1 82 THE MIGRATION OF BRITISH BIRDS 



which particulars hav^e been given has this Emigration 

 been attended by Migration — the latter has helped the 

 former ; in fact it is doubtful whether a successful exten- 

 sion could have been established at all had the species re- 

 mained non-migratory, or had not already been addicted 

 to migratory habits, for the winter conditions would have 

 been fatal to the colonists, and the range gained in 

 summer would have been lost in the following winter. 

 The sedentary species must therefore have increased 

 their area more slowly, not extending their range north 

 into districts until the winter conditions were favour- 

 able. The Migration taking place within our islands is 

 precisely the same as that Migration which progresses 

 beyond them ; the flight south of a Song Thrush in 

 autumn from the north of Scotland to more southerly 

 areas in the same country is precisely the same move- 

 ment as the flight of a Knot from Grinnell Land to South 

 Africa ; the difference is only one of degree. One can 

 readily understand how individual birds and their off- 

 spring gradually extend their range year by year, spring 

 by spring resorting to a locality to breed, yet compelled 

 by the severity of the ensuing winter to return to their 

 more southern base — as the range expands northwards 

 the migration flight lengthens, until instead perhaps of 

 a few miles onh* the journey slowly becomes one of 

 many miles. The route followed is the line of extension 

 in which the breeding range has been increased, each 

 individual bird following a road back in autumn which 

 it traversed north in spring — a route which has been 

 slowly acquired b\- the species mile by mile, even field 

 by field, or wood by wood, and taught to the young that 

 journey south in company with the old birds in autumn, 



