88 THE MIGRATION OF BIRDS 



equal. Mr Clarke understands his subject. It 

 appears, however, to me that he may put rather 

 too much weight upon the barometric influence, 

 and too little on one side of the wind question. 

 Are we yet in a position to say that birds do not 

 make direct use of certain winds ? It may be that 

 the use of the prevailing winds at migration time 

 is far more unconsciously intentional (if such an 

 expression can be used) than is at first apparent. 



One or two points must be kept well to the front 

 which are often ignored by observers. Firstly, 

 very much visible migration is abnormal ; that is 

 to say, most of the incidents of passage which are 

 noticeable, especially observations at the lightships 

 and lighthouses, are during spells of weather which 

 are described as unfavourable ; it is the " hold-ups/' 

 checks, and " rushes/' which attract attention far 

 more than the even-flowing normal migration. 



Mr J. Tomison, in his valuable notes on observa- 

 tions made at Skerry vore (52), shows that in ordinary 

 clear weather birds pass at a great height, beyond 

 the power of vision. He proves this by instances 

 of the diurnal passage of redwings, birds which 

 are generally supposed to migrate at night, and 

 undoubtedly do so frequently. He heard the 

 well-known passage-note in the daytime, but with 

 the naked eye could see no redwings ; he found 

 them with the telescope and later discovered others 



