304 YEARBOOK OF THE DEPARTMENT OF AGRICULTURE. 



With the disappearance of green herbage in fall, reducing their 

 food to roots and bark, mice move more rapidly from exhausted to 

 fresh lands, and devastate larger areas. It is, then, through the prog- 

 ress of large bodies of mice, which may number thousands to the 

 acre, that large districts are laid waste. 



SUBSIDENCE OF PLAGUES. 



After reaching a final autumnal climax and continuing through the 

 winter in gradually lessening numbers, mouse plagues have usually 

 abated early in the following spring, or at most have endured only 

 through the ensuing summer. Like their development, the subsiding 

 of such hosts is so gradual in the early stages as to be scarcely per- 

 ceptible, though apparent enough a little later. 



Most noticeable among the agencies which finally overcome them 

 are predaceous birds and mammals. Attracted in large numbers 

 to the feast, they live almost exclusively on mice during these pe- 

 riods, and, particularly in winter, make such severe inroads on the 

 mice as to attract general attention. Still it is doubtful if, unassisted, 

 they have ever overcome a plague. A conservative estimate places 

 the number of predaceous birds which appeared in the stricken dis- 

 trict in Humboldt Valley at 2,000 ; the predatory mammals at 1,000. 

 It may be assumed that these 3,000 natural enemies would each 

 destroy an average of 15 mice per day, or 450 per month, or collec- 

 tively would kill 45,000 mice a day, or 1,350,000 per month. This 

 number, vast as it is, is far too small to put an end to a well-estab- 

 lished plague, although more than ample to check a plague during 

 its early stages, or to completely wipe it out after the numbers have 

 been materially reduced by poisons or other agencies. 



In most of the accounts of mouse plagues the final destruction of 

 the rodents has been ascribed to disease, and it is believed that the 

 abatement of the plague in Humboldt Valley was aided by natural 

 mortality. At intervals from January to March dead and dying 

 mice were noticed in locations where poisoning could not have been 

 the cause, but efforts to prove this mortality due to some specific 

 bacterial disease failed. 



In the spring the mice in this locality failed to reproduce, while 

 the same sj)ecies was breeding prolifically in other localities. In 

 March several hundred females were examined in Humboldt Valle}^, 

 of which very few were pregnant. Moreover, the mice themselves 

 presented a different appearance from those seen when the plague 

 was at its height — a fact noted by many ranchmen in the valley. 

 During the fall of 1907 larger and much more vigorous individuals 

 predominated, while in the spring of 1908 scarcely any of these re- 

 mained. They continued in destructive numbers until the middle 



