446 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



THE VALUE OF BIRDS IN FORESTS. 



Omitting all mention of many another notable instance of the 

 quelling of insect outbreaks by birds, I will pass at once to the con- 

 sideration of those perennial services which act as a constant check 

 on the undue increase of insects, rodents, weeds, and other pests. 



Birds attain their greatest usefulness in the forests, because the 

 conditions there closely approach the primeval. 



Forest trees have their natural insects foes, to which they give 

 food and shelter, and these insects in turn have their natural enemies 

 among the birds, to which the tree also gives food and shelter. 

 Hence it follows that the existence of each one of these forms of life 

 is dependent upon the existence of the others. But for the trees the 

 insects would perish, and but for the insects the birds would perish, 

 and but for the birds the trees would perish; and, to follow the 

 inexorable laws of nature to the conclusion of their awful vengeance, 

 but for the trees the world would perish. 



Consider for a moment the life of a tree in connection with the 

 insects that prey upon it. At the very beginning, before the seed or 

 nut has germinated, it may be entered by a grub which destroys it. 

 Should, however, the seed or nut be permitted to grow, the roots of 

 the seedling may be attacked by bettles. Escaping this danger, a 

 worm lays its eggs in the cracks of the bark. On hatching, the worm 

 or borer perforates a hole in the stem. This hole, admitting water 

 from every passing shower, causes a decay in the wood to commence, 

 from which the tree may never recover. Other borers feed upon the 

 bark, eating the soft inner layer and the sap. The twigs are affected 

 by the larvae of certain bettles, which act as girdlers, sometimes de- 

 stroying limbs over an inch in diameter. Weevils bore under the 

 bark and into the pith, making excavations in which the eggs are 

 laid. For the same purpose the cicada makes a terrible wound, 

 which often proves fatal. The limbs of trees are affected by aphides, 

 which puncture them and feed upon their juices, exhausting the sap. 

 Many species of plant lice and scale insects infest trees, doing great 

 damage, while over 100 different species of gall flies are parasitic 

 upon them. The buds of trees are entered and destroyed by the larvae 

 of certain moths, while the leaves are devoured by caterpillars. To 

 take the oak as an example, it is known that altogether over 500 

 species of insects prey upon it. Finally, be it remembered that in the 

 bark and in the underlying tissues lie the vital energies of a tree. 



It is difficult to perceive the usefulness of these insects which feed 

 on the different parts of the tree, though they may, perhaps, when in 

 normal numbers, exert a useful influence by a healthful and neces- 

 sary pruning. It is certain, however, that if they were not in turn 



