304 University of California Publications in Zoology. t v L - 6 



the very arbitrariness of the grouping tends to be misleading. 

 A list of all perching birds that have color features tending to 

 conspicuousness in flight (whether white or some other color), 

 would correspond much more nearly to a complete list of the 

 flocking birds. It would include the gregarious species of the 

 family Icteridae which mainly lack white patterns. It would 

 include the pine siskin (Spinus pinus) which has yellow wing 

 markings instead of white. Yet, even as it stands, the list is very 

 suggestive. When we consider the value to all birds ranging in 

 the open foliage of instant recognition at a distance, and sight- 

 clues for the purpose of keeping together, w r e shall not easily 

 believe that wing and tail white are solely features of concealing 

 coloration. Their revealing function during flight is entirely in 

 harmony w r ith their concealing functions when at rest. 



An apt illustration of this harmony of functions is found in 

 the following description of the western evening grosbeak 

 (Bailey, 1902, p. 308) : 



"On a Sierra grade we have passed a flock busily gathering 

 wild cherries in a bush beside the road, and when camped under 

 the firs of Mt. Shasta have had wandering bands stop for a drink 

 from the camp brook, delighting us by their striking yellow 

 and w r hite plumage. Although they are so highly colored and 

 in flight their white wing patches make such prominent directive 

 marks, this very yellow and white coloration often becomes posi- 

 tively protective. While watching the birds on Mt. Shasta one 

 day, I was struck by the conspicuousness of one that flew across 

 an open space. As it lit on a dead stub whose silvery branches 

 were touched with yellow lichen, to my amazement it simply van- 

 ished. Its peculiar greenish yellow toned in perfectly with the 

 greenish yellow of the lichen. " 



It is of interest to note that the above observation was made 

 before the disruptive effect of white patterns had been demon- 

 strated by Thayer (1909), or the theory of their directive 

 function seriously questioned. There could be no better 

 instance than the one cited, of a double office performed by a 

 single color feature revealing in flight, concealing when in 

 repose. Precisely this relation, I believe, exists through the 

 group as a whole 



