BIRDS AND THE WEB OF LIFE 403 



birds (Pitangus) and the Guira Cuckoos preyed on nothing 

 but mice. Carnivorous mammals also fared sumptuously 

 on the rodents. Countless numbers of storks and of 

 short-eared owls came to assist at the general feast, and they 

 became destructive to birds as well as to mice. Diets were 

 changed, habits were changed, numerical proportions 

 were changed, and then the tide turned. 



" By August (1873) the owls had vanished, and they had, 

 indeed, good cause for leaving. The winter had been one 

 of continued drought ; the dry grass and herbage of the 

 preceding year had been consumed by the cattle and wild 

 animals, or had turned to dust, and, with the disappearance 

 of their food and cover, the mice had ceased to be. In spring 

 it was hard to find a survivor, even in barns and houses. 

 It was pitiful to see the little burrowing owls ; for these 

 birds, not having the powerful wings and prescient instincts 

 of the vagrant Otus brachyotus, were compelled to face the 

 poverty from which the others escaped. They became 

 tame with hunger, and so reduced as scarcely to be able 

 to fly." 



This is a very instructive case, showing how wave 

 follows wave, how far-reaching the changes they effect 

 may be, and how the waves die away — though not without 

 leaving their mark. 



§ 3. Reproductive Inter-relations 



When one creature is essential to another for the con- 

 tinuance of its kind we may call this a reproductive inter- 

 relation. Thus the thrush is wrapped up with the continu- 

 ance of the mistletoe. The berry includes a layer of viscid 

 substance which hardens in air. It is used in making 

 bird-Hme. When the thrush eats the berries it rejects the 

 seeds, but these often stick to its bill, and have to be rubbed 

 off against a branch. " The lime sets and the seed is fixed 

 till it germinates, often months later, when in May the 

 temperature is sufliiciently high. Professor Keeble tells 

 us of a certain telegraph wire in Ceylon where, yearly, 



