The chief characteristic of the Swallow, the "bird of freedom," is that it cannot endure captivity, but is forced by instinct to pass from one country to another for the purpose of preserving itself in a tolerably equable temperature, moving northwards as the spring ripens into summer, and southwards as autumn begins to sink into winter. By some marvellous instinct it traces its way over vast distances, passing over hundreds of miles where nothing but the sea is beneath it, and yet at the appointed season returning with unerring certainty to the spot where it was hatched. How it is guided no one knows, but the fact is certain, that Swallows, remarkable for some peculiarity by which they could be at once identified, have been observed to leave the country on their migration, and to return in the following year to the identical nest whence they started. Its habit of making its nest among the habitations of mankind is mentioned in a well-known passage of the Psalms: "The sparrow hath found an house, and the swallow a nest for herself, where she may lay her young, even Thine altars, O Lord of Hosts, my King and my God" (Ps. lxxxiv. 3). The Swallow seems in all countries to have enjoyed the protection of man, and to have been suffered to build in peace under his roof. We find the same idea prevalent in the New World as well as the Old, and it is rather curious that the presence of the bird should so generally be thought to bring luck to a house. In some parts of our country, a farmer would not dare to kill a Swallow or break down its nest, simply because he thinks that if he did so his cows would fail to give their due supply of milk. The connexion between the milking of a cow in the field and the destruction of a Swallow's nest in the house is not very easy to see, but nevertheless such is the belief. This idea ranks with that which asserts the robin and the wren to be the male and female of the same species, and to be under some special divine protection. Whatever may be the origin of this superstition, whether it be derived from some forgotten source, or whether it be the natural result of the confiding nature of the bird, the Swallow enjoys at the present day the protection of man, and builds freely in his houses, and even his places of worship. The heathen temples, the Mahometan mosques, and the Christian churches are alike inhabited by the Swallow, who seems to know her security, and often places her nest where a child might reach it.