294 FACULTIES OP BIRDS. 



a mile a minute, which is less than I have actually 

 ascertained them to do over land, ten or twelve 

 hours would be sufficient to accomplish the jour- 

 ney." 



The periodical migration of the gannet affords an 

 instance of a mixed nature. That bird arrives early 

 in spring, and is located in four or five spots along 

 the British coasts, of which may be mentioned the 

 Bass Rock, Ailsa Craig, and St. Kilda. In autumn 

 the gannets leave their breeding-places, and are 

 seen along the coast of England and in the Chan- 

 nel. In mild winters some individuals often re- 

 main, and even the whole flock has been known 

 to winter in their summer residence. Even when 

 they all leave the breeding-places, many individuals 

 do not extend their migration beyond the southern 

 coasts of England, but where the extreme point of 

 the range may be has not yet been ascertained. 



In all these cases, the distribution of food seems 

 to be the principal cause of the movements of the 

 birds ; but in other cases it is clear that the rigour 

 of the winter also acts as an exciting cause ; yet it 

 is doubtful whether cold alone be sufficient to drive 

 birds from their northern haunts. Fieldfares and 

 redwings, no doubt, leave the northern parts of Eu- 

 rope at the end of autumn, because at that period 

 the ground begins to be covered with snow, so that 

 they are unable any longer to procure food ; but 

 they merely shift, so as to place themselves on the 

 limits of the storm, their object being apparently 

 more to obtain the necessary supplies than to evade 

 the cold. In mild and open winters they remain 

 until late in spring ; whereas, after snow has con- 

 tinued several weeks on the ground, it is seldom 

 that any are to be seen. As to swallows, it is evi- 

 dent that the same cause operates most powerfully 

 on them, because, as we have seen, they are capa- 

 ble of bearing as much cold as other small birds. 



How far the migrations of birds may extend, has 



