BIRDS OF THE PANAMA CANAL ZONE 



ments (usually based on several or many 

 specimens) which that author gives. The 

 dimensions given also have been converted 

 from millimeters to the nearest tenth of an 

 inch. It must be understood that individual 

 birds vary more or less above or below these 

 averages, the usual range of variations being, 

 in small birds, only a few millimeters, but 

 often many centimeters in large birds. 



It is of course very difficult to estimate the 

 actual length of a bird seen at a distance, but 

 by comparing its size mentally with that of 

 some familiar species whose dimensions may 

 be looked up (if they do not happen to be re- 

 membered) , an approximation may be arrived 

 at near enough to assist in identifying the 

 bird. 



Where the males and females differ in 

 color, young males usually first resemble fe- 

 males in plumage, and acquire their adult 

 plumage more or less gradually. During 

 this process they pass through intermediate 

 stages in which various combinations of the 

 adult male and female colors may occur. It 

 has not been possible to deal with these stages 

 in the descriptions. Though they sometimes 

 prove puzzling to the bird student, the diffi- 

 culty can usually be cleared up by considering 

 the plumage of the adults of both sexes. 



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