The common chiffchaff is a small, dumpy, 10–12 centimetres (3.9–4.7 inches) long leaf warbler. The male weighs 7–8 grammes (0.28–0.31 oz), and the female 6–7 grammes (0.25–0.28 oz). The spring adult of the western nominate subspecies P. c. collybita has brown-washed dull green upperparts, off-white underparts becoming yellowish on the flanks, and a short whitish supercilium. It has dark legs, a fine dark bill, and short primary projection (extension of the flight feathers beyond the folded wing). As the plumage wears, it gets duller and browner, and the yellow on the flanks tends to be lost, but after the breeding season there is a prolonged complete moult before migration. The newly fledged juvenile is browner above than the adult, with yellow-white underparts, but moults about 10 weeks after acquiring its first plumage. After moulting, both the adult and the juvenile have brighter and greener upperparts and a paler supercilium.
The common chiffchaff breeds across Europe and Asia east to eastern Siberia and north to about 70°N, with isolated populations in northwest Africa, northern and western Turkey and northwestern Iran.[44] It is migratory, but it is one of the first passerine birds to return to its breeding areas in the spring and among the last to leave in late autumn. When breeding, it is a bird of open woodlands with some taller trees and ground cover for nesting purposes. These trees are typically at least 5 metres (16 feet) high, with undergrowth that is an open, poor to medium mix of grasses, bracken, nettles or similar plants. Its breeding habitat is quite specific, and even near relatives do not share it; for example, the willow warbler (P. trochilus) prefers younger trees, while the wood warbler (P. sibilatrix) prefers less undergrowth. In winter, the common chiffchaff uses a wider range of habitats including scrub, and is not so dependent on trees. It is often found near water, unlike the willow warbler which tolerates drier habitats. There is an increasing tendency to winter in western Europe well north of the traditional areas, especially in coastal southern England and the mild urban microclimate of London. These overwintering common chiffchaffs include some visitors of the eastern subspecies abietinus and tristis, so they are certainly not all birds which have bred locally, although some undoubtedly are.
The male common chiffchaff is highly territorial during the breeding season, with a core territory typically 20 metres (66 feet) across, which is fiercely defended against other males. Other small birds may also be attacked. The male is inquisitive and fearless, attacking even dangerous predators like the stoat if they approach the nest, as well as egg-thieves like the Eurasian jay. His song, given from a favoured prominent vantage point, appears to be used to advertise an established territory and contact the female, rather than as a paternity guard strategy.
Beyond the core territory, there is a larger feeding range which is variable in size, but typically ten or more times the area of the breeding territory. It is believed that the female has a larger feeding range than the male. After breeding has finished, this species abandons its territory, and may join small flocks including other warblers prior to migration.
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