8 UNIVERSITY OF COLORADO. 



to attract many species of birds which are looking for a plentiful food sup- 

 ply, this acting as a check upon the insects. The setting out of large orchards 

 affords an immediate opportunity for a great increase in insect pests, by 

 furnishing them both food and breeding places, while the birds which feed 

 largely upon the insects increase very much more slowly. Thus the natural 

 adjustment of species is disturbed, and it then may require many years to 

 restore the equilibrium. 



The true function of insectivorous birds is not so much to destroy this or that 

 insect pest as it is to lessen the numbers of the insect tribe as a whole- to reduce to 

 a lower level the great flood tide of insect life. 1 hat this is the true relation of birds 

 and insects would be inferred from the fact that the two have lived together for count- 

 ies:: ages, and tho balance: oi nature has; been preserved except as disturbed by the 

 operations of man. Birds have not wholly destroyed predaceous and parasitic insects 

 on the one hand, nor on the other have they, so far as we know, exterminated any 

 vegetable-eating pest, but they have successfully held the balance between the two, 

 and kept both at such a level of relative abundance as has subserved the best interests 

 oi both thu animal and tho vegetable world ; and it is only where man has interfered 

 with this balance that oscillations have taken place which have resulted in damage to 

 him and to tho products of his labor (7). 



Because of the fact that some species of insects and other pests which 

 are normally kept within reasonable bounds, may, during especially favorable 

 seasons or cycles of seasons increase so rapidly as to become very harmful, 

 those birds which take a mixed diet are particularly valuable. Thus a bird 

 species which ordinarily is largely vegetarian lives through seasons of scarcity 

 of insects, and then is ready to turn its attention to an insect diet whenever 

 the insects become overabundant; whereas, if the birds were all exclusively 

 insectivorous, comparatively few would survive seasons unfavorable to in- 

 sects and thus be on hand to aid in the warfare against those pests when 

 seasons favorable to insect life occur. 



A species which is harmless, or even beneficial, in its natural habitat, may 

 become injurious when transported to a new environment, where the natural 

 checks upon its increase are wanting, or the lack of its natural food, or the 

 presence of a food which is more enjoyable, may cause a marked change 

 in its food habits. It is said that the skylark, green linnet and black thrush 

 are considered beneficial in Europe, where they are native, but in New Zea- 

 land, into which country they have been introduced, "they have developed 

 traits which render them far from desirable additions to the fauna of that 

 island" (S). It will be interesting to watch the effect of the introduction of 

 the Little Gray Owl into New Zealand for the purpose of fighting the 

 English sparrow (9). Such importations are now, as a result of disastrous 

 experience, prohibited by law in some countries, including the United States. 



The introduction of a species may disturb the balance as much as the 

 extermination of one. European rats, accidentally introduced into Jamaica, 

 became a great nuisance. The Indian mongoose was imported to destroy tho 

 rats. It turned its attention to game birds, poultry, young pigs and lambs, 

 and became a great pest, also destroying many birds, snakes and lizards, 

 which feed largely upon insects. This was followed by an increase of insert 

 pests. Disaster also followed the introduction of the mongoose into Hawaii, 

 and its importation into the United States was barely prevented. Rabbits, 



(7) Deal, F. E. L., The Relations Between Birds and Insects, U. S. Dept. Agric., 

 Yearbook for 1908, pp. 347-348, 1909. 



(8) Palmer, T. S., The Danger of Introducing Noxious Animals and Birds, U. S. 

 Dept. Agric., Yearbook for 1898, p. 106, 1899. 



(9) Drummond, Jas., Introduced Birds of New Zealand, The Condor, Vol. XIV, p. 

 227, 1912. 



