Old World warblers are a large group of birds formerly grouped together in the bird family Sylviidae. They are not closely related to the New World warblers. The family held over 400 species in over 70 genera, and were the source of much taxonomic confusion. Two families were split out initially, the cisticolas into Cisticolidae and the kinglets into Regulidae. In the past ten years they have been the subject of much research and many species are now placed into other families, including the Acrocephalidae, Cettiidae, Phylloscopidae, and Megaluridae. In addition some species have been moved into existing families or have not yet had their placement fully resolved. A smaller number of warblers, together with some babblers formerly placed in the family Timaliidae and the parrotbills, are retained in a much smaller family Sylviidae.
Characteristics
Most Old World warblers are of generally undistinguished appearance, though some Asian species are boldly marked. The sexes are often identical, but may be clearly distinct, notably in the genus Sylvia. They are of small to medium size, varying from 9 to 16 centimetres in length, with a small, finely pointed bill. Almost all species are primarily insectivorous, although some will also eat fruit, nectar, or tiny seeds.
The majority of species are monogamous and build simple, cup-shaped nests in dense vegetation. They lay between two and six eggs per clutch, depending on species. Both parents typically help in raising the young, which are able to fly at around two weeks of age.
Systematics
In the late 20th century, the Sylviidae were thought to unite nearly 300 small insectivorous bird species in nearly 50 genera. They had themselves been split out of the Muscicapidae. The latter family had for most of its existence served as perhaps the ultimate wastebin taxon in the history of ornithology. By the early 20th century, about every insectivorous Old World "songster" known to science had at one point been placed therein, and most continued to be so.
Only after the mid-20th century did the dismantling of the "pan-Muscicapidae" begin in earnest. However, the Sylviidae remained a huge family, with few clear patterns of relationships recognisable. Though by no means as diverse as the Timaliidae (Old World babblers) (another "wastebin taxon" containing more thrush-like forms), the frontiers between the former "pan-Muscicapidae" were much blurred. The largely southern warbler family Cisticolidae was traditionally included in the Sylviidae. The kinglets, a small genus in a monotypic family Regulidae, were also frequently placed in this family. The American Ornithologists' Union includes the gnatcatchers, as subfamily Polioptilinae, in the Sylviidae.
Sibley & Ahlquist (1990) united the "Old World warblers" with the babblers and other taxa in a superfamily Sylvioidea as a result of DNA–DNA hybridisation studies. This demonstrated that the Muscicapidae as initially defined were a form taxon which collected entirely unrelated songbirds. Consequently, the monophyly of the individual "songster" lineages themselves was increasingly being questioned.
More recently, analysis of DNA sequence data has provided information on the Sylvioidea. Usually, the scope of the clade was vastly underestimated and only one or two specimens were sampled for each presumed "family". Minor or little-known groups such as the parrotbills were left out entirely (e.g. Ericson & Johansson 2003, Barker et al. 2004). These could only confirm that the Cisticolidae were indeed distinct, and suggested that bulbuls (Pycnonotidae) were apparently the closest relatives of a group containing Sylviidae, Timaliidae, cisticolids and white-eyes.
In 2003, a study of Timaliidae relationships (Cibois 2003a) using mtDNA cytochrome b and 12S/16S rRNA data indicated that the Sylviidae and Old World babblers were not reciprocally monophyletic to each other. Moreover, Sylvia, the type genus of the Sylviidae, turned out to be closer to taxa such as the yellow-eyed babbler (Chrysomma sinense) (traditionally held to be an atypical timaliid) and the wrentit (Chamaea fasciata), an enigmatic species generally held to be the only American Old World babbler. The parrotbills, formerly considered a family Paradoxornithidae (roughly, "puzzling birds") of unclear affiliations also were part of what apparently was a well distinctive clade.