proper time for commencing their migrations. In its general disposition, the Rock Swallow is less alert and brisk than its congeners, and its voice has a deeper and rather hoarse sound. THE SAND MARTIN. The SAND MARTIN (_Cotyle riparia_), one of the smallest members of its family, is only five inches long and eleven broad; the wing measures four, and the tail two inches. The plumage is greyish brown above, white beneath, and marked on the breast with a greyish brown ring. The sexes are nearly alike, but the young are darker than the adults. These birds inhabit and breed in all parts of Europe, except the extreme northern countries, and usually frequent such rocks or hills as overhang streams and rivers. The wonderful nests that have rendered the members of this group so famous, are made either in natural hollows, or in holes excavated with enormous labour by the builders; they appear, however, to prefer the cavities which they have themselves prepared, and are most careful to dig their retreats at such an elevation as to be above high-water mark. "It appears," says Naumann, "almost incredible that a pair of these small birds, with no other instruments than their delicate beaks, can dig, as they do, a horizontal passage several inches in diameter, and from three to six feet deep, in the space of two, or sometimes three days. The male and female both assist in this, for them, gigantic undertaking, and work with the utmost energy and ardour, disposing of the loose earth by throwing it out behind them with their feet; and yet, strange to say, it is not uncommon for them suddenly to leave one of these excavations when almost finished, and commence another; occasionally, they will even dig a third. Why they do this has never been satisfactorily ascertained, for it is only the passage to the chamber in which the nest is made that is ever occupied either by the parents or the young family. Many pairs invariably work close together, thus forming an extensive settlement, and it is most amusing to watch the earth flying out of a number of their holes as it is ejected by the busy labourers, who are usually quite out of sight." It is to these settlements that Pliny alludes in the following amusing terms: "At the Heracleotic mouth of the Nile in Egypt, the Swallows present an insuperable obstacle to the inroads of that river, by the embankment formed by their nests in one continuous line, nearly a stadium in length--a thing that could not possibly have been effected by the agency of man. In Egypt, too, near the city of Coptos, there is an island sacred to Isis; in the early days of spring, the Swallows strengthen the angular corner of this island with chaff and straw, thus fortifying it in order that the river may not sweep it away. This work they persevere in for three days and nights together, with such unremitting labour that it is a well-known fact that many of them die in consequence of their exertions;