402 THE BIOLOGY OF BIRDS 



Very careful observations have been made by Forbes on 

 American birds, large numbers of specimens having been 

 examined at different times of year. Two-thirds of the food 

 of the American robin consists of insects, about 29 per cent, 

 of garden fruits. Insects form 43 per cent, of the food of the 

 catbird, fruits 52 per cent. Seventy-eight per cent, of the 

 bluebird's food for the year consists of insects, 8 per cent, of 

 spiders, i per cent, of millipedes, 13 per cent, of vegetable 

 matter, including only i per cent, of fruit. Ninety per cent, 

 of the food of the Kingbird consists of insects. Many birds 

 are entirely insectivorous, and their usefulness in checking 

 the multiplication of insects has been proved up to the hilt. 

 This applies particularly to injurious insects, for they are apt 

 to multiply in proportion to the abundance of food afforded 

 by fields, plantations, and orchards (see Folsom, 1923, 

 p. 239). 



§ 2. Waves of Life 



Wild Nature is in a state of moving equilibrium, like that 

 of a top ! There is no static balance, but an ever-oscillating 

 adjustment. Usually, however, the oscillations in the 

 old-established arrangements of eating and being eaten 

 are within narrow limits, unless man interferes, or unless 

 some big change occurs in climate and other physical con- 

 ditions. When there is a succession of hard winters or 

 of fine summers, the ripples may become waves. Let us 

 take an instance from W. H. Hudson's " Naturalist in La 

 Plata." 



The summer of 1872-73 in La Plata was exceptionally 

 fine — rich in sunshine and showers, blossoms and bees, 

 and mice. In autumn the earth so teemed with mice that 

 one could scarcely walk anywhere without treading on them ; 

 while out of every hollow weed-stalk lying on the ground 

 dozens could be shaken. They were so abundant that the 

 dogs subsisted almost exclusively on them ; the fowls 

 also, from incessantly pursuing and killing them, became 

 quite rapacious in their manner ; while the sulphur tyrant- 



