The thick-billed warbler (Arundinax aedon) breeds in the temperate east Palearctic. South Siberia to West Mongolia.It is migratory, wintering in tropical South Asia and South-east Asia. It is a very rare vagrant to western Europe.

This passerine bird is a species found in dense vegetation such as reeds, bushes and thick undergrowth. 5-6 eggs are laid in a nest in a low tree.

This is a large warbler, at 16–17.5 centimetres (6.3–6.9 in) long nearly as big as great reed warbler. The adult has an unstreaked brown back and buff underparts, with few obvious distinctive plumage features. The forehead is rounded, and the bill is short and pointed. The sexes are identical, as with most warblers, but young birds are richer buff below. Like most warblers, it is insectivorous, but will take other small prey items.

The song is fast and loud, and similar to marsh warbler, with much mimicry and typically acrocephaline whistles added.

It was sometimes placed in the monotypic genus Phragmaticola (or Phragamaticola ) and for a long time as Acrocephalus and in 2009 suggested as being within the Iduna clade but a 2014 phylogeny study based on more loci suggested that it did not fit into the Iduna clade suggesting a resurrection of the genus Phragamaticola or Arundinax, the oldest available genus name which has priority.

Keyserling and Blasius gave no explanation of the genus name Iduna. The specific aedon is from Latin aëdon or Ancient Greek aedon and means nightingale. In Greek mythology Aëdon was changed into a nightingale after killing her own son while attempting to murder one of the sons of her sister Niobe.