Part II.] NATURAL ENEMIES OF BIRDS. 235 



their multiplication, hardly a bird's nest in the woods would 

 escape them, from the ground to the treetops. Deer mice are 

 so small and light that they can climb to any nest where even 

 a squirrel might fail, and they are quite as fond of young birds 

 as are the squirrels. Field mice of various species have from 

 four to six litters of young each year, with from two to thirteen 

 young in each litter, therefore the estimate by Professor Lantz 

 that a single pair of our common meadow mice is potentially 

 capable of producing nearly a million young in five years is not 

 excessive.^ 



Knowing the capacity of a single field mouse to be from 

 twenty-four to thirty-six pounds of green vegetation annually, 

 Professor Lantz calculates that a thousand field mice (which 

 might ordinarily inhabit a meadow) would require at least 

 twelve tons of grass or its equivalent each year. A million 

 would require twelve thousand tons annually. History shows 

 that under favorable conditions countless numbers are produced 

 in a few years, and that when such invasions occur they destroy 

 or ruin grass, clover or alfalfa, hay in stacks, all small grain 

 growing or in shocks or stacks, garden and hotbed plants, po- 

 tatoes, beets, turnips, carrots, cabbage, celery and other vege- 

 tables, apples, pears and other standard fruits, small fruits 

 and the plants that bear them, orchard trees and shrubbery, 

 nursery stock, young forest trees, and nearly all kinds of bulbs, 

 tubers and roots. These mice ruin lawns and pastures for the 

 time being, and become at times in the old world the most 

 important of all pests. 



The great swarms of lemmings that have appeared from time 

 to time on the Scandinavian peninsula, and the destruction 

 brought about by their numbers, are historic. In my "Useful 

 Birds and their Protection," pages 76 to 78, I have noted sim- 

 ilar occurrences in Scotland and England, and the effectiveness 

 of owls and other natural enemies of birds in destroying the 

 pests. Figuier says that in France "Whole districts have been 

 reduced to destitution by this scourge," and that the Depart- 

 ment of La Vendee experienced a loss in two years estimated 

 at £120,000 (nearly $600,000), caused by these creatures.^ 



I Lantz, David E.: U. S. Dept. of Agr., Biol. Surv., Bull. No. 31, An Economic Study of Field 

 Mice, p. 12. 



' Figuier, Guillaume Louis: Mammalia, Their Various Orders and Habits, popularly illus- 

 trated by typical species, 1870, p. 445. 



