6 NEW YORK STATE MUSEUM 



absolutely necessary at the present time in order to explain the probable 

 reasons why some birds are abundant in various sections of the State and 

 others are fast disappearing ; and to suggest reasonable means of encouraging 

 desirable birds to increase in our domains. 



FUNDAMENTAL FACTORS OF ENVIRONMENT 



Climatic. Of the natural factors which influence the bird life of any 

 part of the world, those due to the climate are undoubtedly of foremost 

 importance. Among these is light, the effect of which may be illustrated 

 by reference to owls, goatsuckers and woodcocks, which feed mostly by 

 night or in the twilight, and in the daytime must hide away in hollows 

 or dark portions of the forest. To a less extent the influence of this factor 

 is observed in those forest species such as many thrushes and wood warb- 

 lers whose eyes are noticeably larger than those of their relatives which 

 live in more open surroundings, and unquestionably the intensity of light 

 affects their nervous equilibrium to such an extent that it even determines 

 their presence or absence in a given locality, apart from all other factors. 

 Temperature has usually been considered the most important climatic 

 factor and is the principal one taken into account by the United States 

 Biological Survey in mapping the distribution of animals in America. 

 As illustrations of this factor, it is frequently evident when surveying 

 a ravine in central New York or when approaching the outskirts of the 

 Adirondacks, that such species as the Junco, Canada warbler and Hermit 

 thrush will frequently be present or absent according as the average 

 temperature varies 2 or 3 degrees during the six hottest weeks of summer. 

 Of very great importance is the humidity, which is largely dependent 

 upon temperature and, with it, regulates the distribution of many species. 

 As direct examples of this factor, the author has become convinced that 

 such birds as the Wood thrush and the Hooded warbler are usually found 

 in a denser cover than some of their allies merely because the percentage 

 of humidity is higher in the coverts where they are found than in other 

 localities which, by the casual observer, might be considered equally con- 



