FLIGHT OF ANIMALS — GRAY 



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In temperate climates the use of thermal upcurrents by gliding birds 

 is somewhat restricted; nevertheless, birds may sometimes gain con- 

 siderable height over towns or other regions where warm air is rising. 

 In tropical countries, however, cheels, vultures, and birds of this sort, 

 are very good at this kind of glide (fig. 7). During the mornings, in 

 these places, columns of warm air rise from the ground and drift down- 

 wind. The birds appear to glide down a spiral within such a column 

 of air, and as the rate of tlieir downward movement is less than the 

 rate at which the colmnn itself is moving upward, the birds constantly 

 spiral upward, getting higher above the earth as they drift downwind. 

 As we might expect, thermal gliding is characteristic of birds with 

 light bodies and large wings. On these two factors (weight and wing 

 surface), depends the speed at which the bird would lose height when 

 gliding in still air and, consequently, the speed at which the air must 

 rise if the bird maintains a horizontal or upward path. 



Figure 7. — In regions v/here upward currents of warm air alternate with downward cur- 

 rents of cool air, birds can glide upward in the former and downward in the latter. Arrows 

 show direction of air movement. It is seen that a bird can gain considerable height by 

 taking advantage of these air currents. 



So far, the principles we have applied to gliding birds are exactly 

 the same as those made use of by a glider pilot, whose powers of gliding 

 and of staying in the air depend almost entirely upon the use of ob- 

 structional and thermal upcurrents. Certain birds, however, are able 

 to glide in conditions where there seem to be no upcurrents: shear- 

 waters and albatrosses, for instance, glide near the surface of the sea, 



