Perhaps one of the greatest mysteries surrounding bird life, and awaking, even in the most unthinking, some sense of wonder, is the way in which some of the smallest and most delicate of birds cross enormous stretches of land and water twice a year. This mystery of migration has been especially typified in many countries and from olden times in the Swallow. Essentially a bird of the air, choosing the houses of man for nesting-places, and extremely abundant throughout our islands, he cannot fail to force himself on our attention and to become so associated in our minds with summer days that his first appearance in spring is eagerly looked for. As soon as the March winds have died down the first few stragglers make their appearance, and the early October gales are well over before the last has left. During the whole of April they continue to arrive and disperse through the country, and by the beginning of May we shall find them revisiting the same chimney or eave where their brood was hatched in the previous year. They have but a feeble love-song, merely a rapid twittering, which is especially indulged in during the early hours of dawn, while waiting for the sun to call to life the flies and gnats on which they breakfast. Choosing a beam in a barn or outhouse, or a projecting brick in some old chimney as support, they build a neat cup-shaped nest of mud strengthened with straw to bind it together, and line it with bents, dry grass, and feathers. The eggs, generally six in number, are of a white ground colour dotted or blotched with reddish brown. The duties of incubation devolve on the hen, who is frequently fed by her mate, but soon after the young are hatched and she is free once more to seek her own food, both parents take their share in the duties of housekeeping. This bird, eminently adapted for flight, with long pointed wings and short feeble legs, is hardly ever still. Round and round he circles, sometimes high, sometimes low, wherever food is most abundant, only perching for a few moments on some bare twig or telegraph wire to warble his twittering little song, and then once again to glide with graceful ease through the pathless air. Two families are generally brought to maturity, but he is in no hurry to leave his home and so he stays on well into the autumn.