Variations in Bird Migration. 7 



The table shows that the early geese are very variable in 

 their movements ; that early migrants sometimes appear a full 

 month before the usual time of arrival, and that extreme var- 

 iations in different years amount to a month and a half. At- 

 tention is called to the last column, which is the '' probable 

 error" in records of the arrival of the Canada Goose and 

 means that when a good observer sees the first goose of the 

 season, the probability is that the date is within nine to four- 

 teen days of the normal date of arrival for that district. With 

 such wide variation, a large series of observations is neces- 

 sary to establish a reliable normal for any given locality. 



The extremes of variation are considerably greater in the 

 district just north of the winter range, since with these hardy 

 birds, a few warm days may tempt them north even in Jan- 

 uary, and in mild winters, like those of 1888-9 and 1889-90, 

 geese may be seen every few days all winter, many miles north 

 of the usual winter abode. When it is remembered that the 

 probable error in the date of arrival of the late migrants is 

 hardly more than three days, it will be understood how great 

 an influence the changeable spring weather has on the early 

 migrants. 



Birds differ quite decidedly in their tendency to vary the 

 date of their arrival. The extreme is found in the Canada 

 Goose just mentioned, which has an average variation of 11 

 days from the normal date of arrival. At Lanesboro and 

 Grinnell, the birds showed the following as their average 

 variations from the normal time of appearance. 



Plicelie 4.9 days Ruby-crowned Kinglet. 2.1 days 



Black-poll Warbler 4.9 



Bluebird 4.0 



Meadowlark 4.1 



Bronzed Grackle 3.7 



Whippoorwill 4.0 



Purple Martin 3.9 



Robin 2.5 



Chipping Sparrow .... 2.1 



Wood Thrush 1.8 



Chimney Swift 2.0 



Kingbird 2.0 



Baltimore Oriole 2.1 



Black-biHed Cuckoo.... 2.0 



Ovenbird 1.4 



Thus the Canada Goose and the Ovenbird represent the 

 extremes — the one eight times as variable as the other. 



