129 



EXCEPTIONAL MIGRATION PHENOMENA. 



N the present chapter Herr Gatke endeavours to prove 

 that the occurrence in Heligoland or other localities 

 of individuals, of species, whose normal ranges do 

 not extend so far by hundreds, and in some cases 

 thousands of miles, is due not to accident, but to a 

 voluntary and conscious intention on the part of 

 these wanderers, and that this voluntary act is in its turn the 

 result of recurring causes, or in other words, is governed by 

 certain laws which have been previously overlooked. It will be 

 interesting, therefore, to follow him in his explanation of these 

 laws. 



In rejecting the formerly accepted opinion that the occurrence 

 of these casual visitors has no scientific value whatever, the 

 remarks : " This was the outcome of the traditional error, accord- 

 ing to which these strangers were, as a rule, simple and inex- 

 perienced young autumn birds which had either been driven out 

 of their normal course by storms, or were wandering about the 

 world at haphazard" (p. 114). The latter statement seems to 

 be stretching a point. Few naturalists ever imagined that any 

 bird, young or old, wanders about the world at haphazard. In 

 disproof of the statement that this straying from the customary 

 track is most frequent on the part of young birds, Herr Gatke 

 first brings forward the evidence of those species whose breeding 

 ranges lie some hundred of miles to the south east of Heligoland. 

 In alluding to the latter he calls attention to the fact that twelve 

 old against two young summer birds of the Black-headed Bunting 

 (Emberiza melanocephala) have been obtained, and in the case 

 of the Eastern Pied Chat (Sax. morio), the Black-eared Chat 

 (S. aurita), and the Desert Chat (S. deserti), two old examples of 

 9 



