NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 47 



are usually more aggressive toward the latter than toward most other 

 members of their own species — quite the opposite in fact. It is simply 

 because they tend to encounter plain-colored tanagers more frequently, 

 in many environments, than any other members of their own species, 

 except their own mates and other members of their own family groups. 

 The comparative rarity of encounters between palm tanagers is prob- 

 ably an indication that they are usually more aggressive toward one 

 another than toward individuals of any other species. 



(The aggressiveness of palm tanagers toward one another is obvi- 

 ously variable, and often quite different, on the average, in different 

 situations; but they do appear to be more aggressive toward one 

 another in most circumstances than are plain-colored tanagers in the 

 same or similar circumstances. 



I have seen fairly large groups of 8 to 10 palm tanagers, with and 

 without associated individuals of other species, in both the breeding 

 and the nonbreeding seasons; but such groups are relatively rare. 

 Most pairs and family groups of palm tanagers usually maintain 

 territories during both the breeding and nonbreeding seasons — or, at 

 least, keep well apart from one another. Such territorial birds fre- 

 quently join and follow individuals of other species ; but they usually 

 stop following individuals of other species when the latter move into 

 the territories of other palm tanagers.) 



The characteristic variety of reactions by palm tanagers to plain- 

 colored tanagers may be explained as differential responses to different 

 stimuli presented by plain-colored tanagers. 



All, or almost all, reactions between any two birds are probably at 

 least slightly ambivalent. Thus, for instance, whenever a bird of one 

 species sees or hears a bird of another species it is quite likely to per- 

 form some hostile patterns, at least uttering a hostile note or perform- 

 ing an intention movement of some hostile pattern, in addition to, or 

 instead of, approaching or showing indications of a desire to approach 

 the other bird in a nonhostile ("friendly") manner. Any bird presum- 

 ably presents a complex of partly contradictory stimuli to any other 

 bird that sees or hears it. Some of these stimuli are attractive, others 

 are irritating (i.e., tend to release attack by the perceiving bird), and 

 still others are intimidating (i.e., tend to release escape by the per- 

 ceiving bird). 



Palm tanagers seem to react to plain-colored tanagers as if they 

 found them slightly less attractive, much less irritating, and very much 

 less intimidating than other palm tanagers. This is perhaps what 

 would be expected in view of the fact that plain-colored tanagers are 



