The garden warbler is 14 cm (5.5 in) long with a 7.6?8.4 cm (3.0?3.3 in) wing length. The weight is typically 16?22 g (0.6?0.8 oz), but can be up to 35.5 g (1.25 oz) for birds preparing to migrate. It is a plain, long-winged and long-tailed bird with unstreaked olive-brown upperparts and dull white underparts. It has a whitish eyering and a faint pale supercilium, and there is a buff wash to the throat and flanks. The eye is black, the legs are bluish-grey and the strong bill has a gray upper and paler gray lower mandible. The male and female are indistinguishable by external appearance including size. Juveniles have a looser plumage than an adult, with paler and grayer upperparts and a buff tone to the underparts. The eastern subspecies S. b. woodwardi is slightly larger and paler than the nominate form with a grayer tone to the upperparts and white underparts. The subspecies are hard to distinguish visually where they occur together in Africa, but a wing length greater than 80 mm (3.15 in) confirms S. b. woodwardi when birds are trapped. The plain appearance of the garden warbler means that it can be confused with several other species. The melodious and icterine warblers usually have long bills and a yellowish tint to their plumage. The booted warbler is similar in colour, although it is smaller, more delicately built and has a flesh-coloured bill. Western and eastern olivaceous warblers are also relatively small, and have white outer tail feathers as well as a pinkish bill. Juvenile barred warblers, which lack the obvious barring of adults, are much larger than garden warblers and have a pale double wingbar. Juvenile garden warblers have a partial moult mainly involving the body plumage between June and September prior to migration. Adults also have a similar, but sometimes more extensive, partial moult in late summer, and a complete moult in their African wintering areas before the return migration.
Habitat and Distribution
The garden warbler breeds in most of Europe between the 12?28 ?C (54?75 ?F) isotherms and east across temperate Asia to the Yenisei River in Siberia. Its range extends further north than any other Sylvia warbler. All populations are migratory, wintering in sub-Saharan Africa as far south as South Africa.
The garden warbler feeds mainly on insects in the breeding season, although other small invertebrates such as spiders are also eaten. It picks its prey off leaves and twigs, sometimes hovering to do so. It normally forages at less than 6 m (20 ft) above the ground. After nesting, there is a genetically controlled switch to a fruit diet, although insects are still consumed while the birds fatten prior to migration; birds gain weight more rapidly from a diet containing both fruit and insects than either alone. Berries and other soft fruit are preferred, and figs are particularly important for birds preparing to migrate. This predilection gives rise to the Italian beccafico (fig pecker) and Portuguese felosa-das-figueiras (fig-tree warbler) as names for this species. On this diet a bird can gain weight quickly and the liver increases the rate at which it produces fatty acids for storage in adipose tissue. In Africa, the warbler eats insects as well as berries, and the fruits of the introduced Spanish flag is a favourite where present. An increase in body mass occurs again before the northward migration, birds fattening even more rapidly than prior to their southward journey. Most internal organs (including the liver, spleen, intestines, kidneys and heart) and the flight muscles lose weight during the journey over the Sahara, although the testes quadruple in mass in preparation for the breeding season. Unlike drier-habitat species like the common whitethroat, they leave from savanna rather than the treeless Sahel further north.
Calls and Songs
The male's song, usually delivered by birds in dense cover, is a rich musical warbling usually delivered in bursts of a few seconds duration, but sometimes for longer periods. The song is confusable with that of the blackcap, although compared to that species it is slightly lower-pitched, less broken into discrete song segments and more mellow. Both species have a quiet subsong, a muted version of the full song, which is much more difficult to separate. The most frequent call of the garden warbler is a sharp kek-kek, which is repeated rapidly when the bird is alarmed. A quiet rasping tchurr-r-r-r resembling the main call of the common whitethroat is also sometimes heard. The juvenile has a quia alarm vocalisation. The garden warbler will occasionally mimic other birds, and is itself frequently mimicked by the blackcap. Both Sylvia warblers will also sing against common nightingales, which have a similar song despite being unrelated. Subsong may be heard on the wintering grounds in Africa, developing into the full song in March and April prior to the return to Europe.