The adult Common Grasshopper Warbler has a length of about 12.5 cm (5 in). It is a very secretive bird and seldom seen, 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. This bird seldom takes to the wing but spends its time scurrying through dense vegetation, flitting from twig to twig or running along the ground. It has a peculiar high-stepping gait and long stride as it moves along horizontal stems, looking slender and tapering. It seldom flies, soon diving back into cover, and when it alights it often raises and flares its tail to show its streaked under-tail coverts. It has been known to feign injury in order to distract a potential predator.
Habitat and Distribution
The Common Grasshopper Warbler breeds in north west Europe and parts of western Asia. The range includes Spain, France, central Italy, Romania, Yugoslavia, the British Isles, Belgium, Holland, Germany, Denmark, southern Sweden, southern Finland, the Baltic States and western parts of Russia. Further east it is replaced by related species. In late summer, it migrates to north west Africa where it overwinters. In the breeding season, the Common Grasshopper Warbler is found in damp or dry places with rough grass and bushes such as the edges of fens, clearings, neglected hedgerows, heaths, upland moors, gorse-covered areas, young plantations and felled woodland. In the winter, it is usually found in similar locations but information is scarce on its behaviour and habitat at this time.
The Common Grasshopper Warbler is insectivorous, feeding on a wide range of invertebrates. Its diet includes flies, moths, beetles, aphids, dragonflies and mayflies and their larvae. Spiders and woodlice are also eaten and the chicks are fed on aphids, green caterpillars, woodlice and flies.
Male Common Grasshopper Warblers try to attract females by displaying to them. They walk or run along twigs with tail spread, fluttering their wings as they raise and lower them, often carrying a grass or leaf in their beak. In the air, with wings well extended and fluttering, they spread their tail and fluff their feathers. Both sexes take part in nest-building. The nest is well-concealed and built close to the ground in such places as grass tussocks, gorse bushes, osier beds, reed beds, tangled hedgerows, scrub and among coarse heather plants on moorland. It varies in size and shape but is constructed of grasses, sedges and mosses and often lined with fine grasses. A clutch of four to six eggs is laid. These are creamy white speckled with fine reddish spots, usually randomly distributed but sometimes merged into blotches or zones. The eggs measure 18 by 14 millimetres (0.71 by 0.55 in) and weigh about 1.7 g (0.06 oz). Both parents are involved in incubating the eggs which takes about fourteen days. The chicks are altricial and are fed on insects. They fledge in twelve to thirteen days and there are usually two broods in the season. Young birds become mature at a year old and the highest recorded age for this species is five years.
Calls and Songs
The song is an unmusical long, high-pitched reeling trill performed with beak held wide open and the whole body vibrating. It lasts for from a few seconds to two or three minutes with hardly a pause for breath. It varies in volume from a faint hum to a sound resembling a distant mowing machine. It is performed at any time of day from early morning until after the sun has set and is constantly to be heard from the arrival of the bird in spring until late July. The alarm call is a repeated ticking noise that has been rendered as `twkit-twkit-twkit`.