom i Arunachal Pradesh 2009

Systematics of passerine birds

Systematics is the science of evolutionary relationships (phylogeny) among organisms. Until recently, relationships among birds were inferred based on similarities in external and anatomical features. However, in recent years, analyses of DNA have revolutionized these studies. Most of the traditionally deduced relationships have been confirmed by the new methods. However, DNA analyses have also rebutted many of the old opinions and revealed previously completely unexpected relationships. Together with a number of co-workers in several different countries, I study systematics of passerine birds using DNA sequences.


Closely related species look similar – but not always

In general, closely related species are more similar in appearance to each other than to more distantly related species. However, this needs not be the case. Species that are not closely related might evolve similar features because they live under similar conditions (e.g., different groups of swimming birds have independently evolved webbed feet, and desert birds are generally paler, more “sand-coloured” than relatives in less dry areas). This is called parallel or convergent evolution. Moreover, in some groups of birds, some species have evolved strikingly different features from their close relatives, obscuring their relationships.

Secrets revealed by DNA

Analyses of DNA have revolutionized phylogenetic studies. DNA sequences have become easier and cheaper to obtain, and new statistical methods and more powerful computers have enabled more rigorous assessments of relationships. Although most of the traditionally deduced relationships have been confirmed by the new methods, molecular studies have revealed multiple examples of misconceived relationships due to highly complex morphological evolution.


My colleagues and I use DNA sequence data, usually from both mitochondrial and nuclear DNA, to infer relationships of passerine birds. Many studies concern species within a genus, whereas other studies deal with relationships among genera within a family or among families in a superfamily. Most of these studies have discovered surprising relationships. Some examples:

Taxonomy of passerine birds

The number of recognized bird species in the world has increased by 20% in the last half century. Most of this increase is due to improved taxonomic knowledge. This stems mainly from application of new methods, especially analyses of DNA and vocalizations. Together with a number of co-workers, I study the taxonomy of various groups of passerine birds, mainly different “warblers”, using multiple lines of evidence, such as morphology, DNA, vocalizations, other behaviours, ecology, geographical distributions etc.


New methods – DNA and vocalizations

Traditionally, taxonomy has been based on studies of morphology: forms with a certain (subjective) degree of similarity have been treated as subspecies of the same species. However, in recent years,studies of DNA have frequently revealed that forms that are treated as conspecific are highly divergent, and sometimes not even each other’s closest relatives. Moreover, research on vocalizations has shown that many forms treated as subspecies of the same species have highly distinct songs, and do not respond to songs of other forms that are considered to belong to the same species. Especially vocal studies have also frequently revealed the existence of previously unknown so-called “cryptic species” (i.e. different species that are very similar in appearance).

Exploding species

Studies involving DNA and vocalizations, often in combination with morphological (plumage, structure), behavioural, ecological and distributional data, have shown that certain forms warrant recognition as separate species rather than as subspecies of the same species.The increase in the number of recognized species due to recent research based on these modern taxonomic methods has often been dramatic. For example, in the family Phylloscopidae (comprising the genera Phylloscopus and Seicercus), which is one of the groups that I have been studying, the number of species has almost doubled in the last 25 years.


Most of the taxonomic work that I am involved with concerns passerine birds, especially various groups of “warblers”, larks, pipits and wagtails. My co-workers and I use as many different lines of evidence as possible in evaluations of taxonomic status ("integrative taxonomy"). The research has revealed many previously unrecognized species, including a few that were previously unknown to science. Some examples:

Evolution of modern birds

As part of a large-scale international collaborative effort, the Avian Phylogenomics Project, the genomes of 48 bird species representing all 30 neognath orders, were analysed. A well resolved and well supported phylogeny was recovered, although – surprisingly in view of the enormous amount of data – a few branches were still “hanging a bit loose”, e.g. the position of owls and the enigmatic Hoatzin. Most of the “odd” relationships that have recently been suggested, such as between parrots and passerines and between these two groups and falcons, were confirmed.

A novel finding was a group named Columbea comprising flamingos and grebes (sisters) together with pigeons, sandgrouse and the Malagasy mesites, which was sister to the other neognath birds (named Passerea). Other interesting results were a clade containing bustards, turacos and cuckoos, as well as a close relationship between the pelagic tropicbirds and the Neotropical “oddball” Sunbittern (this group should also include the equally peculiar New Caledonian Kagu, which has previously been shown to be a close relative of Sunbittern).

The phylogeny suggests an explosive radiation after the Cretaceous-Paleogene mass extinction event about 66 million years ago.

Evolution of the Sino-Himalayan avifauna

The Himalayas is one of the world's biodiversity hotspots, and the eastern Himalayas probably has the highest diversity of birds in the world. In one study, we analysed DNA sequences from all Himalayan passerine birds to ask which factors limit the speciation in that area. We concluded that the accumulation of new species in the region is now much lower than it has been in the past. As new species have become established in the area it has become increasingly more difficult for additional ones to move in. We argued that the east Himalayas has become filled up with bird species, because it is hard for new species to find a unique niche. We also found that the peak elevational distribution of bird species, which was formed by insectivorous species, coincided with the abundance of the food of these birds, namely arthropods.


Rediscovery of the enigmatic Blackthroat Luscinia obscura 

In June 2011, together with Dr Gang Song, Institute of Zoology, Chinese Academy of Sciences, Beijing, Dr Xue-bin Gao, Shaanxi Institute of Zoology, Xian, Mr Huisheng Gong, Foping National Nature Reserve, and Mr Yong-wen Zhang, Changqing National Nature Reserve, I rediscovered the breeding area for the poorly known Blackthroat Calliope obscura (Luscinia obscura), in the Qinling mountains, Shaanxi province, north central China. Seven singing males were observed in Foping and seven more in Changqing National Nature Reserves – which almost equals the total number of individuals observed of this species since its discovery in the late 19th century. Nearly all of the birds were on mountain slopes at 2400–2500 meters above sea level in large, dense expanses of bamboo in open coniferous and mixed coniferous-broadleaved forest.

See Alström et al. (2013) and Song et al. (2014), in publication list below, and links to BBC Nature 2011 and 2014.


Lectures and interviews


Per Alström
Swedish Species Information Centre
Swedish University of Agricultural Sciences
Box 7007
750 07 Uppsala

Presentation av Per Alström


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