Garden warbler


The garden warbler is a common and widespread small bird that breeds in most of Europe and in the Palearctic to western Siberia. It is a plain, long-winged and long-tailed typical warbler with brown upperparts and dull white underparts; the sexes are similar and juveniles resemble the adults. Its two subspecies differ only slightly and interbreed where their ranges overlap. Due to its lack of distinguishing features, this species can be confused with a number of other unstreaked warblers. The garden warbler's rich melodic song is similar to that of the blackcap, its closest relative, which competes with it for territory when nesting in the same woodland.
The preferred breeding habitat in Eurasia is open woodland with dense low cover for nesting; despite its name, gardens are rarely occupied by this small passerine bird. The clutch of four or five blotched cream or white eggs is laid in a robust cup-shaped nest built near the ground and concealed by dense vegetation. The eggs are incubated for 11–12 days. The chicks are altricial, hatching naked and with closed eyes, and are fed by both parents. They fledge about 10 days after hatching. Only about a quarter of young birds survive their first year. The garden warbler is strongly migratory, wintering in sub-Saharan Africa. A wide range of habitats are used in Africa, but closed forest and treeless Sahel are both shunned. Insects are the main food in the breeding season, although fruit predominates when birds are fattening prior to migration, figs being a particular favourite where available. These warblers have a mixed diet of insects and fruit in their African wintering grounds.
The garden warbler is hunted by Eurasian sparrowhawks and domestic cats, and its eggs and nestlings are taken by a variety of mammalian and avian predators. It may be host to various fleas, mites and internal parasites, and it is a host of the common cuckoo, a brood parasite. The large and fairly stable numbers and huge range of the garden warbler mean that it is classed as least concern by the International Union for Conservation of Nature. Despite a small population decline in much of its European range, the bird's breeding distribution is expanding northwards in Scandinavia.

Taxonomy

The genus Sylvia, the typical warblers, forms part of a large family of Old World warblers, the Sylviidae. Fossils from France show that the genus dates back at least 20 million years. The garden warbler and its nearest relative, the blackcap, are an ancient species pair which diverged very early from the rest of the genus, between 12 and 16 million years ago. In the course of time, these two species have become sufficiently distinctive that they have been placed in separate subgenera, with the blackcap in subgenus Sylvia and the garden warbler in Epilais. These sister species have a breeding range which extends farther northeast than all other Sylviid species except the lesser whitethroat and greater whitethroat.
The nearest relatives of the garden warbler outside the sister group are believed to be the African hill babbler and Dohrn's thrush-babbler.
The garden warbler was given the binomial name Motacilla borin by the Dutch naturalist Pieter Boddaert in 1783. The current genus name is from Modern Latin silvia, a woodland nymph, related to silva, a wood. The specific borin is derived from a local name for the bird in the Genoa area of Italy; it derives from the Latin bos, ox, because the warbler was believed to accompany oxen.
In the 19th and early 20th centuries there was some uncertainty as to the correct authority for this species. Various names were used including Sylvia salicaria Linnaeus 1766, and Sylvia simplex Latham 1787. Confusingly, the species was also known as Sylvia hortensis Bechstein 1802, where hortensis is the specific epithet of the western Orphean warbler.
There are two recognised subspecies.
Intermediate birds occur where the recognised forms meet and interbreed, and have sometimes been given subspecies status, including S. b. kreczmeri in Poland and S. b. pateffi in Bulgaria, but these are not generally accepted as valid taxa.

Description

The garden warbler is long with a wing length. The weight is typically, but can be up to 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 grey upper and paler grey lower mandible. The male and female are indistinguishable by external appearance including size. Juveniles have a looser plumage than an adult, with paler and greyer 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 greyer tone to the upperparts and whiter underparts. The subspecies are hard to distinguish visually where they occur together in Africa, but a wing length greater than 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.

Voice

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. 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.

Distribution and habitat

The garden warbler breeds in most of Europe between the 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. Birds from central Europe initially migrate to the southwest, reorientating to the south or southeast once in Africa, although Scandinavian migrants may head south through the Alps and across the Mediterranean Sea. S. b. woodwardi reaches Africa by a more easterly route, many birds passing through the Arabian Peninsula. When garden warblers cross the Sahara, they fly at night, resting motionless and without feeding in suitable shade during the day. During their journey, they can metabolise not only body fat but also up to 19% of their breast and leg muscles and 39% of their digestive tract. Many birds pause for a few days to feed after the desert crossing before continuing further south.
The nominate subspecies occurs in the western and central parts of the winter range, although some birds occur as far east as Kenya. S. b. woodwardi winters in eastern and southern Africa. Movements in Africa are poorly known, although at least some birds return to the same location in subsequent years. There are only a handful of records of birds recorded in Europe in winter, from Corsica, the UK and Ireland. Spring migration routes are poorly known, but appear to lie more directly across the Mediterranean. This warbler has occurred as a vagrant in Afghanistan, Djibouti, Iceland, São Tomé and Príncipe, Somalia, Yemen, Svalbard, Jan Mayen and Madeira.
The breeding habitat of the garden warbler is open areas with dense bushes, including thickets and woodland edges. Shady areas and a bushy or herbaceous undergrowth are preferred, as are woods adjacent to rivers or reed beds; in Ireland it favours thickets on the shores of small lakes. A tolerance of willow, alder and birch allows it to breed farther north and at higher altitudes than any other European Sylvia warbler. Mature conifers and dense plantations are avoided, although young conifer plantations with thick undergrowth are suitable for nesting. Despite its name, it is not a bird of gardens. In Africa, a wide range of habitats with trees are used, although closed forests and arid areas are again avoided. This warbler occurs at altitudes of up to in suitable mountain woodland, although in East Africa it is usually found at a lower altitude than the blackcap, and in moister areas than the common whitethroat.