There is a great trade to China in the nests of these birds. They are of texture resembling isinglass, and are, in shape, somewhat like a saucer with one side flatted. Their thickness is little more than that of a silver spoon, and their weight from a quarter to half an ounce. They are very brittle, and have a shining gummy appearance internally when broken; and are wrinkled or slightly furrowed externally. The best and clearest of these nests are nearly as white as writing paper, and, semi-transparent, having a few downy feathers hanging about them; but their general colour is white inclining to red. They are usually packed one within another, to the length of twelve or fifteen inches, and secured with split canes to prevent their breaking. The use to which they are principally applied is for the thickening of soups and broths, and to these they are said to communicate an exquisite flavour. Or, after having been softened in water, they are mixed with ginseng, and put into the body of a fowl, and the whole is stewed together, and constitutes a very favourite dish with the Chinese epicures. It has been calculated that the island of Batavia alone exports to China more than twelve tons' weight of these nests annually. A few are brought into Europe as curiosities and presents. Sir George Staunton speaks of having seen great numbers of them in two caverns which ran horizontally into the side of a rock, in the Island of Cass, near Sumatra. They adhered to each other and to the sides of the cavern, mostly in rows, without any break or interruption. The nests are not taken until after the young ones are fledged; and, in general, this is done by persons who descend to the places where they are situated, by rope ladders. 152. _The WILD PIGEON, or STOCK-DOVE_ (Columba ænas), _is distinguished by its bluish plumage, the neck being of a glossy green colour above; by the wings being marked with two black bars, and the tip of the tail blackish._ These birds are in some degree migratory; large flocks of them arriving in England from the northern regions of the Continent at the approach of winter, and returning in the spring. Many of them, however, remain in this country during the whole year, and only change their quarters to procure food. Similar, but much more extensive, flights of wild pigeons are observed in some parts of Italy, where great numbers of them are caught for sale as food. They build their nests in the holes of rocks, in old castles, churches, and towers, and sometimes in the hollows of trees, but never on the boughs; and they lay two white eggs. The name of Stock-dove has been given to these birds, from their being the stock or origin of our _domestic pigeon_. In a domesticated state artificial cavities are formed for them to breed in; and they are frequently known to have young ones eight or nine times in the year. Thus, although they have only two eggs for each brood, their increase is sometimes extremely rapid.