MOVEMENTS OF BIRDS IN RELATION TO THE WEATHER. 383 



estimated at 750,000, while the number of lifeless bodies scattered 

 over the 1,500 square miles of territory covered by the disaster was 

 beyond computation. Such tragedies are fortunately rare, and yet 

 so fickle is the spring- weather that early migrants, to be successful, 

 require constitutions hardy enough to withstand wide variations in 

 temperature. 



There is no definite temperature to which a bird is confined during 

 migration. The insistent crescendo call of the ovenbird is associated 

 in our minds with the full verdure of May woods, and yet the bird 

 has been known to arrive in a snowstorm. While it prefers a tem- 

 perature of about 55 F., the thermometer at the time of its appear- 

 ance in southern Minnesota varies from near freezing to full summer 

 warmth. Computations of the temperature at the time of arrival of 

 several other common species (so well known and conspicuous that 

 they could hardly fail to be seen as soon as they reached the home 

 of the observer) show variations of from 14 to 37, the average 

 variation being 24. During March, April, and May, in the Missis- 

 sippi Valley, the thermometer rises about 1 for each two days, so 

 that a difference of 24 would be equivalent to about 48 days' 

 variation in the time of migration. 



The later migrants are able to adapt themselves to temperatures 

 of from 70 to 40 F., while the early ones ordinarily experience 

 variations of from 45 to 15, and are able to endure still greater 

 cold. Thus the hoarse caw of the earliest crows is heard in southern 

 Minnesota at an average temperature of 12 below freezing, and the 

 birds can survive a sudden drop of as many degrees below zero. 



MIGRATION GOVERNED BY AVERAGE WEATHER. 



Birds do not migrate by chance. The habit of migration has been 

 evolved through countless generations, and during this time birds' 

 physical structure and habits have been undergoing a process of 

 evolution to adapt them to the climatic conditions of their summer 

 homes. In spring and early summer the climatic conditions are de- 

 cidedly variable, and yet there must be some period that has on the 

 average the best weather conditions for the bird's arrival. During 

 the ages habits of migration have been developed under whose in- 

 fluence the bird so performs its migratory movements that on the 

 average it arrives at the nesting site at the proper time. 



The word " average " needs to be emphasized. It is the average 

 weather at a given locality that determines the average time of the 

 bird's arrival, and the average subsequent weather is the governing 

 factor in deciding when the nest shall be constructed and the eggs 

 laid. In obedience to physiologic promptings, the bird migrates at 

 the usual average time and proceeds northward at the usual average 

 speed unless prevented by adverse weather. But, unfortunately for 



