16 



A HISTORY OF 



set out upon such a great undertaking. It is 

 probable that the same instinct which governs 

 all their other actions operates also here. 

 They rather follow the weather than the coun- 

 try; they steer only from colder or warmer 

 climates into those of an opposite nature ; and 

 finding the variations of the air as they pro- 

 ceed in their favour, go on till they find land 

 to repose on. It cannot be supposed that 

 they have any memory of the country where 

 they might have spent a former winter ; it 

 cannot be supposed that they see the country to 

 which they travel, from their height in the 

 air ; since, though they mounted for miles, 

 the convexity of the globe would intercept 

 their view ; it must therefore only be, that 

 they go on as they continue to perceive the 

 atmosphere more suitable to their present 

 wants and dispositions. 



All this seems to be pretty plain : but 

 there is a circumstance attending the migra- 

 tion of swallows which wraps this subject in 

 great obscurity. It is agreed on all hands, 

 (hat they are seen in migrating into warmer 

 climates, and that in amazing numbers, at the 

 approach of the European winter. Their re- 

 turn into Europe is also as well attested about 

 the beginning of summer; but we have ano- 

 ther account, which serves to prove that num- 

 bers of them continue torpid here during the 

 winter, and like bats, make their retreat into 

 old walls, the hollow of trees, or even sink into 

 the deepest lakes, and find security for the 

 winter season by remaining there in clusters 



White has remarked, in his Natural History of Sel- 

 borae, that little stress may be laid on the difficulty and 

 hazard that birds must run in their migrations, by rea- 

 son of vast oceans, cross winds, &c., because, says he, if 

 we reflect, a bird may travel from England to the eqn- 

 ator without launching out or exposing itself to bound- 

 less seas, and that by crossing the British Channel at 

 Dover and the Mediterranean at Gibraltar; thus select- 

 ing the narrowest points of passage. It is, however, 

 certain that migrating birds in their flight are often sub- 

 ject both to disasters and considerable fatigue. This in. 

 deed has been instanced by the settling of birds in an 

 exhausted state on the rigging f>nd decks of vessels at 

 sea. Certain birds, as the moorhen, rail, &c., being 

 unable to fly for any considerable distance, travel partly 

 on loot. Some even (as the great auk or penguin, diver, 

 and guillemot) migrate by water. Ornithologists have 

 observed, that, in Europe, birds migrate in autumn to 

 the south-west, and in spring towards the north-east; 

 yet the courses of rivers and chains of mountains exer- 

 cise considerable influence on the direction of their 

 flight. It is remarkable, also, that the young of certain 

 species do not make the same journey as the old birds ; 

 they go more to the south, so that it is very common to 

 find, in the south of Europe, only the young birds of a 

 certain species, whilst the older ones remain more to the 

 north. In other species, the females go farther south. 

 It was formerly believed that the birds of the tropical 

 regions never migrate, and that they never pass the line; 

 but Humboldt has shown that this is not the case. He 

 observed, moreover, that the migration there took place 

 with the periodical rise of rivers. 



at the bottom. However this latter circum- 

 stance may be, their retreat into old walls is 

 too well authenticated to remain a doubt at 

 present. The difficulty, therefore, is to ac- 

 count for this difference in these animals thus 

 variously preparing to encounter the winter. 

 It was supposed that in some of them the 

 blood might lose its motion by the cold, and 

 that thus they were rendered torpid by the se- 

 verity of the season; but Mr Buffon having 

 placed many of this tribe in an ice-house, 

 found that the same cold by which their blood 

 was congealed was fatal to the animal ; it re- 

 mains, therefore, a doubt to this hour, whether 

 there may not be a species of swallows to all 

 external appearance like the rest, but differ- 

 ently formed within, so as to fit them for a 

 state of insensibility during the winter here. 

 It was suggested, indeed, that the swallows 

 found thus torpid, were such only as were too 

 weak to undertake the migration, or were 

 hatched too late to join the general convoy ; 

 but it was upon these that Mr Buffon tried 

 his experiment; it was these that died under 

 the operation. 



Thus there are ^some birds which by mi- 

 grating make an habitation of every part of 

 the earth ; but in general every climate has 

 birds peculiar to itself. The feathered inha- 

 bitants of the temperate zone are but little re- 

 markable for the beauty of their plumage ; 

 but then the smaller kinds make up for this 

 defect by the melody of their voices. The 

 birds of the torrid zone are very bright and 

 vivid in their colours ; but they have scream- 

 ing voices, or are totally silent. The frigid 

 zone, on the other hand, where the seas 

 abound with fish, are stocked with birds of the 

 aquatic kind, in much greater plenty than in 

 Europe ; and these are generally clothed with 

 a warmer coat of feathers ; or they have large 

 quantities of fat lying underneath the skin, 

 which serves to defend them from the rigours 

 of the climate. 



In all countries, however, birds are a more 

 long-lived class of animals than the quadru- 

 peds or insects of the same climate. The life 

 of man himself is but short, when compared to 

 what some of them enjoy. It is said that 

 swans have been known to live three hundred 

 years ; geese are often seen to live fourscore ; 

 while linnets and other little birds, though 

 imprisoned in cages, are often found to reach 

 fourteen or fifteen. How birds, whose age of 

 perfection is much more early than that of 

 quadrupeds, should yet live comparatively so 

 much longer, is not easily to be accounted for : 

 perhaps, as their bones are lighter, and more 

 porous, than those of quadrupeds, there are 

 fewer obstructions in the animal machine; and 

 Nature, thus finding more room for the opera- 

 tions of life, is carried on to greater extent. 



