MAPPING MIGRATORY BIRDS. 105 



in some chalk-pit, with a lot of bushes, there must be a 

 pair of Shrikes, or that in some park with a number of 

 old trees, there must be Nuthatches. But this obviously 

 does not mean that the same Nuthatches and Shrikes 

 live there perpetually. Not long ago, for instance, all 

 Nuthatches vanished from the Backs at Cambridge. 

 The place is so ideal for them that I was sure some would 

 soon turn up again there ; and within a year two new 

 pairs had arrived. Now all this makes it seem extremely 

 probable that it is not always the same pair of a migratory 

 species that comes to a place, even when there has been 

 no gap of a year. It is, in fact, clearly impossible to 

 determine the longevity of birds by noting how many 

 return year after year to the same spot. 



