Part II.] NATURAL ENEMIES OF BIRDS. 213 



that accompany crowding, starvation and weakness. Natural 

 enemies tend to prevent this by disposing of the surplus indi- 

 viduals. 



Darwin says, truly, that the struggle for existence is greatest 

 between individuals or varieties of the same species. Natural 

 enemies protect a species against itself by keeping its numbers 

 low enough to prevent serious competition for food. 



NATURAL ENEMIES PRESERVE THE FITNESS OF THE 

 ANIMALS ON WHICH THEY PREY. 



Natural enemies also serve to preserve the fitness of a 

 species by (a) acting to check the spread of disease and (fc) 

 operating to preserve by selection the most active, agile, 

 cautious or otherwise efficient and mentally and physically fit 

 individuals. 



The spread of epidemics or contagious diseases is checked 

 by the natural enemies of a species, which readily capture and 

 destroy those slightly weakened by disease, as such animals 

 are slower to act or react than those in robust health. Sick 

 birds, for example, readily are captured, not only by enemies 

 sly, swift or strong enough to catch healthy birds, but also by 

 an additional number of slower enemies, which birds in full 

 vigor would escape with ease. 



On this subject Professor Spencer F. Baird of the Smithso- 

 nian Institution wrote as follows : — 



It has now been conclusively shoun, 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 gamekeepers were most active in destroy- 

 ing what they considered vermin.^ 



Mr. James Henry Rice says that in South Carolina the bob- 

 white sometimes is attacked by a deadly disease which afi'ects 

 the head of the bird in such a way that it becomes stupid, 



1 Letter from Prof. Spencer F. Baird to Mr. J. W. Shorton, published in the Jour, of the Cin- 

 cinnati Soc. of Nat. Hist., 1882, Vol. V., pp. 69, 70. 



