The Olivaceous Warbler (Hippolais pallida) is a "warbler", formerly placed in the Old World warblers when these were a paraphyletic wastebin taxon. It is now considered a member of the acrocephaline warbelrs, Acrocephalidae, in the tree warbler genus Hippolais . As a result of modern taxonomic developments, it is now usually considered to be two species, Western Olivaceous or Isabelline Warbler, Hippolais opaca, and (Eastern) Olivaceous Warbler, Hippolais pallida.
These small passerine birds are found in dry open country, including cultivation, with bushes or some trees. 2-3 eggs are laid in a nest in low in undergrowth or a bush. Like most warblers, Olivaceous is insectivorous.
These are medium-sized warblers, more like a very pale Reed Warbler than their relative the Melodious Warbler. The adults of both species have a plain pale brown back and whitish underparts. The bill is strong and pointed and the legs grey. The sexes are identical, as with most warblers, but young birds are more buff on the belly. Both species have a characteristic downward tail flick.
Eastern Olivaceous Warbler breeds in south east Europe and the Middle East, and Western Olivaceous Warbler in Iberia and north Africa. Both species are migratory, wintering in sub-Saharan Africa or Arabia. Both are rare vagrants to northern Europe.
Western Olivaceous Warbler is larger and has a browner tinge to the upperparts than Eastern Olivaceous Warbler. It also has a larger bill. Eastern Olivaceous Warbler sometimes has a greenish tinge to its upperparts, and can be very difficult to separate from Sykes's Warbler, Hippolais rama. The song of both species is a fast nasal babbling, but they are not the same.