BIRDS. 375 



countries where they were hred. These birds are named, jpermanent 

 birds [aves manentes). Such, for example, are with us and in the 

 temperate parts of Em-ope, the sparrow [Fringilla domestica), the 

 magpie {Corvus pica), &c. Others migrate from one place to 

 another without a determinate direction, and do not remove far; 

 they are named wandering birds {aves erraticce), as the wood- 

 peckers (Pici). Finally, other birds undertake, as we have said, 

 at certain times of the year, mostly united in numerous packs, 

 long journeys, and pass the winter in warm regions. These are 

 properly onigratory birds {aves migratorice), as, for instance, the 

 stork, the cuckoo, the lark, most singing-birds, the swallows, &c. 

 This distinction, however, applies only to particular comitries, for 

 the same species, which with us are migratory, remain in warm 

 countries through the whole year. Hence it is in tropical countries 

 that the largest number of permanent birds are found. 



Most of our migratory birds pass the winter in northern Africa. 

 Swallows winter in Africa, where Adanson saw them arrive in the 

 Senegal country in October^; in this wandering they cross the 

 Mediterranean Sea as well as the Atlantic Ocean along the western 

 coast of Europe; the idea that these birds conceal themselves during 

 winter at the bottom of water, in holes under ground or in hollow 

 trunks of trees, and become torpid there, does not require serious 

 refutation at this day. 



In the new world, as well as in the old, birds of passage, whose 

 abode is in the northern hemisphere, migrate on the approach of 

 winter towards the tropic of Cancer ; those of the southern hemi- 

 sphere, when winter sets in, migrate towards the tropic of Capricorn. 

 This simple statement of the matter in its greatest generality leads 

 of itself to the idea, that the cause of migration is to be sought in 

 the necessity that birds of passage are under to continue always in 

 the same mean temperature. For when in the northern hemisphere 

 during winter the warmth decreases and the snow-line descends, 

 birds, which at that time migrate southwards towards the tropic, 

 stop, Within certain fixed limits, where the mean temperature of 

 winter does not differ much from the mean temperature of summer 

 in those northern regions which they have left. So the migra- 

 tion of birds is in connexion with the general diminution of life in 



^ Hist, nat, du Senegal, p. 67. 



