distances up to lOO miles. Permanent magnets were then affixed to 

 the under side of the manus part of the wings of half of the birds while 

 copper plates of equivalent weight were attached to the wings of the 

 other half. All birds were released singly at an airline distance of 

 about 65 miles from the loft. The results were most suggestive, as 

 only two of the birds carrying magnets returned to the loft, whereas 

 eight of the controls returned. 



With certain minor modifications, this experiment was repeated by 

 Gordon. In this case 60 pigeons were used and releases were made 

 from points up to 58 miles, where the direction of flight was such that 

 the birds had to navigate across the gradient of the magnetic field. 

 Every bird returned to its loft on the day of release regardless of 

 whether it carried magnets or unmagnetized bars of the same weight. 



Attempts to demonstrate the effect of radio waves on the navigational 

 ability of birds also have produced contradictory results. In some of 

 these tests, homing pigeons released near broadcasting stations have 

 appeared to be hopelessly confused, whereas in others, apparently con- 

 ducted in the same manner, no effects could be discerned. It is obvi- 

 ous that before the electromagnetic theory can be accepted or rejected, 

 much additional experimental work is necessary. 



In concluding this discussion of orientation it is pertinent to point 

 out that the migratory instinct appears to be more or less transitory, 

 that it is not persistent over an extended period. Migratory birds may 

 be arrested en route, either by natural conditions, such as unusual food 

 supplies, or forcibly by the act of man, and detained until the end or 

 nearly the end of the migratory season, and then may not attempt 

 to finish the journey, apparently having lost the migratory impulse. 

 In the fall and early winter of 1929, abundant food and an open season 

 caused an unusual number of mallard ducks to arrest their migration 

 and remain in western Montana and northern Idaho. Later, however, 

 a heavy snowfall with subzero temperatures suddenly cut off the food 

 supply, with the result that great numbers of the birds starved to death, 

 when a flight of a few hours would have carried them to a region of 

 open water and abundant food. 



Segregation during migration 



During the height of the northward movement in spring the woods 

 and thickets may be suddenly filled with several species of wood 

 warblers, thrushes, sparrows, flycatchers, and others, which it is natural 



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