25O ECONOMIC MAMMALOGY 



outbreak. The numerous descendants of the thousands of mice thus 

 saved by the destruction of their enemies, running through the suc- 

 ceeding generations and multiplying rapidly, must have contributed 

 largely to the plague, if they were not the chief element in it, though 

 there are other probable factors, such as weather, food and shelter, to 

 be considered. 



The question as to whether the natural enemies of rodents can pre- 

 vent such plagues has been much discussed. 14 Among the more ac- 

 tive enemies may be mentioned hawks, owls, gulls, ravens, crows, 

 shrikes, herons, coyotes, wolves, foxes, skunks, weasels, badgers and 

 wildcats. 15 The number of rodents taken by all these and other birds, 

 mammals and snakes each year is simply enormous. In Ohio 20,000 

 hawks were killed under a bounty act in 1916, and in a year or so it 

 was said that ''already the unfavorable effects of this law are shown in 

 the large number of complaints of damage done by rats and mice in 

 Ohio and adjoining states." 16 If natural enemies cannot entirely pre- 

 vent the occasional more serious outbreaks, they certainly do so much 

 toward keeping the pests in check during ordinary times as to perhaps 

 justify the assertion that were it not for the predatory birds and mam- 

 mals there would be an almost continuous plague of rodents. When 

 the rodents become abundant in a given locality, they at once enter 

 more largely into the diet of the various predators, both birds and 

 mammals, which tend to congregate in areas of mouse abundance, as 

 attested by many observers. Thus a direct relation between mouse 

 abundance and the value of the fur catch has been asserted. 17 Also, 

 it has been noted that when mice are abundant in Labrador they fur- 

 nish abundant food for wolves and other beasts of prey, which then 

 leave ptarmigan, caribou and other game alone. 18 



Most writers upon the subject of the injurious rodents of the western 

 plains in the United States have agreed that since the settlement of 

 the region began the rodents have increased in numbers, largely on 

 account of the wholesale destruction of predatory mammals and birds. 

 Couch, in view of the fact that while coyotes were being protected as 

 enemies of jack rabbits, and were rapidly increasing in Grant County, 



14 See, for example: Brooks, Can hawks prevent mouse plagues?, The Condor, 

 xxix, 249-250, 1927. Tyler, Can hawks prevent mouse plagues? A reply, The Con- 

 dor, xxx, 124-125, 1928. Henderson, The practical value of birds, 1927, many refer- 

 ences cited in the footnotes. 



15 Piper, Farmers' Bull, No. 352, p. 21, 1907. 



10 Lantz, The house rat: The most destructive animal in the world, Yearbook 

 U. S. Dept. Agric. for 79/7, pp. 235-251. 



17 Innis, The fur trade of Canada, pp. 88-91, 1927. 



18 Cabot, Labrador, 1921. Journ. Mammalogy, in, 58-59, 1922. 



