DISTIUBUTION. 161 



to a cooler climate, instead of travelling hundreds of miles 

 north, they go up into the mountains a mile or two and find 

 just the degree of coolness they desire. So the three-toed 

 woodpecker and the arctic ptarmigan, the leucostictes and the 

 snow buntings, drift southward along the lines of lofty peaks 

 in the Western ranges, and in the East the red-breasted nut- 

 hatch, the chickadee, and the junco follow down the Appa- 

 lachian system, finding a climate that is in most respects 

 the same as their Canadian home. 



In the northward extension of southern birds, we find that 

 they follow plains and river valleys. The flat, barren plains 

 reflect the heat, and the winds across them are often burning 

 hot, withering all vegetation like a fire. Such winds and 

 such heat bear the lines of summer temperature far to the 

 north, even to the plains of the Saskatchewan and the interior 

 of British America. Here the birds find a summer as hot as 

 that upon the seacoast fifteen or twenty degrees farther south. 

 It does not matter that in winter these same plains are many 

 times colder than the seacoast, for the bird's distribution is 

 influenced by the summer climate. 



The map of faunal provinces and subprovinces shows us 

 very nearly the course of the summer isotherms marking 

 each ten or fifteen degrees difference in average heat. The 

 north and south division through the centre of the map is, 

 however, not a temperature division, but it cuts off the dry 

 western half from the moist eastern half of the country — the 

 green and luxuriant prairies and woodlands from the parched 

 and scanty herbage of the plains. And, of course, with the 

 change in amount of rainfall and the consequent change 

 in vegetation, follow changes in the insect life and in the 

 birds that live upon insects and berries. There is often a 

 very close connection between birds and certain plants or 



