Xxii INTRODUCTION. 



they will return to the spot where they are fed. They also 

 can be taught to repeat certain words and sentences, and 

 even songs. 



The migrations of birds are among the most remarkable 

 effects of their instincts ; and the cause thereof has given 

 rise to numerous speculations. It appears to me that the 

 want or scarcity of food, together with the decline of tem- 

 perature, are the chief causes that impel most migratory 

 (and perhaps all our Wading and Water-birds) from the 

 northern regions where they breed, to the more genial 

 climate of the winter of the Tropics ; and it is the ever- 

 continued flow onwards of the birds from the North that 

 impel so many to the extreme South. W^ere it not for this 

 continued stream from the North, many would stop far 

 short of the usual extent of their wanderings south. The 

 Northward migration, however, is dependent on another 

 cause ; viz.^ the rapid enlargement of the sexual organs in 

 spring, causing the wish of returning to their homes (for 

 such their birth-place must be considered), for the purpose 

 of breeding ; and this appears to become a most powerful 

 and irrestrainable impulse, even birds in confinement and 

 incapable of flight, showing great uneasiness at this time. 

 Why such birds as the Cuckoo, immediately after her eggs 

 are laid, and the male perhaps before that, should proceed 

 to the south, is a more diflicult problem. Cuvier's explana- 

 tion of the cause of migration being dependent on the 

 sensibility of birds to the variations of the atmosphere to 

 an extent of which we can have no idea, no more affords 

 an efiicient cause for this, than for that of migration in 

 general. 



In India we have several variations in the time of 

 arrival and departure of migratory birds. In lower 

 Bengal the Kites quit Calcutta and its vicinity during 



