Chapman on Ike Origin of Bird Migration . 



13 



is doubtless more thoroughly established than that a warm 

 temperate or sub-tropical climate prevailed down to the close of 

 the Tertiary epoch, nearly to the Northern Pole, and that 

 climate was previously everywhere so for equable that the 

 necessity of migration can hardly be supposed to have existed. 

 With the later refrigeration of the Northern regions, bird life 

 must have been crowded thence towards the tropics and the 

 struggle for life thereby greatly intensified. The less yielding 

 forms may have become extinct ; those less sensitive to climatic 

 change would seek to extend the boundaries of their range by a 

 slight reinoval northward during the milder intervals of summer, 

 only, however, to be forced back again by the recurrence of 

 winter. Such migration must have been at first 'incipient and 

 gradual,' extending and strengthening as the cold wave receded 

 and opened up a wider area within which existence in summer 

 became possible. What was at first a forced migration would 

 become habitual and through the heredity of habit give rise to 

 that wonderful faculty we term the instinct of migration." 



This theory gives us, I think, as satisfactory a working 

 hypothesis of the origin of bird migration in North America as 

 we can hope to have. The few words I have to say relate to the 

 influences which may have aided climatic conditions in establish- 

 ing the habit of migration and which are probably effective in 

 governing it to-day. 



Most animals have an instinctive desire for seclusion during 

 the season of reproduction, and when this season approaches will 

 seek some retired part of their range or haunts in which to rear 

 their young. Even our domesticated hens, turkeys, ducks, and 

 pea-fowl, if given freedom, often travel a greater or less distance in 

 search of a place where they may conceal their nests. Many 

 species of tropical sea-birds resort each year to some rocky islet, 

 situated perhaps in the heart of their habitat, where they may 

 nest in safety. This is not migration in the true sense of the 

 word, but nevertheless the object is the same as that which 

 prompts a Plover to migrate to the Arctic regions, and, be it further 

 noted, the movejTient is just as regular. These sea-birds pass 

 their lives in the tropics, their presence or absence in any part 

 of their range being largely dependent upon the food-supply. 

 But, as in the case of a Warbler which nests in Labrador, they 



