Introductory. 17 



And, as most people know, the migratory birds are 

 more plenteous in one year than another, notably the 

 nightingales in summer, and the fieldfares and redwings 

 in winter. Their presence or absence, however, can be 

 accounted for by the temperature, with other climatic 

 changes ; and, no doubt, these are the chief influencing 

 causes throughout all, though we are ignorant of the 

 modus operandi. 



