680 BIRDS. 



a year, as the bird changes a summer diet of grain and seeds for a 

 winter diet of fish, and vice versa. In the case of canaries, bullfinches, 

 parrots, etc., it has been noted that the food influences the colouring of 

 the plumage. 



Migration of birds. Migration remains in no small degree a 

 zoological mystery. On certain points we need more facts, and even 

 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. 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 of 

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

 thrush and redbreast in Britain. In Europe the spring migration is 

 on the whole northwards and north-eastwards, in autumn southwards 

 and south-eastwards, but the paths are great curves. 



3. There is a 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 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 

 country 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 



