2 THE BIRDS OF THE BRITISH ISLES. 



Winter Visitors, 46 ; Birds of Passage, 30 ; Occasional Visitors* 

 61 ; Rare Visitors, 149 ; Extinct, i. The difference between 

 "occasional" and "rare" was taken as over or under 20 

 occurrences. The birds described in the following pages are 

 taken from this list, although geographical races or sub-species, 

 where trinomials are used, are treated under species headings, 

 and later additions are included. 



Grouping of birds according to time and frequency of appear- 

 ance is misleading ; many species cannot be placed in a single 

 group. Individuals of certain species remain with us all the 

 year round as permanent residents, though others of the same 

 kind are visitors in summer or winter only, and some migrating 

 through our islands come into the fourth group. Numbers of 

 species, like the Song-Thrush and Starling, are in the first four 

 classes, and even the Swallow, a typical summer visitor, is also a 

 bird of passage. 



The classification and nomenclature adopted is that of the 

 B.O.U. list, for though there are many differences of opinion 

 the majority of British ornithologists are agreed about these 

 difficult problems. In the list strict laws of priority have been 

 overruled in a few cases in order to prevent confusion ; for 

 example, the Turdus viusicus of Linnaeus in the loth edition of 

 the Systeina Natures is certainly the Redwing, but in the 12th 

 he gave this name to the Song-Thrush, and it has been so 

 generally accepted for the latter bird that the Committee decided 

 to retain it. 



Systematic "ringing" or marking birds has thrown much 

 light on the complicated problems of migration, but more is 

 needed. Geographically the British Isles are of service to 

 migratory birds ; they form the true home of some, the winter 

 home of others ; the resting place for long distance travellers 

 and a temporary refuge for lost wanderers or birds of passage 

 storm-driven or wind-drifted from their normal routes. Theories 

 as to height and speed at which birds travel have received 



