^^^^^ Tracy: White Markings in Birds. 309 



DIRECTIVE MARKINGS OUTSIDE THE ORDER 

 PASSBRIFORMES. 



An instance of revealing coloration outside the order Passer- 

 if ormes seems worth citing since the very existence of a principle 

 of directive coloration has been questioned (Thayer, 1900). The 

 wings of the nighthawks, both eastern and western species, are, 

 as is well known, marked with a single white spot in each. 

 Far from tending to merge the bird's contour with anything in 

 the background, these spots easily reveal and characterize the 

 bird to observers. Even were there need for the nighthawks in 

 their swift, crepuscular flights to be hidden from winged pur- 

 suers, it is hardly credible that the spots should serve this 

 purpose. The Texas nighthawk (Chordeiles acuiipennis tex- 

 ensis) is known to have a habit which gives revealing effect to 

 the white throat-patch, as well as that on the wing. This throat 

 patch is concealed as the bird takes its daytime rest in the 

 open; but when surprised upon its nest it adopts the familiar 

 wounded bird tactics to divert attention. After fluttering a 

 short distance it faces the intruder, elevates and depresses its 

 breast, thus appearing to make every effort to hold the attention 

 of its enemy. Such a motion adds decidedly to the conspicuous- 

 ness of the white patch. This effect of a pattern and correspond- 

 ing behavior of the bird is perhaps unique, but it seems at least 

 to establish a case of the revealing function of white. 



CONCLUSION. 



This paper has attempted to test the validity of the older 

 interpretation of white markings in birds by analyzing their 

 mode of occurrence in a single order, and to harmonize it, if 

 valid, with a newer and apparently contradictory interpretation. 

 It has shown that there is good ground for believing that flight- 

 exposed markings, whatever their mode of evolution, are of 

 actual utility to birds as sight-clues, whether occurring in the 

 comparatively uniform rear markings of the open ground species, 

 or the varied top patterns of the arboreal. It has called atten- 

 tion to a decided correlation of the markings with the habit of 



