I20 THE MIGRATIONS OF BIRDS 



northern California, being found only casually 

 south to the region of San Francisco Bay; while the 

 last of the forms under consideration, Juliginosa^ 

 from Vancouver Island and the Puget Sound region, 

 may perform a vertical migration down mountain- 

 slopes, but aside from that wanders only a com- 

 paratively few miles beyond its breeding range. The 

 northern forms thus pass completely over those in 

 intermediate regions in veritable leapfrog fashion. 



We can readily understand why birds might wish 

 to leave the inhospitable north with the coming of 

 winter, but why many individuals press on across 

 the equatorial region to south temperate lands can 

 be explained only by ease in accomplishment of 

 flight, and an instinct to wander. Any one who has 

 worked among the diversified faunas of the tropics 

 will agree that in the region of the Equator may be 

 found areas suitable to almost any species of bird, 

 where there is no question of crowding or shortage 

 of food supply. Insects swarm, and vegetable food, 

 in form of seeds and fruits, is abundant. A climate 

 that varies from humid to arid fosters heavy for- 

 ests, dry, open scrubs, broad savannas with high 

 growths of grasses, or areas of plain with herbage in 

 tufts or clumps. Sandy beaches, or broad mudflats 

 with low growths of mangroves, are found on the 

 coasts, and lakes and streams are common. Where a 

 local dry season is encountered, it is necessary only 



