SWIFT  , a

bird so called from the extreme speed of its flight, which apparently exceeds that of any other British species, the Hirundo apus of Linnaeus and Cypselus apus or murarius of modern ornithologists . Swifts were formerly associated with swallows (q.v.) in classification, but whilst the latter are true Passeres, it is now established that swifts are Coraciiform birds (see BIRDS) and the sub-order Cypseli has been formed to include them and their nearest allies, the humming-birds . The four toes are all directed forwards, whereas in the Passeres the hallux is directed backwards and by opposing the other three makes the foot a grasping organ . In the swifts, moreover, the middle and outer digits have only three joints and the metatarsi and even the toes may be feathered . Swifts are divided into three sub-families: Macropteryginae, the true swifts, of tropical Asia, which form a nest gummed by saliva to branches of trees; Chaeturinae, building in rocks or houses, and with an almost world-wide range: it includes Chaetura palagica, the " chimney-swallow " of the United States, Collocalia fuciphaga which obtained its specific name from the erroneous idea that its edible nests were formed by partly digested seaweed; Cypselinae, also world-wide and containing Cypselus apus, the common European swift . All the swifts are migratory . Well known as a summer visitor throughout the greater part of Europe, the swift is one of the latest to return from Africa, and its stay in the country of its birth is of the shortest, for it generally disappears from England very early in August, though occasionally to be seen for even two months later . The swift commonly chooses its nesting-place in holes under the eaves of buildings, but a crevice in the face of a quarry, or even a hollow tree, will serve it with the accommodation it requires . This, indeed, is not much, since every natural function except sleep, oviposition and incubation, is performed on the wing, and the easy evolutions of this bird in the air, where it remains for hours together, are the admiration of all who witness them . Though considerably larger than a swallow, it can be recognized at a distance less by its size than by its peculiar shape . The head scarcely projects from the anterior outline of the pointed wings, which form an almost continuous curve, at right angles to which extend the body and tail, resembling the handle of the crescentic cutting-knife used in several trades, while the wings represent the blade . The mode of flight of the two birds is also unlike, that of the swift being much more steady, and, rapid as it is, ordinarily free from jerks .

The whole plumage, except a greyish

white patch under the chin, is a sooty black, but glossy above . Though its actual breeding-places are by no means numerous, its extraordinary speed and discursive habits make the swift widely distributed; and throughout England scarcely a summer's day passes without its being seen in most places . A larger species, C. melba or C. alpinus, with the lower parts dusky white, which has its home in many of the mountainous parts of central and southern Europe, has several times been observed in Britain, and two examples of a species of a very distinct genus Chaetura, which has its home in northern Asia, but regularly emigrates thence to Australia, have been obtained in England (Prot . Zool . Soc., 188o, p . 1) . Among other peculiarities the swifts, as long ago described (probably from John Hunter's notes) by Sir E . Home (Phil . Trans . 1817, pp . 332 et seq., pl. xvi.), are remarkable for the development of their salivary glands, the secretions of which serve in most species to glue together the materials of which the nests are composed, and in the species of the genus Collocalia form almost the whole substance of the structure . These are the " edible " nests so eagerly sought by Chinese epicures as an ingredient for soup .

These remarkable nests consist essentially of mucus, secreted by the salivary glands above mentioned, which dries and looks like

isinglass . Their marketable value depends on their colour and purity, for they are often intermixed with feathers and other foreign substances . The swifts that construct these " edible " nests form a genus Collocalia, with many species; but they inhabit chiefly the islands of the Indian Ocean from the north of Madagascar eastward, as well as many of the tropical islands of the Pacific so far as the Marquesas—one species occurring in the hill-country of India . They breed in caves, to which they resort in great numbers, and occupy them jointly and yet alternately with bats—the mammals being the lodgers by day and the birds by night .