BIRDS OF NEW YORK 13 



also be regarded as enemies of the birds, especially the bird-lice which 

 frequently destroy whole broods of phoebes and swallows, and seriously 

 impair the; vitality of others. Internal parasites are also a source of great 

 harm. During one afternoon in the summer of 1900, the author picked 

 up forty terns on the Weepecket islands which had died from the effect of 

 flat-worms growing within the intestine. Thus various kinds of parasites 

 frequently sap the vitality of birds or destroy them altogether. 



The various factors enumerated work together to make up the bird's 

 environment. By a combination of favorable factors, as opposed to the 

 unfavorable ones in any given locality, the balance may be turned to the 

 bird's advantage, so that it may increase like the English sparrow in 

 America. If the natural influences which are unfriendly to the presence of 

 a bird overbalance the favorable factors, it is useless to expect the species 

 to increase. The factors which naturally produce a favorable environment 

 for it must be induced artificially if the species is to be encouraged. 

 A thorough study of the ecologic status of the birds which societies or 

 individuals wish to encourage in a given locality, should be made when 

 any action is taken either to introduce or to encourage the species and by 

 varying those factors which are of the greatest importance to produce 

 a favorable environment they may finally be successfully encouraged. 



Bird Habitats 



For ecological purposes, birds are properly classified according to the 

 nesting habitats which they occupy, but for various reasons these do not 

 always agree with the places chosen for the birds' other life activities, 

 and it is necessary, for purposes of general discussion, to recognize also 

 the feeding habitat. This is different from the breeding habitat of all 

 truly aerial and aquatic species, of which we have a goodly number. All 

 our diving birds, gulls and waterfowl must necessarily make their nests 

 on shore and are usually classified according to the habitat which they 

 utilize for that purpose; but in many of the species, especially the diving 

 ducks, grebes and loons, the food is almost entirely pursued and taken 



