4 THE MIGRATION OF BIRDS 



with striking differences in the performances of 

 individuals of the same species. Take one instance, 

 a song thrush reared in a nest in our own garden. 

 We may see and recognise this bird up to the middle 

 of July, but what trained ornithologist can, yet, 

 say with certainty where that bird will be by the 

 end of the month or in three to four months time ? 

 We know that all through the winter there are some 

 song thrushes near the house, and that they are 

 the birds which not only begin to sing early but 

 actually nest with us ; we know too that before 

 there is any marked immigration of northern 

 thrushes there is a recorded emigration from our 

 southern coasts, presumably of thrushes which 

 have nested with us, beginning towards the end 

 of July ; further we know that there is an autumn 

 immigration of Scandinavian or other northern 

 song thrushes, sub-specifically distinct to the expert 

 eye, and some, small and dark, whose origin is by 

 no means proved, as well as later emigrations of 

 birds to the Continent or Ireland, both regular 

 and occasioned by exceptional weather. Will our 

 young July thrush remain in England or will it 

 join one of these streams, and if so which ? We 

 do not know yet. I repeat " yet," for the study 

 of races, sub-species or local variations is command- 

 ing more and more attention ; the patient work 

 of the " splitters," scorned by the old school of 



