2 46 THE MIGRATION OF BIRDS. 



very considerable, and in many cases composed of 

 individuals of various species driven from one part 

 to another by stress of weather. The usual marvel- 

 lous punctuality of arrival and departure, and per- 

 sistency of following certain routes ; the same order 

 of Migration ; in fact all the minor details of 

 Flight are observed by British Migrants as we have 

 seen is so universally the case in earlier portions of 

 this work, so that they need not be more than 

 alluded to again. We might also, however, remark 

 similar obedience to Meteorological changes and 

 choice of Flight wind. The evidence so far as it 

 goes seems to imply that direction of wind is 

 subservient to a very great extent to change of 

 temperature, the latter, if adverse, initiating Migra- 

 tion even in the face of contrary or unfavourable 

 air-currents. The consequence is, that the pre- 

 vailing winds at the time of Flight, especially in 

 autumn, influence to a great extent the wave of 

 migrants, deflecting it in certain directions, and 

 causing it to be of a broad and expansive character, 

 rendering the tide gentle and continuous, or com- 

 pressed and narrow, rendering it more throbbing 

 and in occasional rushes. 



We will now proceed to glance briefly at each 

 of these great classes of Migrants, beginning with 

 those in spring. Migration at this season begins 

 almost as early in the British Islands as in South 

 Africa, but of course the species afl^ected are very 

 different. The first decided migratory movement 

 is noticeable, say in February, when various birds 



