UTILITY OF BIRDS IN NATURE. 13 



An experience with domestic Pigeons, related to me by 

 Mr. William Brewster, will serve as "proof of this state- 

 ment. He had kept a flock of twenty-five or thirty Pigeons 

 in confinement at Cambridge for many years. Under such 

 protective domestication the individuals of the flock had 

 assumed a variety of shades and colors. There were blue 

 Doves, white Doves, and many pied individuals varying 

 between the two extremes. He removed the flock to his 

 farm in Concord, where they were at liberty to roam at will 

 during the day. Here they were attacked by Hawks, and 

 in five years' time the white and pied birds were practically 

 all weeded out, and the flock consisted of blue rock Doves 

 alone. 



The preservation of birds by the weeding out of sickly 

 or wounded individuals did not escape the notice of Prof. 

 Spencer F. Baird, who wrote : — 



It has now been conclusively shown, I think, that Hawks perform an 

 important function in maintaining in good condition the stock of game 

 birds, by capturing the weak and sickly, and thus preventing reproduc- 

 tion from unhealthy parents. One of the most plausible hypotheses 

 explanatory of the occasional outbreaks of disease amongst the grouse 

 of Scotland has been the extermination of these correctives, the disease 

 being most virulent where the game keepers were most active in de- 

 stroying what they considered vermin. 1 



It appears, then, that under natural conditions the birds of 

 prey destroy merely the unfit and surplus individuals of the 

 species on which they prey, and do not, on the whole, reduce 

 their numbers below what the land will support. 



The relations of birds to insects merit the most profound 

 thought and study. No one can study intelligently the effect 

 produced by birds upon insect life unless he first acquires 

 some knowledge of the habits and transformations of insects, 

 and is able to distinguish the so-called injurious and -benefi- 

 cial groups. A brief explanation here of the transformations 

 of insects will better enable the reader to understand the 

 terms used later in describing them as food for birds. 



1 Letter from Prof. Spencer F. Baird to Mr. 3. W. Shorton, published in the 

 Journal of the Cincinnati Society of Natural History, 1882, Vol. V, pp. 69, 70. 



