250 CHELIDONEP. 



precipices, to which it would be difficult to convey ordinary 

 mortar, construct them generally of so glutinous a substance, 

 that they are sold as a substantive ingredient of soups ; but 

 that the nest is wholly a secretion of the birds, is only said, 

 not proved. The swift is not so prolific as those swallows 

 which inhabit farther down ; its eggs seldom exceed two. 



The swifts have not only more labour to perform in order 

 to procure an equal quantity of food, while they are with us, 

 than any other of the tribe, but they also perform a wider 

 and a more hurried excursion in latitude. The same species, 

 and in all probability the same individuals, range from the 

 northern parts of Europe to the Cape of Good Hope, and 

 back again, in the course of about nine months. They sel- 

 dom appear in any part of Britain before the middle of May, 

 and not in the northern parts till June. The young are 

 hatched, and on the wing, about the beginning of July ; after 

 that time the whole begin to flock, and they disappear about 

 the middle of August, a little sooner or later according to 

 the state of the weather. Their motions do not depend 

 entirely upon the state of the temperature, but on that and 

 the facility of finding food jointly ; and warm showers, with 

 gleams of sunshine between, will protract their stay, whereas 

 very dry weather, though intensely hot, rather accelerates 

 their departure. 



During the short time that they remain with us, the swifts 

 are very interesting birds ; and duly studied, they would no 

 doubt be equally instructive. Migratory birds are all, of 

 course, from the very same instinct that prompts their migra- 

 tions, much more sensitive to atmospheric changes than resi- 

 dent birds, which are tempered by nature to endure those 

 changes, or to change with them, and on that account they 

 are better indices to those changes. But swifts are the most 

 migratory of birds ; their pastures fluctuate more with the 

 state of the weather than those of birds that fly and feed 



