290 THE MIGRATION OF BRITISH BIRDS 



in the past. A chapter on Island Avifaunas, their 

 origin, the conditions under which they are maintained, 

 and their relation to glacial conditions, bring the first 

 part of our subject to a close. 



Having satisfactorily accounted for the presence of 

 the British avifauna, we next proceed to a study of its 

 Migration or Season Flight. Routes of Passage engage 

 our attention first. We learn how gradually a Route of 

 Migration has been formed — how slowly a change of 

 climate might curtail it, or a return to more favourable 

 conditions assist in its expansion — how species never 

 extend their winter area, such expansion invariably being 

 the result of an increase of breeding population — birds 

 Emigrating and Migrating solely to breed ! Then the 

 various routes of migration to the British Islands are 

 divided into classes, each being dealt with and described 

 in turn. First we trace the paths of the Summer 

 Migrants to that area, and show how the most important 

 routes are in the closest vicinity to the continental land 

 masses, the weakest migration taking place in the most 

 westerly areas. The significant bearing of these facts 

 upon the distribution of species within the British 

 Archipelago is next discussed, and many anomalies of 

 dispersal are satisfactorily explained by law. Passing 

 on to the migration across the North and Irish Seas, 

 we endeavour to trace the correlation of routes with 

 submergence in those areas. Wc next deal with the 

 routes of species that winter in our islands or pass over 

 them as coasting migrants to other lands. A brief 

 account of the inland continuation of these routes, 

 showing how such were probably formed, brings that 

 portion of the subject to a close. 



