The Tree Sparrow (Passer montanus), breeds over much of Europe and Siberia. Allied forms of this species also occur in other parts of Asia. It has been introduced to Australia and the United States as well. In the United States it is known as the Eurasian Tree Sparrow or German Sparrow to differentiate it from the native, unrelated American Tree Sparrow. In some parts of its natural range, the Tree Sparrow is declining in population due to changes in farming methods.
The Tree Sparrow is 5 to 5.5 inches long. The adult’s crown and nape are rich chestnut, and on the white cheeks and ear-coverts there is a triangular black patch. The chin and throat are black. Two distinct narrow white bars cross the brown wings. In summer the bill is lead-blue, and in winter almost black. The legs are pale brown. The sexes are practically alike. The young, even in the nest, closely resemble their parents. They are said to be duller, and the face pattern is less distinct. The breast and belly are browner than in the adult.
Though occasionally nesting in isolated trees, it is a gregarious bird at all seasons, and a grove of old trees with a plentiful supply of hollows, or a disused quarry, are favorite sites for the colony. The nesting hole is composed of hay, grass, wool or other material and lined with feathers. Some of the nests are not actually in holes in rock, but are built among roots of overhanging furze or other bushes. The haunts of man are not always shunned, for old thatch in a barn or cottage will shelter a colony. A domed nest, like that of the House Sparrow, is sometimes built in the old nest of a Magpie or other bird.
The four to six eggs, usually five, are smaller, and as a rule, browner than those of the House Sparrow. They vary considerably, and frequently the markings are massed at one end. In most clutches one egg is lighter and differs in markings from the others. The Tree Sparrow’s voice is more shrill than the House Sparrow’s; the call is a shorter chip, and the song, consisting of modulated chirps, is more musical.