But the method of flight which perhaps most of all excites the observer’s admiration is soaring, in which the bird seems to remain almost passive, with outstretched wings and spread tail, and mounts automatically in a spiral course. Exactly how the soaring is performed, only the birds themselves know; but it must be similar in essence to the arrangement of the sails of a yacht so as to select that component of the force of the breeze which will drive the boat in the required direction, even though this be almost “in the teeth of the [Pg 279] wind.” Such spiral soaring may be seen to perfection, among common British birds, in the skylark (p. 316). A breeze is necessary for soaring. In hovering, the motion of the wings is extremely rapid, and the bird remains poised in one place. The kestrel (p. 330) derives its common name of “windhover” from its habit of using this method of flight when looking for food. The pigeon’s air supply.—We know that a man breathes more quickly when he is taking active exercise than at ordinary times; and we might, from this and similar observations, expect to find the most perfect breathing organs in animals which lead the most active lives. Of all vertebrate animals, birds probably perform the most work in proportion to their size; and it is not surprising, therefore, to learn that they have special facilities for quickly replacing fouled air by fresh. The lungs are comparatively small, but they are not the only organs of respiration (p. 242); for the windpipe opens also into several air-sacs, which supply nearly all parts of the body with air, and even communicate with the interior of certain bones. The bone of the pigeon’s upper-arm, for example, is hollow and contains air. By means of the system of air-sacs, the air in the lungs is completely renewed at each respiratory act, and thus the waste carbon-dioxide in the blood can be exchanged for fresh oxygen much more completely than it is in mammals. One result of this is that the blood of birds is much warmer than that of mammals. The air-sacs are also an assistance to flight by rendering the body more buoyant.