HOW BIRDS MAKE THEMSELVES USEFUL 271 
Now, Wrens rear at least two broods a year, and the second 
will be here about 60 days and consume 15,000 insects. The 
hard-working parents will need at least the same rations 
during the five months they spend with us, and that will take 
15,000 more. Adding these we have 55,000 to the credit of 
a single pair of Wrens for the season. 
Chickadees and Cankerworms. — We are all familiar with 
the havoc wrought by cankerworms or measuring worms, for 
occasionally we have seen them strip our trees of their foliage. 
This would probably happen every year and our trees would 
be killed if it were not for the birds. The Chickadee will illus- 
trate this point, for it is fond of adult cankerworms. The 
female cankerworms pass the winter in the ground and in the 
spring crawl up the trunk of the tree and lay their eggs, which 
soon hatch out and become the common measuring worms. 
It is estimated that each Chickadee may eat 30 of the adults 
a day during the 25 days they crawl up the trees, and the 
average number of eggs laid by each female cankerworm is 
185. Thus one bird would destroy 138,750 prospective eggs 
of this noxious insect (30X25 185). 
The Meadowlark as an Example. — Let us now go to the 
prairies and fields and see what the birds are doing there. 
We will take the Meadowlark for our first illustration. The 
principal insects here are the grasshoppers, and the Meadow- 
lark will easily eat 50 of these a day. Now, if the number of 
birds breeding in one square mile is five pairs, and the number 
of young that reach maturity is only three for each pair, 
there will be 25 birds on one square mile, eating 1250 hoppers 
a day; and allowing them 120 days for the season’s campaign, 
this amounts to 150,000 insects. The amount of grass and 
green grain these would consume is computed in this way: 
the average weight of a grasshopper is 15.4 grains, and his 
