40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



THE ROLES OF DIFFERENT SPECIES WITHIN MIXED FLOCKS 



PLAIN-COLORED TANAGER 



Plain-colored tanagers occur in mixed blue and green tanager and 

 honeycreeper flocks very frequently and usually play a very signifi- 

 cant role in such flocks. The presence of these birds in any given 

 area tends to stimulate the formation of mixed flocks in the area ; 

 and their presence in any mixed flock tends to increase the cohesion 

 of the flock. These effects are usually produced by the reactions of 

 individuals of other species to plain-colored tanagers, not the reac- 

 tions of plain-colored tanagers to individuals of other species. Some 

 of the reactions of individuals of other species of plain-colored 

 tanagers seem to be indirect results of the extreme intraspecific gre- 

 gariousness of plain-colored tanagers. 



Intraspecific gregariousness may be contrasted to interspecific gre- 

 gariousness. Throughout this paper, the former term will be used 

 to include all the tendencies of individuals of any given species to 

 associate with one another. The latter term will be used to include 

 all the tendencies of individuals of one species to associate with indi- 

 viduals of other species. 



Flocks of 10 to 16 plain-colored tanagers are common in suitable 

 environments, especially along the edges of fairly mature forest, dur- 

 ing the nonbreeding season. Such flocks tend to break up during the 

 breeding season, when individual males and pairs adopt territories ; but 

 it is still not uncommon to see groups of 4 to 6 moving about together 

 and/or feeding together for more or less brief periods of time even 

 during the height of the breeding season. 5 The frequency with which 



5 The territories of many tropical lowland tanagers (and many other tropical 

 species of other groups) are more difficult to recognize and study than the 

 territories of most related Temperate Zone species. 



The conventional definition of territory is "any defended area" ; but terri- 

 torial defense is comparatively rare among tropical tanagers. Individual birds 

 and/or pairs of the same species in the same general area may keep apart from 

 one another for long periods of time, each individual or pair remaining in its 

 own particular part of the area, without showing any overt hostility to one 

 another. It is probable, however, that such behavior is often or usually the result 

 of previous territorial disputes which do occur occasionally between members of 

 the same species. A few disputes may be sufficient to fix territorial boundaries 

 for many months, or even years. Neighboring birds may learn the boundaries 

 of their territories after a few disputes, and may retain this learning for long 

 periods of time without further reinforcement. The territories of many tropi- 

 cal birds may become "traditional" more easily than those of many migratory 

 Temperate Zone birds simply because the tropical birds are almost completely 



