CAUSE AND ORIGIN OF MIGRATION 17 



a pair has been destroyed. The survivor of any 

 pair might have the home attachment and by bring- 

 ing a fresh mate create an attachment which 

 would be passed on from mate to mate indefinitely. 

 Again it must not be overlooked that certain sites 

 present advantages to particular species which 

 must be evident to all in search of those advant- 

 ages ; it by no means follows that the occupiers 

 of a nesting site are in any way related, except 

 specifically, to those which occupied it in previous 

 years. 



The answer to the argument that birds do not 

 seek fresh nesting places and thus extend their 

 distributional area, is evident when we consider 

 those species which, at the present time, are extend- 

 ing their range. Within the last few years, for 

 instance, the turtle dove and tufted duck have 

 begun to nest regularly in many parts of England 

 in which they were entirely unknown twenty or 

 thirty years ago. The starling has spread and in 

 some parts is spreading still, and many other similar 

 cases might be cited. 



In this manner migration, as we know it to-day, 

 may have originated, and as Mr P. A. Taverner ex- 

 pressed it, " however instinctive their habit may now 

 be, there must have been a time when migrations 

 were intelligent movements, intended to escape 

 some danger or secure some advantage' (51), 

 B 



