The River Warbler (Locustella fluviatilis) is one of Sweden's most elusive songbirds, a plain brown grass warbler that almost always betrays its presence through its strange, insect-like song rather than through sight. At 14–16 cm it is a medium-sized warbler with a uniformly dark olive-brown upperside lacking any distinct markings, a paler underside, and only faint, diffuse streaking on the throat and breast that is easily missed in the field. The tail is broad and rounded, and the bird moves with a mouse-like, creeping gait through dense, damp vegetation, staying so hidden that it is almost always identified by voice rather than appearance.
Its song is the species' defining feature: a persistent, buzzing, insect-like "zr-zr-zr-zr" delivered in long, steady, unbroken series, most often heard at dusk and through the night, and easily mistaken for a grasshopper or a sewing machine. River Warblers breed in dense, damp vegetation — thickets and overgrown ground near water — placing their nest low in grass or bushes, well concealed, where the female lays 5–6 eggs; because the species is so secretive, most Swedish breeding records rest on singing males rather than confirmed nests. The diet consists of insects and other invertebrates such as spiders and larvae, gleaned while creeping through thick cover near the ground. In Sweden the River Warbler is a scarce breeder confined mainly to the southeast, with Gotland and coastal districts holding the core of a small population numbering only a couple of hundred pairs; it is classified as Near Threatened on the Swedish red list. A long-distance migrant, it winters in East Africa and returns to its breeding grounds in May.
Read about this species in Swedish →