88 CLASS AVES. 



possibly in wait for stragglers not yet arrived from the 

 interior, or take directly to the south, and are soon lost sight 

 of altogether for the allotted period of their absence. 



We have already observed that the primary object of emi- 

 gration is a supply of food ; and as in temperate countries, 

 occasionally exposed almost to the extremes of cold as well as 

 heat, the air abounds with insects only at particular seasons, 

 it is natural enough that these birds, whose locomotive 

 powersj to almost any distance, are so striking, should quit 

 such countries when the cold begins to deprive them of a 

 sufficient supply of food. The fact abstractedly is less sur- 

 prising than the mode, the regularity, and the punctuality 

 with which it is performed. The whole of them quit our 

 shores and the whole continent of Europe, and pass simul- 

 taneously in organized bodies over the Mediterranean into 

 Africa ; they arrive in Senegal about the ninth of October, 

 and quit it for the north, in spring; and however difficult to 

 be credited, it seems ascertained beyond doubt, that the same 

 pair which quitted their nest, and thelimited circle of their resi- 

 dence here, return to the very same nest again ; and this, for 

 several successive years, in all probability, therefore, for their 

 whole lives. The, simple experiment of tying a silk thread 

 round the leg of one of these birds (which has been re- 

 peatedly done,) will convince any one of the truth of this 

 astonishing fact. Others have been caught, and carried in 

 covered cages to very great distances, and then turned out. 

 These have, been observed immediately to fly to a great ele- 

 vation, and then to describe a large circle several times, until 

 having decided on the right course, they have proceeded, 

 almost with the rapidity of an arrow, undeviatingly to their 

 nest. 



Many anecdotes are related of the sociability and mutual 

 readiness of these birds to assist each other ; but the story that 

 these birds will unite to close the entrance of any nest which 

 may have been taken possession of,, during their absence, by a 



