Evolution and systematics

The taxonomy of the Passeri, the suborder of oscine passerines (the songbirds) that contains the Sylviidae (Old World warblers), has long been debated. Much of the controversy focuses on the delineation of the apparently closely related families Muscicapidae, Turdidae, Timaliidae, and Sylviidae. Currently available molecular data suggest that widely used, traditional family classifications do not represent the evolutionary history of this large and complex group of birds. Sibley and Ahlquist place the Sylviidae within a large superfamily, the Sylvioidea. This superfamily also contains members of the following traditional families: Sittidae (nuthatches), Certhiidae (creepers), Troglodytidae (wrens), Paridae (tits), Aegithalidae (long-tailed tits), Hirundinidae (swallows), Pycnonotidae (bulbuls), Zosteropidae (white-eyes), and Timaliidae (babblers). The molecular data do not support the long-held belief that the Sylviidae are closely related to the Turdidae (thrushes) and the Muscicapidae (Old World flycatchers), for these two families fall into a separate superfamily, the Muscicapoidea. The data do, however, support the inclusion of some of the babblers (Timaliinae) and laughing thrushes (genus Garrulax) in the Sylviidae family.

The classification used in the present work is based upon the traditional definition of the Sylviidae. The genera may be divided into six groups, based on molecular evidence of their taxonomic affiliation. The first of these are the gnatcatchers and gnatwrens (Polioptila, Microbates, and Ramphocaenus), which comprise the Sylviid subfamily Polioptilinae, but are placed by Sibley and Ahlquist within an expanded Certhiidae, including both creepers and wrens. The remaining five groups are considered to be within the sylviid subfamily Sylviinae. These are: (1) the kinglets (Regulus), regarded as a separate family by Sibley and Ahlquist and others; (2) the cisticolas and allies (Cisticola, Prima, Apalis, Camaroptera), and other allied genera, a distinct group that Sibley and Ahlquist regard as a separate family, and most authors regard as a subfamily, Cis-ticolinae, of the Sylviidae; (3) the Sylvia warblers (Sylvia, Parisoma), a group that, according to Sibley and Ahlquist, is more closely related to the timaliine babblers than to traditional sylviids; (4) the grassbirds and allies (Megalurus, Bowdleria, Cincloramphus, Megalurulus, Chaetornis, Gramnicola, Schoeni-cola), and other allied genera, which comprise the subfamily Megalurinae in the Sibley and Ahlquist system; (5) the remaining genera of the traditional Sylviidae, most of which appear to fall within a clade represented by Sibley and Ahlquist's Acrocephalinae subfamily. Groups three, four and five above, plus the Garrulax laughing thrushes comprise Sibley and Ahlquist's more restricted Sylviidae, hereafter referred to as Sylviidae sensu strictu.

Little is known about the evolutionary history of passerines. The prevailing opinion has been that passerines arose in the Tertiary, specifically in the early Eocene, then underwent a dramatic diversification during the Miocene. The oldest putative passerine fossils are from early Eocene Australia (ca. 54 million years ago), lending support to a Southern Hemisphere origin, since passerine fossils do not appear in the Northern Hemisphere until the Oligocene. By the lower Miocene, passerine fossils greatly outnumber all other taxa in many Northern Hemisphere sites. A recent molecular study suggests that the passerine divergence may be much older and that passerines

Warbler nests: 1. Great reed warbler (Acrocephalus arundinaceus); 2. Common tailorbird (Orthotomus sutorius); 3. Blue-gray gnatcatcher (Po-lioptila caerulea); 4. Fan-tailed cisticola (Cisticola juncidis); 5. Long-billed crombec (Sylvietta rufescens); 6. Tawny-flanked prinia (Prinia subflava). (Illustration by Barbara Duperron)

Warbler nests: 1. Great reed warbler (Acrocephalus arundinaceus); 2. Common tailorbird (Orthotomus sutorius); 3. Blue-gray gnatcatcher (Po-lioptila caerulea); 4. Fan-tailed cisticola (Cisticola juncidis); 5. Long-billed crombec (Sylvietta rufescens); 6. Tawny-flanked prinia (Prinia subflava). (Illustration by Barbara Duperron)

evolved on Gondwana, the Cretaceous supercontinent in the southern hemisphere. If this is the case, oscine passerines may have diverged from suboscine passerines when Australia separated from Gondwana, radiated on Australia, then dispersed throughout the Old World when Australia came in contact with Southeast Asia.

Within the Sylviidae sensu strictu, the first divergence was probably between Sibley & Ahlquist's Acrocephaline subfamily (Acrocephalus, Hippolais, Phylloscopus, Seicercus, Sylvietta, Sphenoeacus, etc.) and the lineage leading to the other three subfamilies. Next to diverge was Garrulax, the laughing-thrushes, which diverged from a lineage leading to the Timali-ine babbers and Sylvia and Parisoma warblers. The genus Sylvia is thought to be at least 12-16 million years old. The Phylloscopus radiation is about as old as Sylvia, while the Acrocephalus/Hippolais radiation is only about half as old. These estimates place much of the sylviid radiation during the Miocene and Pliocene epochs of the mid-Tertiary, consistent with the passerine radiation in the fossil record.

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