38 THE IMPORTANCE OF BIRD LIFE 



references to the descent of hawks and owls upon 

 -the ravaging swarms of rodents and the annihila- 

 tion of them. 



Similar irruptions have taken place in the Uni- 

 ted States. In Humboldt Valley, Nevada, a most 

 notable one broke out in 1907 and lasted for nearly 

 a year. Hundreds of acres of alfalfa were ruined, 

 and a quarter of a million dollars' damage was 

 done. From eight to twelve thousand field-mice 

 an acre were estimated as gnawing at the roots. 

 Although at that time large numbers of hawks 

 and owls were being shot throughout the country 

 and the population of the various species had 

 greatly diminished, about two thousand managed 

 to gather in the infected region. They consumed 

 millions of mice. Finally, because there were not 

 enough birds to do the work, the farmers were 

 compelled to fall back on poisons to stamp out the 

 pests. 



This "plague" happened only fifteen years 

 ago, but even at that late date the farmers of 

 Humboldt Valley did not realize what the birds 

 of prey were accomplishing for them. Although 

 each bird devoured about 700 mice a month, a 

 number of hawks were shot in that very valley that 

 year. 



The Department of Agriculture at Washington 

 has studied for more than half a century our 

 native raptorial birds (hawks and owls) to deter- 

 mine which species are harmful to the work of 



