Most birds use their wings in flight, the feather-covered arms being raised and depressed with great rapidity by means of the breast muscles. Every one who has watched birds is familiar with the marked differences in rapidity and mode of flight. It has been calculated that a common average of rapidity is about 40 to 60 feet per second, but records of the feats of carrier-pigeons, etc., certainly greatly surpass this. It seems probable that strong-winged birds, like eagles, can cover about 80 feet in a second. Buffon noted that they disappeared from sight in about three minutes. Strong birds, like the albatross and birds of prey, can not fly very rapidly, but can sustain their exertions for long periods, while many other birds rarely take prolonged flights, except during migration. The ostrich uses its wings to help it along in its rapid race; some aquatic birds, like the steamer-duck, use them as paddles, auxiliary to their legs. On the ground, birds vary greatly in rate and manner of progression: the swift strides of the ostrich, the rapid run of the partridge, the hopping of the sparrow are well-known illustrations of different gaits. That many birds are expert divers and climbers is also a familiar fact. The great activity of birds is associated with very efficient respiration. Expiration, or the expulsion of used air, is managed by the contraction of breast and abdominal muscles, which compress the inclosed cavities and force the air from the sacs and lungs. When these muscles are relaxed the cavities again elastically expand, and fresh air rushes in by the windpipe to lungs and air sacs.