The grass warblers are small passerine birds belonging to the genus Locustella. Formerly placed in the paraphyletic "Old World warbler" assemblage, they are now considered the northernmost representatives of a largely Gondwanan family, the Locustellidae.
These are rather drab brownish "warblers" usually associated with fairly open grassland, shrubs or marshes. Some are streaked, others plain, all are difficult to view. They are insectivorous.
The most characteristic feature of this group is that the song of several species is a mechanical insect-like reeling which gives rise to the group's scientific name.
Species breeding in temperate regions are strongly migratory.