Vol \\\ 



19 



f : ; xx ] Cooke, The Relation of Bird Migration to the Weather. 215 



The above temperatures probably include nil thai would influence 

 the bird in t In- flight which broughl i1 to the place of observation. 

 A careful examination of these tables will convince anyone thai 

 these temperatures with their greal variations could nol have been 

 the cause of the migration. A bird that arrives with an average 



temperature of 50° F. may appear one year when the temperature 



is below 10° and is just as likely to be seen for the lirsl time the 



nc\t year with a temperature far ahove 60°. Even omitting the 

 extreme variations, yet the average variations are far more vari- 

 able than the movements of the birds and demonstrate that tem- 

 perature alone does not cause the birds to move northward. 



Conversely these figures show that no one of these birds is re- 

 stricted to an_\ single temperature for the performing of its migra- 

 tion, hut that each one can and does migrate with a wide range of 



temperature. 



It is interesting to note in passing, the wide differences between 

 the average temperature of the day of arrival, and the average of 



the temperatures of the days of arrival. Thus the average date 



of arrival of the Robin for the years L885 L890 was at Lancshoro, 



Minn.. March 16, and the average temperature of March Hi al 

 Lanesboro is 31°F. Bu1 the average of the temperatures of the 

 days during L885 1890 on which the Robin was first seen at Lanes- 

 boro was 1 1 ° l'\ This indicates that the Robin had varied its arrival 

 both before and after March L6 so as to arrive on those days that 



were warmer than the average. An extreme difference of 10° was 



found in the ease of the Robin which is an early migrant and often 



encounters severe storms. With birds like the Brown Thrasher 

 which move aboul the middle of the season these differences are 



only aboul half as great, while in the case of late migrants like the 

 Baltimore Oriole and the Scarlet Tana^er these differences disap- 

 pear, since in the latter pari of the season feu storms arc sex ere 



enough to interfere seriously with migration. 



In addition to all the local temperatures at the time of arrival, 

 it is possihle that the total heal for the previous month or the total 

 heat of the whole spring mighl he a determining factor. All of 

 these differenl temperatures were examined and to show how the\ 

 work out in detail, all these temperat tires are given for a single 

 species; the bird selected is the Baltimore Oriole hecause that 



