Certhia familiaris

The Eurasian treecreeper or common treecreeper (Certhia familiaris) is a small passerine bird also known in the British Isles, where it is the only living member of its genus, simply as treecreeper. It is similar to other treecreepers, and has a curved bill, patterned brown upperparts, whitish underparts, and long stiff tail feathers which help it creep up tree trunks. It can be most easily distinguished from the similar short-toed treecreeper, which shares much of its European range, by its different song. The Eurasian treecreeper has nine or more subspecies which breed in different parts of its range in temperate Eurasia. This species is found in woodlands of all kinds, but where it overlaps with the short-toed treecreeper in western Europe it is more likely to be found in coniferous forests or at higher altitudes. It nests in tree crevices or behind bark flakes, and favours introduced giant sequoia as nest sites where they are available. The female typically lays five or six pink-speckled white eggs in the lined nest, but eggs and chicks are vulnerable to attack by woodpeckers and mammals, including squirrels. The Eurasian treecreeper is insectivorous and climbs up tree trunks like a mouse, to search for insects which it picks from crevices in the bark with its fine curved bill. It then flies to the base of another tree with a distinctive erratic flight. This bird is solitary in winter, but may form communal roosts in cold weather. Similar in appearance, all treecreepers are small birds with streaked and spotted brown upperparts, rufous rumps and whitish underparts. They have long decurved bills, and long rigid tail feathers that provide support as they creep up tree trunks looking for insects. The Eurasian treecreeper is 12.5 cm (4.9 in) long and weighs 7.0–12.9 g (0.25–0.46 oz). It has warm brown upperparts intricately patterned with black, buff and white, and a plain brown tail. Its belly, flanks and vent area are tinged with buff. The sexes are similar, but the juvenile has duller upperparts than the adult, and its underparts are dull white with dark fine spotting on the flanks. The contact call is a very quiet, thin and high-pitched sit, but the most distinctive call is a penetrating tsree, with a vibrato quality, sometimes repeated as a series of notes. The male's song begins with srrih, srrih followed in turn by a few twittering notes, a longer descending ripple, and a whistle that falls and then rises. The range of the Eurasian treecreeper overlaps with that of several other treecreepers, which can present local identification problems. In Europe, the Eurasian treecreeper shares much of its range with the short-toed treecreeper. Compared to that species, it is whiter below, warmer and more spotted above, and has a whiter supercilium and slightly shorter bill. Visual identification, even in the hand, may be impossible for poorly marked birds. A singing treecreeper is usually identifiable, since short-toed treecreeper has a distinctive series of evenly spaced notes sounding quite different from the song of Eurasian treecreeper; however, both species have been known to sing the other's song. Three Himalayan subspecies of Eurasian treecreeper are now sometimes given full species status as Hodgson's treecreeper, for example by BirdLife International, but if they are retained as subspecies of Eurasian, they have to be distinguished from three other South Asian treecreepers. The plain tail of Eurasian treecreeper differentiates it from bar-tailed treecreeper, which has a distinctive barred tail pattern, and its white throat is an obvious difference from brown-throated treecreeper. Rusty-flanked treecreeper is more difficult to separate from Eurasian, but has more contrasting cinnamon, rather than buff, flanks.