differently. Thus not all birds arrive at this condition at the same 

 time. 



It has been demonstrated experimentally that Andean sparrows, 

 resident in equatorial regions, come into breeding condition twice 

 annually entirely independent of changing light periods (Miller 

 1963); evidently the breeding cycle is controlled by periodic internal 

 stimuli. Probably northern migrants that winter in equatorial 

 regions and beyond have their migratory urges controlled by similar 

 rhythms or biological clocks. Also, no evidence suggests that the 

 southward migration of birds is controlled by changing periods of 

 light even among species such as white-crowned sparrows, for which 

 this is a controlling factor in the spring. The fall stimulus is probably 

 an innate cyclic occurrence brought on by a biological mechanism of 

 unknown nature (King, Barker, and Farner 1963). 



It is pertinent to point out that the migratory instinct appears to be 

 more or less transitory and not persistent over an extended period. 

 Migratory birds may be delayed en route, either by natural 

 conditions such as unusually abundant food supplies or forcibly by 

 man. If detained until the end of the migratory season, migrants may 

 not attempt to finish the journey because they apparently lose the 

 migratory impulse. In the fall and early winter of 1929, abundant 

 food and open water caused an unusual number of mallards to arrest 

 their migration and remain in western Montana and northern Idaho. 

 Later, however, when a heavy snowfall with subzero temperatures 

 suddenly cut off the food supply, great numbers of the birds 

 subsequently starved to death; a flight of a few hours could have 

 carried them to a region of open water and abundant food. 



16 



