28 THE MIGRATION OF BIRDS 



with the life of the species. That this impulse has 

 not always sufficient strength to force them to per- 

 form the whole journey is apparent from the fact 

 that many non-breeders, young or sexually mature, 

 on their northward journey through our islands or 

 along our coasts, never reach the breeding area ; 

 the food supply on the way attracts them more than 

 the memory of home ; they linger with us until the 

 breeding season is over and the return journey has 

 begun. Knots, sanderlings, turnstones and many 

 other waders may be seen on passage late in June, 

 and some remain on our mud-flats throughout the 

 summer ; in July the tide of migration has turned. 



It has been suggested that some of the sexually 

 mature non-breeders may be actually enjoying their 

 winter during our summer ; in other words that they 

 have bred in southern breeding-stations whilst their 

 congeners wintered in the same zone. This means 

 a double breeding - area for certain species a 

 possible explanation, but one hardly supported by 

 known facts. When a bird had so cosmopolitan a 

 range that in the course of its dispersal its breeding 

 areas were separated, we almost invariably find that 

 the birds inhabiting these two areas are distinguish- 

 able geographical forms or sub-species. Mr W. H. 

 Hudson, in his " Naturalist in La Plata/' refers to 

 the godwit, Liwosa haemasiica, which spends the 

 southern summer in La Plata and breeds in the 



