18 BIRDS OF THE NEW YORK CITY REGION 



SEASONAL VARIATION IN BIRD-LIFE 



The bird-life of our vicinity varies widely during the 

 course of a year. A few are always present. Others nest and 

 then retire southward; others come in winter only, and still 

 others pass through twice each year to and from southern 

 winter quarters and northern summer homes. Many of them, 

 however, do not fit into any of these groups with any degree of 

 definiteness, and others are in different groups in different 

 parts of the territory. I have not, therefore, adhered to 

 strict definitions of terms, but have consulted utility, putting 

 each species in that group in which it belongs either in the 

 greatest extent of territory or in the greatest numbers. Thus 

 the Hermit Thrush is technically a permanent resident, as it is 

 undoubtedly present somewhere at every season. But in no 

 one locality will this be true, except casually, so it is classed as 

 a transient, as under this head it is known to the majority of 

 students and is most abundant. In many cases, the inclusion 

 of a species in any particular group must be quite arbitrary. 

 There is no answer, for instance, to the question : when does a 

 summer resident become a permanent resident? Every pos- 

 sible stage of intergradation occurs. Mr. J. T. Nichols' 

 opinion has been of assistance in many of these doubtful 

 cases. 



I. Permanent Residents. This group includes species 

 regularly present throughout the year. Some, like the 

 Bob-white, Ruffed Grouse, and certain Owls, are doubtless 

 strictly non-migratory. The majority, however, do migrate, 

 and it is probable that wintering individuals come from 

 further north, replacing breeding individuals who retire 

 southward. Bird-banding, now happily gaining rapidly in 

 popularity, can alone definitely answer this question. 



