MIGRATION OF BIRDS 763 



of the migrants. In spite of the immense distances which many of our 

 immigrants travel, and in spite of unpropitious weather, they are often 

 punctual within a day or two to their average time of arrival for many 

 years. Similarly some birds, such as the swifts, are hardly less precise 

 in leaving our shores. 



4. It has been proved in a few cases that individual birds may find 

 their way back to where they made their nest in previous years. Not 

 less marvellous is the security with which the flight from country to 

 countr}' is continued in darkness, at great heights, and over the track- 

 less sea. At the same time it must be noticed that the mortality 

 during migration is very great. 



Having stated a few of the outstanding facts, let us note some of the 

 interpretations and suggestions which help us to understand them. 



The impulse to migrate is instinctive ; but it is likely that there are 

 always immediate causes which prompt the instinct, such as scarcity 

 of food, the shortening daylight, and the increasing cold in the case of 

 many birds which leave us in autumn. It is more difficult to recognise 

 the immediate causes prompting their return. In leaving Britain the 

 young birds usually fly first ; in returning, the sexual adults lead the 

 way. 



It seems likely that the origin of the migrating habit is wrapped up 

 with the history of climates, and we can understand how the setting in 

 of glacial conditions from the north would gradually force birds, century 

 by century, to a longer flight southwards. And if the climatic condi- 

 tions limit the area of safe and comfortable breeding to one country (the 

 more northerly), and the possibility of food during winter to another 

 country (the more southerly), we can understand, with Wallace, " that 

 those birds which do not leave the breeding area at the proper season 

 will suffer, and ultimately become extinct ; which will also be the fate 

 of those which do not leave the feeding area at the proper time." In 

 short, given environmental changes of climate on the one hand, and a 

 measiu"e of plasticity and initiative on the part of the organism, the 

 instinct of migrating would be perfected in the course of natural 

 elimination. 



But while this view is so far satisfactory, it leaves us face to face with 

 the problem how birds migrate as safely and siirely as they do on their 

 pathless way. For to point out that the merciless elimination which 

 continually goes on keeps up the standard of racial fitness, leaves us 

 still wondering how any became fit at all. 



Birds learn or have learned to find their way. The way-finding has 

 been compared to the " homing " of carrier pigeons, but it is likely 

 that these carefully trained and also selected birds are guided largely 

 by noticing landmarks, which could hardly be done over 10,000 miles 

 of land, and obviously not over 1000 miles of sea, or during the night. 

 Some have urged that birds follow river valleys, the lines of old " land 

 bridges " connecting continents, the roll of the waves, and so forth, 

 but the difficulty remains of flight by night and at very great heights. 

 Attractive is the suggestion that birds are guided by what may be 

 called a " tradition " based on experience ; those guide well one year 

 who have followed well in previous years. But many young birds 

 fly apart from their parents, and some birds do not fly in flocks at all» 



