NO. 7 FLOCKS OF NEOTROPICAL BIRDS MOYNIHAN II3 



or most of the regular members of the blue and green tanager and 

 honeycreeper flocks in the same (see below) and other habitats; but 

 the actual frequency of such reactions is usually not very high. Crim- 

 son-backed tanagers certainly follow and join yellow-rumped tanagers 

 much less frequently, on the average, than palm tanagers join and 

 follow several other species of the blue and green tanager and honey- 

 creeper alliance in central Panama during the nonbreeding season. 

 What the crimson-backed tanagers usually manage to do is stay in the 

 vicinity of yellow-rumped tanagers without appearing to follow them 

 in any regular manner or coming very close to any particular indi- 

 vidual. 



The responsiveness of crimson-backed tanagers to yellow-rumped 

 tanagers is probably not strong enough to be considered a special inter- 

 specific preference. It is not a more frequent occurrence than would 

 be expected as a result of generalized gregariousness, in view of the 

 similar notes and preference for scrub of the two species. 



Yellow-rumped tanagers also tend to follow and join crimson- 

 backed tanagers relatively more frequently than they are followed and 

 joined by the latter ; but their reactions to crimson-backed tanagers are 

 even more obviously not the results of a special interspecific prefer- 

 ence. Yellow-rumped tanagers tend to follow and join almost all 

 other tanagers and finches that occur in or near the scrub along river 

 banks. Their responsiveness to individuals of so many other species 

 seems to be a reflection of their extreme gregariousness among them- 

 selves. Their tendency to join and follow one another seems to be 

 so strong that it frequently "overflows," to be vented upon suboptimal 

 stimuli. 



It has already been mentioned that special signal patterns and some 

 related characters that seem to be adaptations to promote intraspecific 

 gregariousness will usually or always tend to attract individuals of 

 other species also, but that this attraction may be weakened by other 

 factors. Most species are usually more strongly attracted to other 

 species that are more or less similar to themselves than to other species 

 that are very different. The relations between crimson-backed tanagers 

 and most other species of the blue and green tanager and honeycreeper 

 alliance were cited as an example. Most species of this alliance are 

 probably attracted by the restlessness and general conspicuousness of 

 crimson-backed tanagers, but not as strongly as they would be if the 

 latter were not so distinctive in color and voice. 



The relations between yellow-rumped tanagers and the species of 

 the blue and green tanager and honeycreeper alliance may be an even 



