1 6 Chapman on the Origin of Bird Migration. [Tan. 



had to do if we suppose them to have become North American 

 since the advent of the members of the second class. Doubtless 

 they may once have represented the first class and perhaps at 

 that time all our migrants were confined to our southern 

 borders, — this being presumably the condition of things during 

 the period of glaciation, — but as a gradually changing climate 

 advanced the isotherm which bounded the northern limit of 

 their range, and with it the conditions they required, they fol- 

 lowed it northward until even the southern limit of their summer 

 home was carried further north than the northern limit had pre- 

 viously been, except where altitude gave them the surroundings 

 needful to their existence. 



As an illustration of how a northern habitat might be acquired 

 I will instance the case of our Common Tern (Ster?ia hirtindo) 

 on the Atlantic coast. It is only a few years since this species 

 was an abundant breeder along the greater part of the coast, 

 but a demand arose for these birds for millinery purposes and, 

 as the result, they are now restricted during the breeding season 

 to comparatively few localities. On Long Island, for example, 

 this Tern was a common summer resident but those birds which 

 nested on the mainland were easily accessible to hunters and 

 were soon exterminated, until at present few or no Terns nest 

 on Long Island except a colony of about iooo pairs confined to 

 the small, uninhabited, isolated islet known as Big Gull Island. 

 On the Massachusetts coast, practically the same thing has hap- 

 pened and Terns are now largely restricted to Muskeget Island. 



What has occurred on Long Island and in Massachusetts will 

 doubtless take place throughout the larger part of this Tern's 

 American range. It breeds now from the Gulf of Mexico to 

 the Arctic regions, but is the day far distant when the Common 

 Tern will be unknown as a breeding bird in that part of its 

 present summer habitat inhabited by man ? Then its breeding- 

 range in America will be a boreal one, and just as the Terns of 

 Big Gull and Muskeget Islands return year after year to the 

 home of their birth, so will these northern breeding Terns return 

 to their Arctic home, and have thus established a habitat similar 

 to those of the birds in the second class of migrants I have 

 mentioned. 



But we may learn another lesson from these island-nesting birds. 



