191 ] Tracy: White Markings in Birds. 289 



though sometimes passive, is justified by the facts as to the dis- 

 tribution of white patterns in passeriform birds of the United 

 States. 



WHITE MARKINGS AS VISUAL CLUES. 



Coloration in birds, whatever its cause or the mechanism of 

 its production, is conceded to be adaptive; it responds to their 

 needs, forms a part of their life adjustments. 



Concealment from its enemies is not the only need in a bird's 

 life, not the only adjustment that affects color-patterns. The 

 bird also needs to be made known to other individuals of its 

 kind, and to other species associated with it; and this need has 

 certainly been met. Just how it has been met depends upon 

 the bird 's manner of life, and upon its chosen habitat ; this much 

 we know. Without entering upon any disputed phase of the 

 subject, we may state at the outset that the need exists and has 

 been met, if not by special provision in coloration, at least by 

 peculiarities of form and manner, or by qualities of voice. 



The vital importance to a bird of seeing and being seen by 

 its companions, hardly finds a parallel among the lower verte- 

 brates ; certainly not among most fishes, though a school of fishes 

 bears a superficial resemblance to a flock of birds. Neither in 

 organization nor in instinct does the one approach the high 

 development of the other. A comparison of the brain of a perch 

 with that of a pigeon shows the remarkable superiority of the 

 latter in respect to cerebral development. The phenomena of the 

 associations of a bird's life, the well-known facts of mutual in- 

 dependence in feeding, nesting and migration among nearly all 

 of the class Aves, indicate the degree to which individualism 

 has been subordinated, and cooperation of a certain kind 

 developed. 



There are, however, great differences within the class. If we 

 are to make any comparison between these and the lower verte- 

 brates, let it be between gregarious sea-birds and fishes, not 

 between oscines and herring. Delicate life-adjustments are to 

 be found among the higher genera of the perching birds that are 

 missing from the more primitive divers or long-winged swim- 

 mers, as well as from fishes. The number and variety of perils 



