BIRDS AND THE WEB OF LIFE 411 



have found abundant food, convenient and safe nesting 

 places, practically no natural enemies, and unrivalled means 

 of dispersal. Aside from an early and brief period of foster- 

 ing care, they have been left to shift for themselves ; natural 

 agencies have since been at work, and in the relatively 

 short space of forty years a continent has been, not merely 

 invaded, but inundated by an animal which, in its native 

 habitat, has been fairly subservient to the regulations 

 imposed by competing life." 



Biologically regarded, this is an interesting instance of 

 the practical suspension of the sifting operations of Natural 

 Selection, but also of the results of rapidly altering the 

 moving equilibrium of Nature. The sparrows multiplied 

 exceedingly. In some measure, it is true, they checked 

 the elm-tree caterpillars, but only to become themselves 

 a greater pest. They did great damage to the crops, 

 including buds, they drove away native insectivorous birds, 

 and they are blamed for spreading among poultry certain 

 diseases, such as " blackhead," due to parasitic Coccidia. 

 They often became a nuisance about houses. What has 

 happened in the past goes on to-day, for no successful 

 method of checking or exterminating sparrows has been 

 discovered. In his " Economic Zoology " (Philadelphia, 

 1 9 19), Professor A. M. Reese notes that, according to some 

 computations, 98 per cent, of the sparrow's food is vegetable 

 and 74 per cent, of this is grain of some sort. "It is of 

 some sHght value about towns as a scavenger and possibly 

 does some good as a destroyer of insects and weed seeds, 

 but the total balance seems to be strongly against it." 



Apart from domesticated forms, man has introduced 

 into Britain the pheasant, the red-legged partridge, the 

 capercailzie, and a number of other birds. 



Man's hand is often heavier indirectly than directly. 

 The destruction of the Scottish forests has changed the 

 whole face of the country, and deprived many birds of their 

 old homes ; drainage has driven marsh birds away ; and 

 the extension of agricultural ground has meant a great 

 reduction of the wild places that afford shelter to nests. 



