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.! 
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 
ternis used later in describing them as food for birds. 
1 Letter from Prof. Spencer F. Baird to Mr. J. W. Shorton, published in the 
Journal of the Cincinnati Society of Natural History, 1882, Vol. V, pp. 69, 70. 
