290 University of California Publications in Zoology. ["^°'"- ^ 



that daily surround our smaller land-birds, and the extent to 

 which these may be diminished by the birds' keeping in touch 

 with one another, point to the need of something more than 

 concealing coloration, and admit of special adaptions that shall 

 act in harmony with it and yet serve to reveal the bird to its 

 kind. 



Notwithstanding this distinction there will be some who 

 fail to find in the life-relations of the Passeriformes anything 

 to occasion the development of marks whose main function shall 

 be that of revealing the birds to each other. Thayer (1900), 

 exposing the weakness of the "banner-mark theory," indirectly 

 implicates all theories of directive coloration, as well as that 

 of warning colors, and has since made them the object of special 

 attack (A. H. Thayer, 1909). The ground for his criticism — and 

 it is a good o;ne — is that birds, in order to profit by such aids as 

 signs and signals, or at least to need them, must be less acute 

 than human observers, who easily recognize species of birds by 

 slight hints, such as are afforded by silhouette, by mode of flight, 

 by mannerisms of one sort or another, rather than by special 

 marks. This is so true that it must and does discredit the crude 

 interpretations such as the title "banner-mark" suggests. We 

 must distinguish between hypothetical functions, the creation 

 of fancy, and a series of well-defined stripes, bars, or checks, 

 which may be interpreted by any one of at least three categories, 

 of which that of "concealing coloration" is only one. 



Starting out with a presumption in favor of some form of 

 revealing clues among the higher land-birds, and eliminating a 

 terminology which has been misleading, it remains for us to 

 determine, if possible, what these clues are, and whether color 

 features form a part of them; if so, how this harmonizes with 

 the function of the same or similar color features as concealing. 



How do the birds of our woods and fields actually keep track 

 of one another? Obviously to a great extent by vocal sounds 

 which they utter frequently when moving about in the foliage, 

 or in unison when leaving a feeding ground as a flock; by call- 

 notes, when moving in pairs or companies; by location notes, 

 when separated and seeking to come together. No one doubts the 

 existence of such vocal clues or their vital necessity to birds 

 belonging to the order under discussion. 



