50 THE PENTLAND SKERRIES. 



It will be seen from our list that already some birds which 

 were at one time quite scarce in the islands are now getting 

 commoner. A species may fly over a certain area for years on 

 migration, and, there being nothing to attract it either for food 

 or breeding purposes, it will naturally pass on without a halt, 

 unless driven to do so by stress of weather. Should, however, any 

 part of our area be so altered by art or nature as to become suit- 

 able for that species, during one of its involuntary visits the bird 

 might be more inclined to linger, and eventually, though not all 

 at once, become a breeding species. 



From the foregoing we think we may safely say in regard 

 to the distribution of birds during migration, as opposed to their 

 distribution at other times, that the former on their fixed and 

 well-known fly -lines indicate future breeding areas, if the places 

 become so altered at any time as to suit the species. We trust this 

 will explain to a certain extent the changes that are now going on 

 in the Orcadian avi-fauna, as regards the increase of some birds. 



Though the fact is now well known to most ornithologists, that 

 nearly all the birds in our islands are to a certain extent migratory, 

 this may possibly be news to many of our Orcadian friends. In 

 some instances this may be more easily seen in those islands than 

 in many other parts of Scotland. Take, for example, the Chaf- 

 finch, a bird that is seen in most parts of Scotland the whole year 

 round, and where, for that reason, its migratory habits might be 

 easily overlooked ; yet in the Orkneys, though breeding in a few 

 rare instances, in winter it is, in many places, almost a common bird. 



The autumn migration seems always vaster and denser to the 

 ordinary observer than the spring migration, and there are several 

 reasons which will readily account for this : 



First, For every pair of birds which pass north in spring, 

 which are not barren, or which do not have their eggs or young 

 destroyed, we may say from four to ten more come south in 

 autumn. 



Second, In autumn and winter the areas where food-supplies 

 are obtainable by species are more restricted, and so the birds, 

 especially those that are most noticeable during migration, their 



