MIGRATION OF BIRDS. 629 



where facts are abundant we but imperfectly understand them. Let us 

 first state some of the outstanding facts. 



(1) Most birds seem to be more or less migratory, but the range 

 differs greatly. It is said that the dotterel may sup on the North 

 African steppe and breakfast next morning on the Arctic tundra, and 

 although the alleged rate may not be demonstrable, there is no doubt 

 that a distance of about 2000 miles is traversed by this bird and by 

 many others. Indeed, flights of 7-10,000 miles are said to occur. 

 In the Tropics, on the other hand, the migration may simply be from 

 valley to hillside. 



(2) Observers in temperate countries long ago noticed that the birds 

 they saw might be grouped in reference to their migrations. Thus (a) 

 some arrive in spring from the South, remain to breed, and leave for the 

 South in autumn, e.g.^ swallow and cuckoo in Britain ; {b) some arrive 

 in autumn chiefly from the North, stay throughout the winter, and fly 

 northwards again in spring, e.g., the fieldfare and the redwing in 

 Britain; (c) some — the " Birds of Passage " — are seen only for a short 

 time twice a year on their way to colder or warmer countries in spring 

 or autumn, e.g., sandpipers ; and [d) some seem to deserve the name ot 

 "residents," but really exhibit a partial migration, such as the song- 

 thrush and redbreast in Britain. In spring the tide is on the whole 

 northwards, in autumn southwards, but the paths are great curves, and 

 easterly and westerly waves pass from one country to another. The 

 migrants always breed in the colder countries included in their range. 



(3) There is striking regularity in the advent and departure of many 

 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 jDrecise 

 in leaving our shores. 



(4) It is beyond all doubt that many individual birds find their way 

 back to the same district, even to the same spot, where they had made 

 their nest in previous years. Not less marvellous is the security with 

 which the flight from country to country is continued in darkness, at 

 great heights, and over the trackless 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 ; it is exhibited by well-fed 

 caged birds ; migrating is an inherited habit. But it is likely that there 

 are always immediate causes which prompt the habit, such as scarcity of 

 food, and to a less degree, increasing cold in the case of many birds 

 which leave us in autumn. It is more diflicult to recognise the immed- 

 iate causes prompting their return. 



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 



