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Greenfinches are classified as passerines, a family of birds that includes a variety of wrens, warblers and jays as well as ravens, shrikes and crows. Common in the British Isles, the greenfinch's native breeding range extends into mainland Europe and parts of Africa and Asia; relocation has extended the bird's population into both North and South America as well as Australia. Although territorial about the area immediately around its nest and known for aggressively defending any feeder it claims as its own, the greenfinch is sociable bird otherwise, and tends to live close to humans. Greenfinches are popular as pet songbirds, due to their small size, coloration and pleasing vocalizations.
These small birds have wingspans not exceeding 10.6 inches (27 cm) and weigh no more than 3.5 ounces (99 grams). Females have a grayer coloring than males, which have bright, yellow-green breast feathers. Young greenfinches look like the females, but have a less even coloring. The plumage of adult males tends to fade somewhat during the winter months.
The greenfinch will feed on insects, but it prefers seeds, which comprise most of its diet. During the winter, as natural food stocks become harder to come by, they will sometimes flock with other birds and will rely more on feeders filled by humans. This reliance on bird feeders can lead to a parasitic disease called trichomoniasis that interferes with their ability to feed and swallow. According to a 2010 news report, trichomoniasis killed approximately 20 percent of the British greenfinch population in a five-year period. Greenfinches, however, are not threatened birds, and experts do not expect them to become so in the near future.
In the UK, greenfinches are present in most places with trees. These birds form small nesting colonies in bushes, shrubs and trees of all varieties. They commonly nest in backyards, orchards and gardens, all close to human population centers. In the country, greenfinches will nest in hedgerows and forested areas. Greenfinches migrate only in areas where the winters are especially severe.
Although the European greenfinch is the most common, four other varieties are recognized by ornithologists. These are the black-headed, Vietnam, yellow-breasted and oriental greenfinches. Each has slightly different colorations and habitats. The black-headed greenfinch's range extends from China to Vietnam, while the Vietnam greenfinch is not known to inhabit any other country. Oriental and yellow-breasted greenfinches make their homes in several Asian countries, including China, Thailand and India. Those introduced into the Americas and Australia are of the European variety.