The Sand Martin flies quickly, but not with the arrow-like speed of the Chimney Swallow. It dwells on the waterside, where it nests in colonies of hundreds, even thousands. The nest is composed almost exclusively of earth, and is placed in the steep high bank or in the walls of a landslip, and it is remarkable as to its architecture. The little bird excavates a long horizontal tunnel in the side of the bank, at the end of which is an oven-like cave, in which it builds its nest of vegetable fibre, roots, feathers and hair. The neighbours build so close together that the bank in many places appears to be completely honeycombed. These nests are built at least 12 inches from the surface of the bank. This bird visits the neighbouring streams and ponds in flocks, circling and darting here and there as is necessary in the pursuit of the winged water-insects. On its return in the spring it seeks and enlarges its old nest hole. It is widely distributed and occurs in great numbers. * * * * * The Sand Martin arrives in Great Britain often as early as the last week in March; it is also one of the first species to leave us. The Sparrows often oust whole little colonies of these birds from their dwellings, but when the colony is a large one they get the better of the hectoring intruders. As soon as the young are able to leave the nest they go to spots where there is water, as they find their food all day long in localities where there is an abundance of insects--gnats especially. Most useful they are in marshy localities, where the [Illustration: USEFUL. THE SAND MARTIN.] atmosphere would be intolerable for human beings but for the work of these little creatures. A little dry grass and a quantity of feathers supplies material for the nest which, being in a little chamber up a tunnel, out of the disinfecting wind, gets flea-infested and very unpleasant. Railway cuttings are much frequented both by Martins and Wagtails because the passing of a train stirs up insect life in it. The gnat is frightfully prolific; it would soon poison our water as well as render it hard for men to breathe. A mother gnat is said to lay from 200 to 300 eggs at one time, and in two weeks the young from these are able to lay eggs themselves. Gnats must themselves be needed in the economy of nature, but if not kept in check they would render our life absolutely unbearable; they form the food for fishes, however, as well as for birds.