96 ANIMAL ECOLOGY 



day experience, but the details of the changes, and the reasons 

 for them, are at present little understood. A study of seasonal 

 changes in the fauna leads us on directly to a number of 

 problems, one of the biggest of which is that of bird migration — 

 a vast subject which presents us with a number of smaller 

 problems which are essentially ecological. The arrival of 

 certain groups of birds in the spring, e.g. warblers, and their 

 departure again in autumn, are linked up with the summer 

 outburst of insect life, which in turn depends directly upon the 

 rise in temperature, and indirectly upon temperature and light 

 changes acting through plants. For instance, one of the 

 biggest key-industries in many animal communities on land is 

 that formed by aphids which suck the juices of plants. Many 

 small birds depend for their food either directly upon aphids, 

 or indirectly upon them through other animals. The aphids 

 which form this food-supply are only abundant during the 

 height of summer (June, July, and August), and thus their 

 seasonal occurrence has enormously important effects upon the 

 birds. So far the attention of ornithologists has been directed 

 to the accumulation of facts about the actual dates and routes 

 of migrations. This work has resulted in the setting of a 

 number of problems, and the asking of a number of questions. 

 A further advance which will throw light upon the ultimate 

 reasons for the migration-behaviour of birds must be sought 

 along ecological lines, and will only be attained by a careful 

 study of the relations of each species to the other animals and 

 plants amongst which they move in nature, and upon which 

 they are vitally dependent. 



21. Since the biological environment is constantly shifting 

 with the passage of the seasons, it follows that the food habits 

 of animals often change accordingly. In the case of many 

 higher animals, a different food is required for the young. 

 The adult red grouse feeds upon the shoots of heather, 

 but the young eat almost entirely insects and other small 

 animals. The food of adult animals also changes in a regular 

 way, especially when they are omnivorous or carnivorous. 

 Niedieck'^2 describes how the big brown bear of Kamskatka 

 varies his diet as the seasons pass. When he comes out in 



