NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 41 



plain-colored tanagers associate with one another does not seem to be 

 affected by the presence or absence of individuals of other species. 

 None of the other species of tanagers that frequently occur in mixed 

 flocks with plain-colored tanagers are as gregarious apart from mixed 

 flocks as plain-colored tanagers. 



Some of the social behavior patterns of plain-colored tanagers seem 

 to be very specialized. Among these specializations are increased fre- 

 quency and exaggeration of wing-flicking and tail-flicking movements 

 (stereotyped "intention movements" of flight), extreme restlessness 

 (plain-colored tanagers are more nearly constantly active than most 

 other tanagers), increased frequency of call notes, loss of song (which 

 seems to have been replaced by a variation of the call-note patterns), 

 and various modifications and/or reductions of hostile behavior. 



As a result of their frequent call notes and vigorous movements, 

 plain-colored tanagers are usually very conspicuous — probably more 

 so, on the average, than individuals of many other species of the blue 

 and green tanager and honeycreeper alliance. 



All or most of the specialized social behavior patterns of plain- 

 colored tanagers seem to be adaptations to promote gregariousness 

 (see the discussion in Moynihan, i960) and were probably evolved 

 originally to promote intraspecific gregariousness. Many other species 

 of birds that are gregarious among themselves, but not usually asso- 

 ciated with mixed species flocks, have evolved similar characters, which 

 may promote associations between individuals of the same species in 

 several different ways. If nothing else, a bird that is very conspicuous 

 is more often noticed and more easily recognized by other individuals 

 of the same species than it would be if it were less conspicuous. 



Some of these specialized social behavior patterns of plain-colored 

 tanagers also seem to affect the behavior of individuals of many other 



sedentary. There is evidence that the same pairs of some species of tropical 

 tanagers retain, or revert to, the same territories year after year. 



In the case of the more highly gregarious species, the situation is further 

 complicated by the fact that territorial birds frequently abandon their territories 

 at more or less regular intervals in order to join and/or follow flocks of the 

 same or other species. Thus, the territories of many tropical tanagers may be 

 both very stable over long periods of time, and very unstable, i.e., held only 

 intermittently, over short periods of time. 



The groups of plain-colored tanagers that are formed during the breeding 

 season may be composed of birds that have abandoned their territories com- 

 pletely for the time being, and/or birds that have met in a neutral area between 

 their territories, which they are still prepared to defend. In some cases, at 

 least, these groups are not composed of an adult pair plus their still juvenile 

 young of the previous breeding season. 



