76 THE SCOTTISH NATURALIST 



Orkney Islands. 



A rare visitor. 



A rare visitor. 



Shetland. 



As will be seen from the above evidence the headquarters 

 of the Great Crested Grebe in Scotland are the faunal 

 areas of Clyde, Forth, and Tay ; north and south of this the 

 species becomes much more sporadic in its distribution. 

 Whence these centres were colonised it is difficult to say, but 

 bearing in mind the localities and dates given above, it would 

 appear to us to have been by immigration from the east. 

 This would naturally follow the valleys of the Tay and 

 Forth, the former leading to the lochs in the neighbourhood 

 of Dunkeld, the latter through Fife and Stirlingshire to the 

 Clyde breeding places. The fact that the lochs along the 

 sea-board were not the first to be colonised does not militate 

 against this theory ; in the case of the Tufted Duck similar 

 facts were observed, and Dr Harvie-Brown in the Annals of 

 Scottish Natural History, 1896, p. 6, writes : " It is significant 

 to find that the lochs of Wigtownshire, or those nearer to the 

 coast-line, are not the first to have become populated by 

 breeding birds. Are the individuals arrested by the 

 more favourable localities visible to them, first on their 

 autumn N.E. to S.W. migration, or does the occupation 

 follow only upon their second or spring observations? i.e., 

 are they arrested and induced to breed more by the 

 amenities of the place in spring than autumn ? We incline 

 to the former belief, and that they are ' brought up ' or 

 arrested in spring by the unsuitability of the areas beyond. 

 Once acclimatised, or once they have reared young, the wave 

 of dispersal, as it were, rolls back upon itself over the ' lines 

 of least resistance,' and localities formerly passed over 

 become occupied. This we consider is a well-founded belief, 

 built upon the facts we are able to bring forward not only 

 here but in other areas we shall treat of" A similar back- 

 wash would seem to have occurred in the Great Crested 

 Grebe. 



Another factor in the distribution of the species is the 



