inodifiecl both in the case of species and of individuals as to render it certain that 

 the habit of making the extended northward migrations now undertaken by cer- 

 tain of our American birds has been acquired recently and by degrees. We know 

 that until aboat thirty years ago such birds as the Meadowlark, Bobolink, Balti- 

 more Oriole and others did not extend their flight beyond our southern borders, 

 because the interior and northern part of the province was then heavily wooded 

 and unsuitable to their requirements, but now these birds migrate in increasing 

 numbers every year as far north as and even beyond the Ottawa Eiver. They have 

 taken advantage of the clearing of the forest and the cultivation of the land to 

 disperse themselves over an area which was previously not adapted to their way of 

 living. In the early eighties I noticed a similar movement in Manitoba. As the 

 land there was brought under cultivation and the prairies were peopled. Bluebirds, 

 Purple Martins, Cliff Swallows, and other birds which were previously unknown 

 came in as migrants and established themselves as regular summer residents. 

 Failure of the food supply and the severe cold of these northern regions drive these 

 birds southward for the winter, where they remain until returning spring gives the 

 impulse for their return flight to the north. 



If all the individuals of the so-called migratory species were in the habit of 

 entirely leaving their winter quarters and resorting to some northern region pecu- 

 liarly adapted to their requiremeuts during the breeding season, we might well 

 assume that migration was an inherited instinct transmitted from remote ancestors 

 who had acquired it by reason of climatic changes, which had forced them at cer- 

 tain seasons to leave what had been originally their permanent habitat. This is 

 true, however, of only a few American species, the majority of which, in greater 

 or lesser numbers, breed almost all through their range. 



It seems to me, therefore, that the impulse to migrate is the result of a natural 

 law which provides for the dispersal of birds over the world during the season 

 when their services are most required in maintaining the balance in nature, and 

 that when the physical features of a country are changed, as ours have been, from 

 heavy forests to open fields, the species of birds Which mi.grate into it will change 

 also, so that the land will be occupied by those best specialized to perform the 

 functions required of them in nature's economy. 



As the study of migratory birds has progressed and the peculiarities of method 

 adopted by each species have been traced the difficulty of assigning any general 

 cause for the habit except that already stated becomes greater. The movement 

 from the north in the autumn presents many instances showing that various species 

 act upon an impulse which differs from that of others closely allied to them. Genera- 

 ally speaking, it is assumed that birds leave the northern regions, where they have 

 nested, at the approach of winter; when cold weather is imminent and their food 

 supply is failing. Many species do linger in their summer homes, until it would 

 seem as if they required to be driven out, but others again leave while food is most 

 abundant and the temperature at its highest. Among the Thrushes this difference 

 is very marked. Wilson's Thrush, the Hermit Thrush, and the Olive-backed Thrush 

 resemble each other very much in appearance and in all their habits except their 

 migration. Wilson's Thrush arrives here early in May and breeds abundantly from 

 our southern border northward. About the middle of August their return flight 

 begins and by the twenty-fifth of the month they have all gone. The Hermit 

 arrives early in April, breeds sparingly in Southern Ontario and remains until 

 the beginning of November. The food of these two species is exactly the same, 

 consisting of insects and such small berries as are to be found in the woods. The 

 Olive-backed Thrush moves at the same time as the Hermit, but goes further north 



