Binomial name: Passer montanus, Carolus Linnaeus, 1758
The Eurasian Tree Sparrow (Passer montanus) is a passerine bird in the sparrow family with a rich chestnut crown and nape, and a black patch on each pure white cheek. The sexes are similarly plumaged, and young birds are a duller version of the adult. This sparrow breeds over most of temperate Eurasia and Southeast Asia, where it is known as the Tree Sparrow, and it has been introduced elsewhere including the United States, where it is known as the Eurasian Tree Sparrow or German Sparrow to differentiate it from the native unrelated American Tree Sparrow. Although several subspecies are recognised, the appearance of this bird varies little across its extensive range.
The Eurasian Tree Sparrow's untidy nest is built in a natural cavity, a hole in a building or the large nest of a European Magpie or White Stork. The typical clutch is five or six eggs which hatch in under two weeks. This sparrow feeds mainly on seeds, but invertebrates are also consumed, particularly during the breeding season. As with other small birds, infection by parasites and diseases, and predation by birds of prey take their toll, and the typical life span is about two years.
The Eurasian Tree Sparrow is widespread in the towns and cities of eastern Asia, but in Europe it is a bird of lightly wooded open countryside, with the House Sparrow breeding in the more urban areas. The Eurasian Tree Sparrow's extensive range and large population ensure that it is not endangered globally, but there have been large declines in western European populations, in part due to changes in farming practices involving increased use of herbicides and loss of winter stubble fields. In eastern Asia and western Australia, this species is sometimes viewed as a pest, although it is also widely celebrated in oriental art.
The Eurasian Tree Sparrow is 12.5–14 cm (5–5½ in) long, with a wingspan of about 21 cm (8.25 in) and a weight of 24 g (0.86 oz), making it roughly 10% smaller than the House Sparrow. The adult's crown and nape are rich chestnut, and there is a kidney-shaped black ear patch on each pure white cheek; the chin, throat, and the area between the bill and throat are black. The upperparts are light brown, streaked with black, and the brown wings have two distinct narrow white bars. The legs are pale brown, and the bill is lead-blue in summer, becoming almost black in winter.
This sparrow is distinctive even within its genus in that it has no plumage differences between the sexes; the juvenile also resembles the adult, although the colours tend to be duller. Its contrasting face pattern makes this species easily identifiable in all plumages; the smaller size and brown, not grey, crown are additional differences from the male House Sparrow. Adult and juvenile Eurasian Tree Sparrows undergo a slow complete moult in the autumn, and show an increase in body mass despite a reduction in stored fat. The change in mass is due to an increase in blood volume to support active feather growth, and a generally higher water content in the body.
The Eurasian Tree Sparrow has no true song, but its vocalisations include an excited series of tschip calls given by unpaired or courting males. Other monosyllabic chirps are used in social contacts, and the flight call is a harsh teck. A study comparing the vocalisations of the introduced Missouri population with those of birds from Germany showed that the US birds had fewer shared syllable types (memes) and more structure within the population than the European sparrows. This may have resulted from the small size of the founding North American population and a consequent loss of genetic diversity.