INSTINCTS OF BIRDS. 287 



by appointing sentinels to give notice of i^- 

 proaching danger. This social disposition, which 

 (with the well known exception of rooks) usually 

 continues no longer than the next pairing season, 

 seems, from the uniformity of the actions that 

 result from it, to be of mstinctive origin; though 

 it certainly would be difficult to bring any direct 

 proof that such is the case. 



In treating of the migration of birds. Dr. 

 Darwin observes, that as all species are capable 

 of remaining throughout the year in those coun- 

 tries in which they were bred, any departure 

 from them must be unnecessary, and therefore 

 cannot be instinctive. This reasoning, however, 

 is extremely fallacious, inasmuch as it restricts 

 the operations of instinct solely to what is neces- 

 sary ; whereas we have seen that the singing of 

 birds, and the practice of concealing their super- 

 fluous food, though not absolutely indispensable, 

 are, nevertheless, decidedly instinctive. It is 

 moreover built on the gratuitous assumption, 

 that several of the periodical summer birds, as 

 the swallow, flycatcher, cuckoo, goatsucker, &c., 

 which feed almost entirely on insects, and con- 

 sequently would not be able to procure a suffi- 

 cient supply of nourishment in the winter months, 

 have the property of passing the cold season in 

 a state of torpidity ; an hypothesis directly at 



