\ ,,l \ \ \ 



Cook] , Tfu Relation of Bird Migration to thi Weather. 21.) 



variation in the time of migration. Tims the temperature under 

 which the birds arc migrating is about four times as variable as the 

 day of arrival of the birds. 



The above table representing the temperature at 10 p. m. of the 

 aighl during which the birds arrived is probably the nearest approxi- 

 mation that ean he obtained to the actual temperature al the time 

 the birds arrived. Since the birds have undoubtedly flown many 

 miles during the night, it might he thai the temperature of the 

 place where the evening flight started would have a controlling 

 influence. 



The average of these last two tables is probably the best statement 

 obtainable of the actual temperature at which the birds migrated. 



It is difficult to see how the mean temperature of the day when 

 the bird was first noted could have had any great influence on its 

 migratory movements of the previous night, but as these condi- 

 tion under which we first see the bird are the ones we are most 

 likely to associate with the bird's arrival, they have also been 

 calculated. 



