Henshaw on Causes affecting the Decrease of Birds. I9 1 
Before advancing an hypothesis to account for the decima- 
tion in the ranks of species occasionally to be noticed, it may be 
well to glance at certain factors in the mortality of bird life, 
natural and other, with a view to ascertaining what part they 
play, if any, in the phenomenon. 
As is well known, foxes, skunks, weasels, snakes, and the 
whole tribe of predatory birds, animals, and reptiles contribute 
towards the reduction of bird life, and unquestionably in wild 
sections, where nature reigns supreme, play an important and 
as usually thought, a beneficent part in preventing the over- 
production of birds. 
While it is true that the aggregate of birds destroyed by these 
agents is large, it nowhere, I think, even during the nesting 
season, the period of greatest danger, amounts to the proportions 
necessary to account for the decrease noticed. This belief is 
changed to certainty with respect to the civilized districts, where 
birds of prey, predatory animals, and even snakes are themselves 
subject to extermination at the hands of man. 
If the above be true of the nesting season, still less can natural 
enemies be supposed to affect to any very appreciable extent the 
ratio of birds at other seasons or in other regions when away 
from our observation. 
Little appears to be known of the number of diseases among 
wild birds and the extent to which they prevail. That diseases 
do actually exist among birds, and to a greater extent than is 
usually supposed, is, I believe, true, and in a few instances I have 
myself found dead birds under circumstances that precluded the 
idea of death from any form of violence.* The almost total 
lack of evidence upon the diseases of birds is, however, conclu- 
sive proof of their comparative rarity, f since if serious diseases 
were common, or anything like epidemics prevailed, we may be 
sure their presence and effects would make themselves visibly 
known. 
* Upon this point see Shufeldt in American Naturalist (Vol. XV, Apr. 1881, pp. 
283-285). The subject is an interesting one and that its fuller investigation would 
result in developing some valuable facts cannot be doubted. 
f As compared, for instance, with mammals, which are well known to be subject 
to epidemic diseases that actually depopulate wide districts. Buffalos among large 
animals, and rabbits among small ones, may be cited in illustration. The latter, par- 
ticularly, fairly swarm in certain parts of the western Territories, and by their numbers 
and the consequent damage they inflict upon the farmer woujd be intolerable nuisances 
were it not for the occasional epidemics that sweep them off by thousands. 
