BIRDS OF NEW YORK 9 



indispensable to the presence of rails, bitterns and numerous species 

 which belong to their community. Rocky cliffs determine the nesting 

 site of Duck hawks and murres. Thus it might be shown that the physi- 

 ography of every locality attracts its own characteristic bird life. 



Soil factors. The character of the soil, whether it is wet or dry, 

 must not be neglected while making a study of bird ecology; and the 

 material of its composition, whether rock, gravel, sand, clay, loam, marl, 

 muck or peaty ooze; also its richness in mineral ingredients such as lime, 

 nitrates, sulphates, phosphates etc. These edaphic conditions influence 

 bird life mainly through their control of vegetation and so affect the 

 breeding and feeding habitats of numerous species. Some are more directly 

 affected, such as the Bank swallows, woodcocks, and snipes which can not 

 breed or find their food supply except in proper soil. 



Biotic factors. Under this heading must be considered first, plants 

 as furnishing nesting sites, food and shelter, and also as controlling the 

 light, heat, humidity, and through the heat and humidity the rate of 

 evaporation which is of great importance in determining the presence 

 of various species of animals in a given habitat. The effect of vegetation 

 upon the nesting site is illustrated in the case of all arboreal species which 

 decline directly in proportion to the deforestation of a region, and of the 

 thicket community which is very quickly affected -by pasturing or the 

 clearing of hillsides and swamps. Illustrating the important effect upon 

 various species by certain kinds of vegetation, I noticed that in 1880 the 

 Purple finch appeared as a common breeding species in the village of Spring- 

 ville at the same time with the growth of numerous spruce and cedar trees 

 which were planted by residents in their dooryards. When these became 

 of a height from ten to twenty feet they were invariably utilized by the 

 finch as breeding sites. Everyone has noticed the influence of the American 

 elm upon the abundance of the Baltimore oriole, which, although it breeds 

 also in various other kinds of trees, succeeds much more often in rearing 

 its young when it chooses the drooping branches of an elm. In driving 

 across the country in springtime everyone must have noticed that colonies 



