CHAP. VIII. MIGRATION OP BIRDS GENERALLY. 257 



allowing two pigeons to the square yard, we have 

 1,115,136,000 pigeons in one flock; and as every 

 pigeon consumes fully half a pint per day, the quantity 

 required to fill such a flock must be 8,712,000 bushels 

 per day."* It is time, however, to return once more to 

 the general subject. 



(270.) Birds in general migrate in very different 

 ways, both as to the direction of their course, the sea- 

 son it is performed in, the number assembled, and the 

 circumstances which accompany their journeys. There 

 is even a variation in the habits of the same species, 

 when spread over widely separated countries ; and 

 numerous others, which are undoubtedly migratory, 

 perform their journey either solitary or in pairs. There 

 is, also, every possible variation both in the extent and 

 in the duration of these migrations. To illustrate all 

 these circumstances would far exceed our limits ; but 

 it may be stated, in general, that in every case these 

 movements are made either for the object of procuring 

 a sufficient supply of suitable food, for the purposes of 

 incubation, or to avoid the extremes of heat or cold. 

 The spring and autumn are the usual seasons ; but 

 there is some variation in the birds of tropical latitudes. 

 The majority of the European land birds which emi- 

 grate, come from Northern Africa (or, more properly, 

 from Asia Minor) in the spring, apparently to avoid 

 the excessive heat of those parched and sandy coun- 

 tries during summer ; but when the season of incuba- 

 tion is over, and the European winter commences, they 

 return again to the Asiatic and African countries, 

 which are then beginning to assume all the verdure and 

 the mildness of our summer : this is proved by the 

 fact of immense numbers annually visiting the shores 

 of the Mediterranean, on their way to Central Europe ; 

 and they are again seen, upon their return, in September 

 and October. These migrations we know, from per- 

 sonal observations, to be as regular as those of the ar- 

 rival and departure of the swallows of England ; and 



* Aud Anim. Biog. 



