72 BIRD FRIENDS 



kind of insect, and this might prove undesirable 

 even if possible, but they help to keep them in check 

 to such an extent that man is able to cope with those 

 that are left and thus raise his crops successfully. 

 If man were suddenly deprived of the services of the 

 birds, this would allow insects to increase to such an 

 extent that it would render it many times more 

 difficult to raise crops; and some bird-students 

 say that without the aid of birds it would not be 

 possible to raise crops at all on account of the enor- 

 mous number of insects that would prey upon 

 them. 



How serious the results would be if we were de- 

 prived of the service of the birds, it is difficult to say 

 exactly, but enough is known regarding the balance 

 that Nature has established between birds and in- 

 sects, so that we are sure that the birds are among 

 man's greatest friends in his warfare on injurious 

 insects. 



Amount of food eaten by birds. One thing about 

 birds that makes them such effective checks on in- 

 sects is the enormous amount of food they require. 

 The temperature of their blood is between 102 and 

 112 degrees, from 4 to 14 degrees higher than that in 

 man, and large amounts of food must be eaten to 

 maintain this high temperature. The blood courses 

 through the vessels with great rapidity, driven by 

 the heart, which beats one hundred and twenty 

 times a minute when the bird is at rest, and faster 



