Common grasshopper warbler

This small passerine bird is found in dense grassland vegetation, often close to water, and usually with a few small scattered shrubs. It is a medium-sized warbler about 13 cm (5 in) long. The adult has a streaked brown back and whitish grey underparts which are unstreaked except on the undertail coverts. The sexes are identical, as with most warblers, but young birds are yellower below. Like most warblers, it is insectivorous. Four to seven eggs are laid in a nest on or near the ground in thick vegetation or in a tussock of grass.

This is a species which skulks in the undergrowth, creeping through bushes and low foliage, and which is very difficult to see except when singing from a prominent position. The song, which gives this species its name, is a mechanical grasshopper-like reeling, often given at dawn or dusk.

The adult is about 12.5 cm (5 in) long, and weighs 11.5–16 grams (0.41–0.56 oz). It is a secretive bird and can be hard to see, but its presence is easily detected because of its characteristic song. The upper-parts are pale olive-brown, each feather having a central darker brown streak. The cheeks are greyish, the irises are brown and there is a faint eye streak behind the eye. The upper mandible of the beak is dark brown and the lower mandible yellowish-brown. The underparts are cream-coloured or yellowish-buff with a few dark brown spots and streaks on the breast and flanks. The wings are brown with the outer edge of the feathers rimmed with paler brown. The tail feathers are reddish-brown with faint transverse bars being visible in some individuals and the under-tail coverts are streaked. The slender legs and the feet are pale yellowish-brown.

The song is a long, high-pitched reeling trill performed with beak held wide open and the whole body vibrating. It lasts from a few seconds to several minutes without any pauses, and is of remarkable speed and complexity, consisting of 52 notes (26 double notes) per second; its speed and high pitch however make it impossible for human ears to analyse without slowed-down recordings. It varies in volume from a sound resembling an angler's reel to a distant mowing machine. It is strongly ventriloqual, making it difficult to pinpoint the location of the singing bird, but has strong carrying power, audible at ranges of up to 500 m to even 1 km. The song can be heard at any time of day or night, but peaks around dawn and dusk, and can be heard from the arrival of the birds in April until early August. The alarm call is a repeated ticking noise that has been rendered as "twkit-twkit-twkit". The song shows similar 'reeling' structure to that of some of its congeners, notably lanceolated warbler (Locustella lanceolata), Savi's warbler (Locustella luscinioides) and river warbler (Locustella fluviatilis), though they are all distinguishable in different tones and speed.