[14] BIRDS OF OREGON 



We can do no more than mention them here. Among those most prom- 

 inently advanced have been the following: (i) seasonal shift of food 

 supply; (2.) response to changes in temperature birds are supposed to 

 have originated in the north and to have traveled south during the ice 

 ages; (3) population pressure birds are assumed to have arisen in the 

 south and moved northward because of population pressure; (4) photo- 

 tropism birds are assumed to have moved to the regions of greatest light; 

 and (5) physiological changes in the sex organs such changes are assumed 

 to be correlated with these great mass movements. No one of these 

 theories has been entirely accepted, and it does not seem necessary to 

 assume that any one of them may entirely account for migration habits. 



MORTALITY AMONG MIGRANTS 



THE LOSS of life among migratory hosts is sometimes enormous, and 

 ornithological literature contains many stories of bird tragedies. Small 

 birds are often blown to sea. Some of them come aboard steamships in 



FIGURE i. Returns of Cackling Geese banded near the mouth of the Yukon River, Alaska, 

 chiefly in the vicinity of Hooper Bay. 



