STIMULUS FOR MIGRATION 



Modern views based on studies of bird behavior and physiology 

 indicate migration is a regular, annually induced movement, 

 modified by local weather conditions, but largely independent of 

 them. Migration is a phenomenon far too regular to be created anew 

 each season merely under stress of circumstances, such as need for 

 food; and it begins before the necessity for a change in latitude 

 becomes at all pressing. Swallows, nighthawks, shorebirds, and 

 others may start their southward movement while the summer food 

 supply in the North is at peak abundance. American robins and 

 bluebirds may leave abundant food in the South and press northward 

 when food supplies there are almost entirely lacking and severe cold 

 and storms are likely to cause their wholesale destruction. Regularity 

 of arrival and departure is one of the most impressive features of 

 migration, and since birds travel in a rather strict accordance with 

 the calendar, we might ask: "What phenomena, other than the 

 regular changes in length of day, occur with sufficient precision to 

 act as a stimulus for migration?" 



Experimental work has abundantly demonstrated the effect of 

 increased light upon the growth, flowering, and fruiting of plants. 

 Similarly, Rowan's (1925) experiments with slate-colored juncos and 

 the work of numerous subsequent investigators showed, at least in 

 some temperate zone species of migratory birds, increasing periods 

 of daylight triggered sex organs to develop, fat to be deposited, and 

 migration restlessness to begin (King and Farner, 1963). When these 

 conditions develop to a certain level, the bird enters a "disposition to 

 migrate" and takes off for its breeding or wintering grounds. There is 

 reason to believe certain weather conditions influence the actual time 

 of departure and especially the rate of progress to the breeding area. 



This explanation of the stimulus for migration may apply very 

 broadly to birds that winter in temperate parts of the world and nest 

 in the same hemisphere but fails in those birds wintering in the 

 tropics, where little change in length of day occurs and even 

 decreases during the spring in regions south of the Equator. It might 

 be asked: "If the lengthening day is the stimulating factor, why 

 should our summer birds, wintering in the tropics, ever start north?" 

 In addition, if daylength influences when birds are stimulated to 

 migrate, why should they not all leave the same locality at the same 

 time? Or, if weather controls the departure of birds from a given 

 area, should not all the migrants leave when conditions are optimal 

 and refrain from departing when conditions are not so? Actually, the 

 conditions that place a bird in a disposition to migrate are probably 

 the result of a combination of factors affecting different species 



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