The genus Arundinax contains only one species, a common winter visitor to the eastern parts of the Empire. It has the general aspect of Acrocephalus, but has really no close affinities for that genus, from which it differs in having a large first primary, a more graduated tail, and a rough forehead. Its affinities seem to be more with Hypolais and Sylvia. Although fond of water and generally found near rivers and marshes, it is not a reed-bird to the same extent as the Reed-Warblers. I have frequently found it among bushes and tins, and Davison remarks that in Tenasserim it is found chiefly in gardens, along the edges of fields where there is cover, on the outskirts of forests, and not infrequently in grass.
It appeal's to have two complete moults a year, but the changes of colour at the two seasons are very slight and not worthy of separate description. The sexes are alike.
In this genus the bill is very strong and rather wide, and there are three strong rictal bristles somewhat diagonally placed. There are some supplementary hairs in front of these bristles, and the feathers of the forehead are disintegrated, soft, and with the shafts somewhat produced. The third primary reaches to the tip of the wing. The tail is very much graduated, and the feathers narrow.