The Garden warbler 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 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.
Garden warbler breeds in most of Europe and east across temperate Asia to the Yenisei River in Siberia. All populations are migratory and spend winter 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. Garden warblers breed in open areas with dense bushes, including thickets and woodland edges. They prefer shady areas and a bushy or herbaceous undergrowth, woods along rivers, or reed beds; in Ireland, they favor thickets on the shores of small lakes. They can also be found in suitable mountain woodland. Despite their name, these birds don't occur in gardens.
Garden warblers are diurnal birds. They forage during the daylight hours picking their prey off leaves and twigs, sometimes hovering to do so. They typically forage at less than 6 m (20 ft) above the ground. Some fruits like mulberry they may take in flight. Despite being solitary birds, Garden warblers migrate in flocks together with other small birds. When Garden warblers cross the Sahara, they fly at night, resting motionless and without feeding in suitable shade during the day. Many birds pause for a few days to feed after the desert crossing before continuing further south. The southward migration starts in mid-July, with larger numbers departing in August and peaking in early September. 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' ' is also sometimes heard. The juvenile has a 'quia' alarm vocalization. The male's song is a rich musical warbling, usually delivered by birds in dense cover.
Garden warblers are carnivores (insectivores) and herbivores (frugivores). They eat mainly insects in the breeding season and also take small invertebrates such as spiders. After nesting, they switch to a fruit diet, although insects are still consumed while the birds fatten prior to migration. They favor berries, soft fruit, and especially figs when preparing to migrate.
Garden warblers are monogamous and form pairs. When males return to their breeding areas, they establish a territory. A male attracts a female to his territory through song and a display that involves rapid wing beating while perched. He will also build a number of simple nests (cock's nests) to show to his mate. The nest is concealed in vegetation, normally between 0.3 and 1.2 m (0.98 and 3.94 ft) above the ground, and very rarely higher than 2 m (6.6 ft). The nest is a cup of dry grass, moss, and twigs, with a soft lining of finer plant material or hair. Garden warblers are solitary nesters. Their breeding season starts in April and continues until July. The female lays 4-5 eggs, which are usually whitish or buff with grey, purple, and brown blotches. The eggs are incubated for 11-12 days by both adults, although only the female stays on the nest at night. The chicks are altricial, hatching naked and with closed eyes, and are fed by both parents. They fledge about 10 days after hatching and leave the nest shortly before they are able to fly. Parents feed them for a further 2 weeks, and the family may stay together for a few days after that. Young Garden warblers become reproductively mature and first breed when they are one year old.
Garden warblers are not threatened at present, although climate change appears to be affecting their migration pattern. They are arriving in Europe earlier than previously, and juvenile (but not adult) Garden warblers are departing nearly two weeks later than in the 1980s.
According to the IUCN Red List, the total population size of the Garden warbler is 41,000,000-67,999,999 mature individuals. In Europe, the breeding population consists of 16,700,000-26,900,000 pairs, which equates to 33,300,000-53,800,000 mature individuals. Currently, this species is classified as Least Concern (LC) on the IUCN Red List, but its numbers today are decreasing.
