680 BIRDS. 
a year, as the bird changes a summer diet of grain and seeds for a 
winter diet of fish, and wzce 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; (4) 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, ¢.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 Jeaving 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 
