BULLETIN 
OF THE 
NUTTALL ORNITHOLOGICAL CLUB. 
vol. vi. October, 1881. No. 4. 
ON SOME OF THE CAUSES AFFECTING THE 
DECREASE OF BIRDS. 
BY H. W. HENSHAW. 
In an extended field experience in Massachusetts and at various 
points along the Eastern coast, I have often noticed marked 
changes in the relative abundance of the species of birds of a 
given locality from summer to summer. A locality that for 
several years has furnished a certain species or a summer resi- 
dent in great numbers will be found after the usual spring mi- 
gration to be inhabited by comparatively few of that species, 
although the associate kinds may continue in undiminished force. 
Or it may be that the reduction will be found to affect several 
species to a varying degree, involving perhaps birds of very 
different habits. As my experience in this particular is by no 
means unique but, on the contrary, as I find, has been shared by 
every ornithologist whom I have consulted, I have been led 
to an inquiry as to the probable cause of these seemingly mys- 
terious fluctuations in the numbers of birds, amounting in some 
instances to almost the complete extermination of a certain species 
over a restricted area. 
It is perhaps scarcely necessary to call attention to the fact 
that the great variation occasionally to be noticed in the number 
of migrants passing a given point can have comparatively little 
weight in an estimate of the actual increase or decrease of the 
several species, the facts concerning migration being too general 
