48 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



much smaller than palm tanagers but rather similar in plumage and 

 even more active and noisy. Size is always an important factor in 

 determining the nature of the hostile reactions between any two birds 

 of any species ; but its direct effect upon positively " friendly" reactions 

 is usually considerably weaker. 



As indicated or implied by the figures in the accompanying tables, 

 palm tanagers tend to join, follow, and supplant blue tanagers less 

 frequently than plain-colored tanagers when the two latter species 

 are even approximately equally abundant. In such environments as 

 the clearing on Barro Colorado Island, palm tanagers also tend to 

 follow blue tanagers less closely than plain-colored tanagers and are 

 relatively less frequently associated with blue tanagers in mixed flocks 

 of only two species. All these facts would suggest that the social rela- 

 tionships between palm tanagers and blue tanagers are usually weaker 

 than those between palm tanagers and plain-colored tanagers. 



This is particularly interesting because palm tanagers and blue tana- 

 gers are strikingly similar to one another in voice, movements, and 

 shape, as well as being very closely related. The fact that palm tana- 

 gers usually react less strongly to blue tanagers than to plain-colored 

 tanagers is an indication that the palm tanager tends to react more 

 strongly to the plumage patterns of the other species it encounters than 

 to their behavior, size, or shape, and/or that the behavior patterns of 

 the plain-colored tanager which are adapted to promote gregarious- 

 ness are really remarkably effective. 



Although less strong and frequent, the reactions of palm tanagers 

 to blue tanagers are similar to their reactions to plain-colored tanagers 

 in one important respect. Palm tanagers seem to find blue tanagers 

 more like other palm tanagers in attractiveness than in irritating or 

 intimidating qualities, and they usually supplant, threaten, and escape 

 from blue tanagers relatively much less frequently than they join and 

 follow them. 



Thus, both the special interspecific preferences of palm tanagers 

 may be said to be expressed primarily by "friendly" reactions and only 

 secondarily by hostility. This is not true of all the special interspecific 

 preferences of some other species (see below). 



Palm tanagers join both plain-colored tanagers and blue tanagers 

 rather frequently without starting to feed immediately afterward. 



Being rather noisy and moderately restless, palm tanagers some- 

 times attract individuals of many other species; but this effect is 

 seldom conspicuous. They induce overt, complete joining and follow- 

 ing responses by individuals of other species comparatively rarely. 



