MIGRATION. 27 



to the north and sometimes to the south to lay their eggs ; in 

 others again this migration is not determined by any appreciable 

 cause. 



13. Some migratory birds perform their journeys through the 

 air alone or only accompanied by their females ; but the number is 

 small comparatively to those that travel in company. We admire 

 in the latter the instinct which induces them to assemble at a cer- 

 tain place, ten or twelve days previous to the time of departure, 

 which is ordinarily an indication of a change of weather; for it 

 is remarked that birds feel the influences sufficiently early to derive 

 prognostics of the change of temperature from their deportment 

 and certain habits. During the whole journey, the most perfect 

 oyder prevails throughout the whole troop; to be convinced of 

 this fact we have only to observe the flight of some large species, 

 such as geese. The conducting of 'the troop is confided to a 

 chief placed at the head of two files, more or less separated from 

 each other, which meet at a point ; the chief is the summit or 

 point of this moving angle, and opposes the first resistance of the 

 air, clears the way, and the whole band follows him, observing 

 the most perfect order. As the efforts of the chief are very 

 violent, and as he cannot support them during the whole voyage, 

 he is perceived, when overcome by fatigue, to yield his post to 

 his next neighbour, and fall into the ranks at the extremity of 

 one or other of the files. The period of these great migrations 

 is fixed by nature for each species of migratory birds, and it is 

 remarked, they follow the same route every year: hence, in 

 certain districts, the fowlers or bird-catchers, count upon their 

 passage as upon a revenue of rent that falls due every quarter, 

 and calculate in advance the period, and the chances. Armed 

 with their nets and all the apparatus of the chase, they station 

 themselves in the gorges and valleys over which the flocks pass, 

 and reach their several points a few minutes before their arrival. 

 These bands or flocks are sometimes so numerous, and the in- 

 dividuals composing them are so close together that they might 

 be readily mistaken for dense clouds. 



14. Certain birds always lead an erratic life, and seem to have 

 no country : these are the most powerful on the wing; many 

 seem not to be impeded by the strongest wind, and appear to 

 delight in the midst of storms. They form a striking contrast 

 with a small number of species, less favoured by nature, which, 



13. How are these migrations formed ? At what period do these migra 

 lions take place ? Are these periods vi ry certain? 



14. Do all birds belong to some one or other country ? Are all birds act ; vo 

 in flight? 



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