CHAPTER VI 



MIGRATION 



Kinds of migration. Routes. The. height at which birds fly, Speed. 

 Lighthouses and their influence. Erratic migration. Causes of migration. 

 Meaning of migration. 



IN whatever part of the world the Ornithologist may live 

 there he will find birds : and further that the birds of any 

 given area may be divided into two more or less well-defined 

 classes — -the resident and the migratory. That is to say, that 

 the ranks of the resident population are constantly receiving 

 new-comers, either in the shape of stragglers which appear 

 suddenly and as suddenly vanish, or of large bodies of more or 

 fewer species whose appearance and disappearance is periodic 

 and apparently governed by the seasons. Of these larger 

 bodies of wanderers, some may stay but a short time, others 

 may remain for months. And thus there will inevitably arise 

 the question — What governs the movements of these migrants ? 

 To this at present no really satisfactory answer is forthcoming, 

 we must perforce remain content with speculation. 



For reasons more or less obvious, it will be best, in cori- 

 sidering this difficult subject, to confine our remarks, in the 

 main at any rate, to observations on this theme made on the 

 birds of the British Islands : they may safely be regarded as a 

 guide to what obtains in more remote regions of the world. 



First of all as to casual vagrants. The ever-increasing list 

 of birds new to the British list shows that these are fairly 

 numerous. With the largely increased army of keen observers 

 the number of species of birds new to Great Britain is not only 

 steadily increasing, but the number of records for each of these 

 " new species " seems to be increasing also. The spring and 

 autumn, it is to be noticed, are the seasons when these vagrants 

 most generally make their appearance, and it is further to be 



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