THE SWIFT Cypselus apus (Linnæus) The causes that govern the migratory movements of birds are still unknown. Some species are doubtless impelled by stress of weather or lack of food to seek other quarters, but what power can it be that brings the Swift thousands of miles from another continent to lay its eggs and rear its young, and then precipitately to retire again when the flies on which they feed are most abundant, and the warmest month of the year has hardly begun? It is one of the last of the summer birds to arrive, May being generally well advanced before we hear its harsh yet pleasant scream of “swee ree,” as it swerves in rapid flight round the cottage or belfry which is to form its summer home. This species is the most aerial of all our native birds, and is never seen to settle except when entering the hole under the eaves where it nests, and, in fact, owing to the length of its wing and shortness of its legs, it cannot raise itself from a level surface should it once settle. Gregarious in its habits, it returns yearly to the same place, and, entering through a hole or crevice under the eaves, nests there in security. Swifts, as a family, differ from all other birds in using a sticky, mucous saliva, with which to bind together rough bits of straw, cobwebs, feathers, etc., which form their nest. The nests of certain foreign species are formed entirely of this saliva, and such nests are eaten with great relish and in large quantities by the Chinese. With our species, however, the amount of saliva used is comparatively small; it does not often collect material for its nest, but makes use of the accumulation of rubbish usually found under roofs, and, hollowing out a shallow depression, cements it into a permanent cup. Two or three dull, oblong, white eggs form the clutch. The young when first hatched are naked; they stay in the nest a long time, not leaving it until fully fledged. When, however, they leave their home and drop into the air for the first time, they fly off at once and appear as much at home on the wing as their parents. For a few days they return to the nest to rest, and then a week or two after they are fledged, about the beginning of August, they leave us for their tropical winter home. These birds occasionally perform curious aerial evolutions on warm summer nights. As darkness comes on they become very restless, screaming round the tower or belfry as they dash by in wide circles; gradually they rise higher and higher in the air till they become mere specks, and are finally lost in the darkness. Here again our ignorance comes in and our story must end. Observers have sat up in vain till two or three in the morning, awaiting their return. Not one has reappeared, and yet the next day they will all be seen back again, apparently unwearied by their restless night, but rather enjoying to the full the marvellous powers of flight which they have inherited.