o 

 O 



64 



MIGEATION OF BIRDS. 



[Ch. XXXIX, 



/ 



It was before stated that birds, not- 



witlistanding their great locomotive powers, form no excep- 

 tion to the general rule, that gronps of distinct species are 

 circumscribed within definite limits. 



In parallel zones of the northern and southern hemispheres 



B 



B 



Tm 



the aquatic and terrestrial birds ; but there is rarely anj spe- 

 cific identity : and this phenomenon is remarkable, when we 

 consider the readiness with which some birds, not gifted with 

 great powers of flight, shift their quarters to different reo-ions 

 and the facility with Avhich others, possessing great strength 

 of wing, perform their aerial voyages. Many species 

 periodically from high latitudes, to avoid the cold of winter 

 and the accompaniments of cold, — scarcity of insects and 



For this purpose, they often traverse the 

 ocean for thousands of miles, and recross it at other periods, 

 with equal security. 



miorate 



vegetable food. 



mi 



Humboldt, of many American w^ater-fowl, from one'part of the 

 tropics to another, in a zone where there is the same tempe- 



Immense flig 



rature throughout the year, 

 the valley of the Orinoco, when the increasing depth of its 



of its shores prevent them from 



waters and the flooding 



catching fish, insects, and aquatic worms. They then betake 

 themselves to the Eio [^Tegro and the Amazons, having passed 

 from the eighth and third degrees of north latitude to the first 

 and fourth of south latitude, directing their course south- 

 south-east. In September, when the Orinoco decreases and 

 reenters its channel, these birds return northwards."^ 



* 



The insectivorous swallows which visit our island would 



^^ r 



perisli during winter, if they did not annually repair to 

 warmer climes. It is supposed that in these aerial excursions 

 the average rapidity of their flight is not less than fifty miles 

 an hour ; so that, when aided by the wind, they soon reach 

 warmer latitudes. Spallanzani calculated that the swallow 

 can fly at the rate of ninety-two miles an hour, the rapidity 

 of the swift being much greater. f Baehman says that the 



Voyage aux E^gions Equinoxiales, 

 torn. vii. p. 429. 



t Fleming, Phil. Zool., vol. ii. p. 43. 





