124 ANIMAL ECOLOGY 



reproduction of the carnivore may be always adjusted to such 

 a low rate that there is hardly ever any danger of over- eating 

 its food supply, and its numbers always remain relatively 

 small. This is not so common, however, as other methods. 



34. The regulation of numbers of terminal animals is seen 

 at its best in some of the birds and mammals which are either 

 at or near the end of food- chains, e.g. hawks and tigers on the 

 one hand, or warblers and insectivorous animals on the other. 

 The fact that animals become less abundant as we pass from 

 key-industry herbivores to the carnivores at the end of the 

 chain makes it possible for animals at a certain point in the 

 series (at a certain height in the number-pyramid) to limit their 

 numbers by dividing up their country (and therefore the 

 available food-supply) into territories each owned by one or a 

 few individuals. For instance, in the EngHsh Lake District 

 each buzzard or pair of buzzards requires a certain stretch 

 of country to supply it with enough food, and the same 

 applies to a bird like the Dartford warbler, which lives in the 

 heather and furze heaths of Southern England. The division 

 of country into territories is especially common at the beginning 

 of the breeding season, when it is necessary not only to provide 

 for the immediate needs of the animals, but also for the needs 

 of the young which will appear later on. 



35. We owe our knowledge of the existence and nature of 

 bird territory chiefly to Eliot Howard,^ whose remarkable 

 studies on the subject, especially among the warblers, have 

 opened up an entirely new field of ecological work. He showed 

 that among birds like the willow wren (which migrates north- 

 wards into England every spring) there is a very regular system 

 of dividing up their habitat into parcels of land of roughly 

 equal value. The arrangement is that the male birds arrive 

 first in the early spring, before the females, and fight amongst 

 themselves for territory ; and are then followed by the females, 

 each one of which becomes attached to one male. Ultimately 

 the nest is built and young produced and reared. At the end 

 of the season the territories are given up and the birds go south 

 again. We are not concerned here with the ways in which 

 this territorial system in warblers and other birds is connected 



