NOCTURNAL MIGRATION 55 



Other species, however, have no reaction against 

 cold. The blue and yellow macaw {Ara ararauna) 

 cannot withstand a fall in temperature, and I have 

 a captive blue-headed parrot {Pionus menstruus) 

 from Bolivia, that is sensitive to a degree, and com- 

 plains of the shghtest degree of cold. 



Wind is as much a deterrent to migration as rain 

 or snow, as head winds greatly increase the labor 

 of flight and, if at all strong, cut down the speed 

 at which migrants are able to travel. Similarly, 

 strong winds blowing in the direction in which the 

 birds are travelling are equally bad, since they 

 interfere seriously with balance, and disarrange 

 feathers, thus hindering flight. Moderate cross, 

 quartering, or light winds from other directions, 

 seem to off'er the best air-movement conditions. 



Cooke has indicated that migration among birds 

 is so adjusted to mean weather conditions that each 

 species moves north in spring at a time when the 

 average weather encountered is not unsupportable. 

 Hardy forms move early, unafraid to brave the 

 blasts of retreating winter. More delicate ones 

 come later, when there is less danger of encountering 

 prolonged cold spells. Occasionally, as in the 

 Canada goose, it is found that the advance in migra- 

 tion coincides with advance of certain isothermal 

 lines. The goose keeps closely abreast of the line of 

 35 degrees average temperature, and moves north as 



