114 A HISTORY OF BIRDS 



Incidentally this remarkable case serves also to illustrate the 

 fact that the numerical level of a species is dependent largely 

 upon the supply of food. And that this is so we may point out, 

 on the authority of Mr. W. H. Hudson, that the Caracaras 

 or Carrion Hawks of South America have largely increased in 

 numbers since the introduction of large herds of cattle into the 

 plains of this vast cattle-raising region. They find abundant 

 food in the offal from the carcasses of the cattle slain for 

 European markets. 



By way of further illustration of the influence of birds on 

 their animal environment, take such a case as that cited by 

 Darwin, of the absence of feral cattle, horses and dogs in Para- 

 guay owing to the abundance of a certain fly which lays its eggs 

 in the navels of these animals as soon as they are born and so 

 destroys them. Darwin contended that these flies must be held 

 in check by insectivorous birds, and that these were in turn 

 kept down by Hawks ; but for which, he argues, the insectiv- 

 orous birds might increase sufficiently to hold the pestiferous 

 flies in check, and so render existence possible for these per- 

 secuted mammals. It may well be, indeed, that the navel- 

 breeding insects owe their amazing abundance to this chain 

 of circumstances. But, be this as it may, the main point — the 

 inability for these mammals to maintain a hold on life in this 

 region — remains the same, and this materially afiects, as he 

 shows, the whole face of the country in so far as its vegetation 

 is concerned. 



A similar case affecting sheep in New Zealand is furnished 

 by the Kea Parrot, if all accounts are true. This bird, originally 

 vegetivorous, has taken to a carnivorous diet, feeding on the 

 backs of the huge flocks of sheep kept by the colonists. The 

 Parrot is said to tear away the wool, and burrow down through 

 the flesh for the sake of devouring the kidneys, though this 

 part of the story savours of the ridiculous, since the bird would 

 certainly never succeed in obtaining these morsels. Neverthe- 

 less, if, as the flock-owners avow, these birds actually devour 

 any part of the flesh of the back, death would certainly ensue. 

 And after such fashion birds may well have played an import- 

 ant part in the past in determining or controlling the mam- 

 malian life in different parts of the world. 



Before man took it upon himself to regulate the balance of 



