In familiar classification systems of the 1950s through the 1970s, the Irenidae were the only bird family entirely restricted to tropical Asia. Traditionally, this family has comprised three genera: the ioras (Aegithina), the leafbirds (Chloropsis), and the fairy bluebirds (Irena). As each is markedly distinct from the other, their inclusion in one family has long been questioned. For years, the leafbirds were commonly considered bulbuls (Pycnonotidae). The status of the fairy bluebirds was particularly controversial. At one time, these birds were popularly known as "blue drongos" (Dicruridae). More recently, various ornithologists have reassigned them to the Old World orioles (Oriolidae), based on skeletal studies. They have also been designated the sole genus in the family Irenidae.
However, the extensive DNA hybridization conducted in the 1980s by Sibley, Ahlquist, and Monroe indicates Irena and Chloropsis comprise a natural grouping, as the great systema-tist Jean Delacour concluded decades before. On the other hand, the same DNA research, published in 1990, suggests Aegithina is not part of this family, but instead forms a subfamily in the great assemblage Sibley, Ahlquist and Monroe classify under the family Corvidae. No fossils can be attributed to the Irenidae, but DNA hybridization suggests the divergence of these birds as distinct lines in the early Oligocene (roughly 30 million years ago), as well as an origin in Australia (where no modern forms have ever occurred).
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