FLIGHT 267 



II. Flight hy active strokes of the wings. It is very much 

 easier for a bird to sustain itself in the air by active flight, if it 

 have a certain initial relative velocity, than if it seek to begin flying 

 from a position of relative rest or Avant of motion. This leads all 

 birds to rise from the ground with their heads towards the wind. 

 If there be not wind enough, the bird seeks to gain initial velocity 

 by running or springing, or both. In the case of aquatic birds 

 the velocity is obtained in part by striking the water with their 

 wing-tips during the first few strokes. In "hovering," which is 

 only possible for very powerful flyers, the great exertion can be 

 seen from the rapidity with which the wing-strokes follow one 

 another. Birds ^Wth short legs and long wings, as the Condor and 

 Albatros, cannot, in the absence of wind, rise at all from the 

 ground unless they have room to run, and Condors can be easily 

 caught by tempting them with food within a narrow enclosure. 

 Even the strong-flying Pigeon, after being made to rise from the 

 gi^ound and fly a short distance for five or six times in succession, 

 refuses to rise again, and remains on the ground panting with open 

 beak. Marey, who observed this in his own Pigeons, calculates 

 that the energy expended per second in a Pigeon when taking 

 flight is five times as great as when it has acquired a certain velo- 

 city. The bird at starting makes rapid strokes Avith its wings, 

 which move through a large angle, — in the case of the Pigeon 

 striking one another above the back at the end of the up stroke, 

 and nearly touching with their tips at the end of the down stroke. 

 When velocity has been acquired, the flaps are slower, and the 

 angle which they describe round the shoulder-joint is a very much 

 smaller one. The reason why so much more energy is required to 

 fly when they have little or no initial velocity relative to the air is 

 due to the increased support afforded by the air if the wing-surface 

 which strikes it be at the same time travelling through the air in a 

 line more or less parallel to its surface. This gain in resistance to 

 the wing-stroke increases, as we have already pointed out, with the 

 square of the velocity of the wing in a direction parallel with the 

 axis of flight. 



With regard to the manner in which a bird uses its wings 

 during flight we are indebted chiefly to Marey, who employed 

 much more exact methods of observation than had previously been 

 made use of. As can be seen from the annexed figure (Fig. 6), 

 which shews a Gull photographed at successive intervals of one- 

 fiftieth of a second, the wings during the down stroke move forward 

 as Avell as downward. It can be shewn that the outer end of the 

 humerus describes a kind of ellipse (Fig. 7) round a straight line passing 

 through the shoulder-joints, the long axis of which ellipse is inclined 

 slightly downwards from the horizontal. During the down stroke, 

 which is made along the front half of the ellipse, the surface of the 



