THE FLIGHT OF BIRDS 305 



hour is no extraordinary pace for Swallows and Swifts, while 36 

 miles an hour is a favourable average for Pigeons. Much higher 

 speeds have, however, been recorded, e.g. 71 miles per hour (for a 

 distance of 82 miles) in the case of a Pigeon, and 106 miles per 

 hour (for 160 miles) in the case of a Swallow. The achievements 

 of many migrant birds, extended as they often are over enormous 

 distances, are remarkable as illustrating both speed and endurance. 

 The American Golden Plover (Charadrms Virginicus), for ex- 

 ample, during its autumnal migration to the South, is credited 

 with being continuously on the wing for a distance of some 1700 

 miles, as it passes oversea from Nova Scotia to South America 

 via the West Indies. In such a case the speed must be very 

 considerable, as birds cannot remain without food for an in- 

 definite time. Of course it must not be imagined that during a 

 prolonged stay in the air the wings are in constant movement. 

 For having once got up speed a bird is able to glide along for 

 a considerable time without effort, especially by taking advantage 

 of air-currents. 



Passive Flight. — Birds may be often observed moving 

 through the air with motionless outspread wings and expanded 

 tail. Two chief kinds of this passive flight may be recognized, 

 known respectively as Gliding and Soaring. 



Glidmg Flight. — This takes place when a bird simply moves 

 onwards and does not describe the upward spirals that are charac- 

 teristic of soaring. It is exemplified by many forms possessing 

 considerable spread of wing, among the most familiar being 

 Gulls and Pigeons. The simplest case is seen when one of the 

 latter birds slides down obliquely from a roof to the ground, and 

 this may be described as a parachute movement (see p. 286). 

 Such a descent may also be made by a bird which is actually 

 flying, as when a Falcon swoops steeply down upon its prey 



(fig- 837). 



Gliding may also take place in an upward direction, and here 

 again the Falcon will serve as an illustration. For in the middle 

 of its swoop it can, if necessary, so adjust its wings and body as 

 to utilize the momentum gained for an upward slide. Supposing 

 that a bird which is flying horizontally forwards with consider- 

 able speed suddenly ceases to move its wings, but allows them 

 to remain extended. It will then glide on for a considerable 

 distance. And it has been proved experimentally that under 



