208 Cooke, The Relation of Bird Migration to the Weather. I April 



As proof that birds are not dependent on any exact temperature 

 for their time of migration, it can be stated that birds do not move 

 north in the spring as soon as the temperature rises to the degree 

 of warmth at which they ordinarily migrate. Thus the Baltimore 

 Oriole arrives at Lanesboro, Minn., at an average temperature of 

 about 55° F. but it does not make its appearance as soon as the 

 temperature has risen to this point. The Oriole was not noted 

 at Lanesboro before May 1 in any of the years from 1884 to 1893, 

 though in 18S4 a temperature of 55° F. was attained on April 25, 

 in 1885 on April 20, and in the following years on April 9, April 8, 

 April 26, April 9, April 11, April 13, April 1, and April 3. Dur- 

 ing the spring of 1886, the temperature from April 13 to April 23 

 averaged 65° F. but no Orioles appeared. 



On the other hand, birds do not always wait for their average 

 temperature before they migrate. In 1893 there had been no three 

 consecutive days during the whole spring with as high a tempera- 

 ture as 55° F. when the Baltimore Oriole arrived at Lanesboro, 

 and during the previous two weeks no temperature higher than 4S° 

 F. either at Lanesboro or in the country a hundred and fifty miles 

 to the southward. 



It thus appears that each species has a wide range of temperature 

 at which it can migrate. In the case of early migrants this varies 

 from 40° F. down to many degrees below freezing, while with the 

 latest from about. 40° F. to 70° F. 



If the movements of migration are caused by the weather, then it 

 should be that a late spring would retard the arrival, and that the 

 birds would appear earlier in an unusually warm season. The facts 

 do not seem to bear our this supposition. During the nine years, 

 1885-1893, at Lanesboro, the larger variation in the time of arrival 

 occurred under the following conditions. In this table a ' warm ' tem- 

 perature is three or more degrees above the normal and a ' cold ' an 

 equal amount below; the intervening temperatures are called 'nor- 

 mal.' 



Bird arrivtib,- at Lanesboro, Minn. 1SS5-1893. 



No. of 

 The birds came three or more days Instances. 



early with a warm temperature 58 



late " " cold " 60 



on time with normal temperature 7 



Arrivals agree with theory 125 



