THE 



WILSON BULLETIN 



NO. 82. 



A QUARTERLY JOURNAL OF ORNITHOLOGY 



VOL. XXV MARCH, 1913. NO. I 



OLD SERIES VOL. XXIV. NEW SERIES VOL. XIX 



VARIATIONS IN BIRD MIGRATION FROM YEAR 

 TO YEAR. 



BY WELLS W. COOKE. 



The records of the Weather Bureau show that the average 

 temperatures of the three spring migration months — March, 

 April and May — vary very considerably from year to year. 

 In some years all three months are below normal. In 

 other years the whole season averages above normal. If 

 the birds depended to any considerable extent on temperature 

 to indicate to them the proper time for migration, their 

 movements should vary up and down with these great 

 changes in average temperature. If on the other hand the 

 later theory is correct that the departure from the winter 

 home has no connection with the weather, and that the 

 average temperature at the breeding grounds is the principal 

 factor that determines the time and speed of migration, then 

 it should follow that local variations in temperature should 

 have only a slight influence in varying the dates of arrival 

 from year to year. 



The latter assumption is borne out most strikingly by a 

 long series of careful migration observations. They show 

 that the average of migration at any locality for the entire 

 season as compared with the average for other years is re- 

 markably uniform. The migration arrivals in any given 

 week may be much retarded by a great storm or a long spell 



