THE FOOD OF BIRDS. 
By S. A. FORBES. 
Excluding the inhabitants of the great seas, birds are 
the most abundant of the Vertebrata, occupying in this 
great subkingdom the some prominent position that in- 
sects do among invertebrate animals. These two classes 
thus constitute exceptions to the general rule that the 
higher and more active animals of each group are the less 
abundant — a fact doubtless largely due to the immense 
advantage given them by their power of flight. It is this 
which, by making migration possible, enables birds to 
choose their climates and their seasons — thus avoiding, 
in a great measure, one of the most destructive checks 
upon the multiplication of animals. Their dispropor- 
tionate number, their universal distribution, the remark- 
able locomotive power which enables them readily to 
escape unfavorable conditions, and their immense activ- 
ity and higher rate of life, requiring for their mainte- 
nance an amount of food relatively enormous, give to 
birds in their relation to the pursuits and interests of 
man a significance which only here and there one seems 
ever fully to have realized. A few figures will illustrate 
and enforce this proposition. 
The careful estimates of three ornithologists and ex- 
perienced collectors give, as an average of the whole 
bird-life of Illinois, three birds per acre during the six 
summer months. That is to say, if all the birds of the 
year, except the swimmers, were concentrated in these 
six months, equally distributed throughout them and 
equally scattered over the State, we should have three 
birds on every acre of land. It is my opinion that about 
two-thirds of the food of birds consists of insects, and 
that this insect food will average, at the lowest reason- 
able estimate, twenty insects or insects ’ eggs per day for 
each individual of these two-thirds, giving a total for the 
year of seven thousand two hundred per acre, or two 
hundred and fifty billions for the State — a number which, 
pfaced one to each square inch of surface, would cover an 
area of forty thousand acres. 
