VII. EXCEPTIONAL MIGRATION 

 PHENOMENA 



The occurrence within a particular district of unusual, or more or 

 less rare, visitors, belonging to a totally distinct avifauna, and gener- 

 ally known under the name of ' casual visitants,' is a fact which 

 has hitherto not received a due amount of attention, owing to our 

 having remained unacquainted with the laws by which these pheno- 

 mena, like others connected with migration, are unquestionably 

 regulated. 



In regard to this question, Heligoland provides material of 

 paramount importance, both bj^ reason of the unexampled number 

 of such interesting strangers from all countries of the northern 

 hemisphere occurring on the island, and also because of the fact of 

 their appearing in such quantities within so limited an area, giving 

 proof of the incomj^arably larger numbers of these rare birds which 

 must annually pass through the whole of the European continent. 



It is remarkable how much our views on such exceptional 

 phenomena have changed from those which prevailed some twenty 

 years ago. At that time, the report of each new stranger shot here 

 used to arouse the most joyful sensation among ornithologists; 

 whereas, subsequently, the views were expressed from manj- sides 

 that ' casual visitants ' of this kind had no scientific significance 

 whatever. 



Such a change in the manner of looking at this question can, 

 however, only be ascribed to the insuthcient amount of attention 

 devoted to the subject. This neglect was itself undoubtedly the 

 outcome of the traditional error, according to which these strangers 

 were, as a rule, simple and inexperienced young autumn birds 

 which had either been driven out of their normal course by storms, 

 or were wandering about the world at haphazard ; and individuals, 

 it was held, which had thus been blown about casually by the wind 

 to one place or another were, of course, not deserving of any further 

 attention. 



With this view my own experience of many years is most 



