42 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



species, to which the tanagers are undoubtedly very attractive. They 

 are joined and followed 6 by individuals of other species very fre- 

 quently, relatively much more frequently than are any other birds of 

 the blue and green tanager and honeycreeper alliance. During the 

 course of the present investigation the attractiveness of plain-colored 

 tanagers was most obvious in the study area on Barro Colorado 

 Island; but it was also quite evident at Frijoles and Gamboa, where 

 these birds were relatively much less abundant (see table 41). 



The attractiveness of the plain-colored tanager is often clearly inde- 

 pendent of its food-finding or food-providing ability. Of all the 

 species of the blue and green tanager and honeycreeper alliance, it 

 is the species most often joined by birds of other species in places 

 where there is little or no food available, and most often joined and 

 followed by other birds that are obviously not in the mood to feed 

 at the time. ( I saw nothing to suggest that plain-colored tanagers are 

 more efficient than individuals of many other species in finding the 

 fruits and insects on which many members of the blue and green tana- 

 ger and honeycreeper flocks feed.) 



These facts would indicate that birds of other species are usually 

 or always attracted to plain-colored tanagers by some aspect of the 

 latter, probably behavior, as the plumage of plain-colored tanagers is 

 particularly dull and probably does not provide many strong stimuli. 

 The plumage may facilitate associations with other species ; but it is 

 probably not directly attractive in itself (see discussion below). The 

 behavior patterns of plain-colored tanagers that attract individuals of 

 other species are almost certainly the patterns that also promote intra- 

 specific gregariousness, as plain-colored tanagers do not perform any 

 other patterns much more frequently or conspicuously than individuals 

 of many other species. 



It has already been mentioned that many or most passerine birds 

 show some tendency to approach birds of almost any other species. 

 This tendency may be called "generalized" gregariousness (one type 

 of interspecific gregariousness, as defined above), and observation of 

 many species would suggest that it is usually more strongly stimulated 

 by the sight and sound of conspicuously active and noisy birds than 



6 Unless specifically stated otherwise, the terms "joined" and "followed" (or 

 "join" and "follow") are used in a rather restricted sense in this and sub- 

 sequent discussions. They are applied only to reactions in which the active 

 bird, the joiner or follower, does not appear to be hostile (or, at least, no more 

 strongly hostile than during all social reactions). 



