Page:The Migration of Birds - Thomas A Coward - 1912.pdf/44

28 with the life of the species. That this impulse has not always sufficient strength to force them to perform 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 distinguishable geographical forms or sub-species. Mr W. H. Hudson, in his "Naturalist in La Plata," refers to the godwit, Limosa haemastica, which spends the southern summer in La Plate and breeds in the