MIGRATION AND WEATHER 101 



which, through geographical position and isolation, 

 are the likely refuges for wind-borne migrants. Also 

 that the accidental departure from the directions 

 aimed at by the birds is, where wind and barometric 

 systems are so variable, far more frequent than is 

 usually suspected. Direct routes are doubtless 

 aimed at, but only accomplished under favourable 

 conditions for the whole journey ; migration is less 

 infallible than we have been led to think. It is, too, 

 an evolving habit, strengthened by those which 

 survive its perils, now as it was in its early days. 



During a long overland journey, winds will prob- 

 ably have less influence, though for rapid passages 

 high flights certainly appear to be not uncommon. 

 There is, however, another aspect of the connection 

 between migration and weather which we have 

 hardly touched, migration synchronal to the change 

 of season. Mr Cooke shows that in North America 

 the push forward in spring is not in most species so 

 soon as the weather permits ; they do not actually 

 move on the spring wave. Many warblers which 

 nest in the Great Slave Lake region in an average 

 temperature of 47, linger in the Tropics, and reach 

 New Orleans when the temperature is about 65 F. 

 Then they hasten northwards, outstripping the ad- 

 vancing spring, finding in Minnesota a temperature 

 of about 55, and 52 in Manitoba, and gain another 

 5 on the season by the time they reach their home. 



