CHAPTER XXVII 
HOW BIRDS MAKE THEMSELVES USEFUL 
Seed-eaters and Insect-eaters. — Birds are roughly divided 
into two classes: seed-eaters and insect-eaters. But it must 
not be understood that the line of division is so sharp that 
birds belonging to one class never touch the food of the other 
class. Many birds that are essentially seed-eaters crave ani- 
mal food for at least a small portion of their rations. Our 
domestic chickens illustrate this. .Other birds that are quite 
strict vegetarians themselves will feed insects to their young. 
On the other hand, some birds insist upon an exclusively 
insect diet, and when the supply is exhausted there is nothing 
for them to do but migrate. But many others adapt them- 
selves to circumstances. Take, for example, the Robin. It 
is said that insects constitute 70 per cent of his diet the year 
round; and yet in the fall, when insects have become very 
scarce, he lingers with us many weeks longer, subsisting largely 
upon seeds and berries. 
What Seeds are Eaten. — Most of the food that the seed- 
eaters find is the seeds of noxious weeds. In helping thus to 
check the multiplication of these pests the birds obviously 
do a great deal of good. But the service rendered in the 
destruction of insects, gophers, mice, etc. is more striking and 
it will be the chief subject of our inquiry here. 
Quantity Eaten, and How We Know. — As a bird cannot be 
followed in its flight to see what it eats, we must get at the 
question indirectly. One way is to experiment upon birds 
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