In taking flight it is very important for the bird to "get a start," as we say; that is, to acquire some relative velocity, and to accom plish this we often see birds which are about to fly run along the ground for a few steps or flap along the surface of the water, while starting head toward the wind with wings properly spread accomplishes the same result. Rising into still air by flapping is very laborious work, and sonre heavy-bodied birds, as the loons, for instance, are utterly unable to take flight when confined within a small area.
When once in the air and fairly started in flight the wing action is far less laborious than at the start and the upward stroke is often relative to the body only and not necessarily a muscular effort. When this stroke is active, the individual feathers, as instantaneous photographs show us, are more or less separated to reduce the resistance of the air on the recovery. Most birds mingle flapping and straight sailing in various ways and when once on the wing flight is mainly a matter of presenting their sail area to the air currents in such a way as to gain the greatest benefit from them.
Soaring is flight in circles with set wings and without any visible muscular action on the part of the bird. In spite of this the bird is able to mount higher and higher in the air, gaining impetus enough on the flight with the wind to carry it above its initial altitude when returning on the other half of the circle against the wind. Many theories have' been advanced to explain the "soaring bird,' some of them purely fan tastic. This method of flight is possible only in the presence of currents of air; the unequal velocity of air at different altitudes doubtless having much to do with it. It has been ascer tained that the flexibility of the tips of the wing feathers of a bird renders soaring pos sible to it. Mr. Everett H. Bickley, after elab orate experiments, claims to have solved the riddle. The flexibility of the wing feathers,' he says, "act like the flap in a pump valve, which offers resistance to fluid motion in one direction and freedom in another?' Aeronauts naturally look to birds for su? gestions in artificial flight. They have paid particular attention to the poise and flight of the herring-gull in arriving at the principles of aerial navigation. The cutting edge of the monoplane from which the machine obtains its lifting capacity corresponds with the cutting edge of a bird's wing, the edge in the case of the bird being composed of living muscle and bone. The rear edges of the plane can be warped up and down in rough imitation of the twisting of a bird's wing' Professor Ernest Huebner, the distinguished German naturalist, in a series of experiments on the shores of the Baltic Sea, has made the discovery that birds "never cross the seas and oceans except on storm currents and sometimes on the storms themselves.' Birds determine from the direc tion of the wind currents when to start in flight over sea. These currents have tremendous pro
pulsive power and a sustained directness that guides the bird to its destination. Huebner's investigations also demonstrated the fact that bird flight is npt affected by temperature, snow or ice and that "only a sudden blizzard from an unknown direction, or a fog, compels the bird to seek refuge on islands or the nearest mainland." These facts have not been lost on aeronauts, and it is believed that the path of the storm will be found materially to assist in stead of interfering with aerial flight. The methods of birds in attacking their prey have been copied by air-craft. Birds vary very much in wing power according to their method of flight; the humming-bird and pigeon being abundantly supplied with wing muscles to main tain their rapid strokes, while the frigate-bird, a notorious "sailer," is remarkably weak in muscular development. The speed of flying birds also varies greatly. The best flyers of which we have definite record are the carrier pigeons, which travel from 30 to 50 miles an hour, while an albatross, caught and tagged by sailors, was recaptured, according to Lucas, 12 days later, 3,150 miles distant. Lanchester, the English engineer, says that "no bird can fly more than 50 miles an hour by its own energy, and therefore that tremendous migrating speed is made only on the swiftest wings.' A plover can fly at least 76 miles an hour between Nova Scotia and South America.
Our familiar small birds do not travel at anything like such a rate, but their endurance is very great, as we can realize in view of their migrations, which often reach from the north ern United States to equatorial South America, while the small waders travel from one end of the hemisphere to the other. See MIGRATION.
Flying creatures occur among mammals, rep tiles and fishes. The extinct pterodactyls were evidently experts on the wing, and some of them constituted the largest flying animals of which we have any record. Of mammals the bats are the only true flyers, the flying squirrels and lemurs having merely parachute-like ex pansions of skin on the sides of the body which when the legs are stretched out enable them to sail obliquely downward from the tree tops to the lower branches.
In the flying-fish (q.v.) there is an enormous development of the pectoral fins which simulate wings. Their flight, however, consists only of a short sail through the air on an impetus gained as they leap from the waves with the fins rig idly extended. Consult Marey, E. J., 'Vol des Oiseaux' (Paris 1890) ; Roy, Chas. S., article "Flight' (in 'Newton's Dictionary of Birds,' 1896) ; Langley, S. P., 'The Greatest Flying Creature' (Smithsonian Report 1901) ; Head ley, 'The Flight of Birds' (London 1912); MacMcchen and Diensthach, 'Bird Flight Aerial Navigation' (in the Century, Vol. LX X X, pp. 297-307).