428 The Outline of Science 



Migrations of Lapwings 



Many birds, such as lapwings, or peewits, marked in Scotland 

 as chicks in summer have been recovered in winter from Ireland. 

 Other Scottish lapwings have wandered further and have been 

 recorded from the west coast of France or from Portugal. In a 

 few cases, too, lapwings were reported during the winter months 

 from their native districts. It is therefore evident that even within 

 a single species, in a single area, there may be resident and migra- 

 tory individuals, and, among the migrants, some which go much 

 further afield than others. 



There is no doubt that many birds on their return make for 

 their birthplace. A swallow marked when it was a young one has 

 been found thus to return to its native farmyard. Birds would 

 appear to return in spring impelled by a greater urgency than in 

 the autumn migration, when we see sometimes a good deal of 

 dallying. Some birds are known to make trial trips and begin 

 their journey with short stages. On the return, some authorities 

 believe there is evidence that the spring journey is more direct, 

 that short cuts are found, and that haste is evident. When weather 

 conditions are very bad there is often great loss of life. "The 

 streets of towns are sometimes strewn with thousands of birds that 

 have gone astray and have perished in the cold. As many as five 

 hundred nightingales have been gathered in a single day from one 

 small town." 



The Purpose Served by Migration 



Migration must serve some good purpose and be of advan- 

 tage to the species which possess the habit. It is, indeed, an ex- 

 pensive habit, involving the perpetuation of a complex instinct, the 

 output of a large amount of energy, and the facing of great risks 

 and a heavy mortality: these factors would surely be enough to 

 wipe out a species in the keen struggle for existence did not some 

 great compensating advantage also accrue. For the departure of 



