442 THE GROUSE IN HEALTH AND IN DISEASE 



all the time. If there is a high point on the moor, rocky and 

 precipitous, it is in extent probably a mere fraction compared 

 with the acreage of good moorland around it. On a Scotch 

 moor you have usually a large acreage above the line of your 

 highest driving ground." l It follows that in Scotland the 

 Grouse is forced to leave the high ground in time of snow to 

 seek his food at a lower elevation, and the same motive will 

 cause him to return again in the spring to the fresh young 

 heather on the " tops," whereas in Yorkshire, where the 

 climatic conditions are much the same on every moor, he 

 would gain little by such migration. 



Whether migration actually results in a crossing of blood 

 has been often debated. Some naturalists contend that the 

 migrating packs do not interbreed to any extent with the 

 birds upon the moor where they have sojourned for the winter, 

 but that they return to their own ground with their ranks 

 unbroken. The evidence available does not altogether support 

 this view, and indeed it is doubtful whether it is always the 

 same birds which departed in the winter that reappear again 

 in the spring. It is difficult to obtain conclusive proof on the 

 subject, but one or two facts are suggestive. In the first 

 place it often happens that on ground where there has been 

 a light stock in the autumn, there is sometimes found to be a 

 heavy stock in the following spring, thus pointing to immigration. 

 This circumstance is usually associated with a moor on which 

 the feeding is good. Conversely a heavy stock may migrate 

 wholesale in the autumn, and only a few birds may return in 

 the spring. The reduction of their numbers may, it is true, 

 be due to disease, but is equally likely that the absentees 

 have become naturalised elsewhere. 



But the most striking evidence on the subject is furnished 

 by the manner in which a moor, which has been entirely denuded 

 of birds, will recover its proper stock in such a remarkably 

 short time that the only possible solution is the immigration 

 of birds from elsewhere. A good example is furnished by 



1 Fur and Feather Series, " The Grouse,'' pp. 152-153. 



