NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 69 



the other species commonly occurring in that type of mixed flock 

 whenever a suitable opportunity occurs (at least during the nonbreed- 

 ing season). Occasional members of a particular type of mixed flock 

 frequently, perhaps usually, do not approach and/or are not ap- 

 proached by the other species commonly occurring in that type of 

 mixed flock when opportunities to do so occur (even during the non- 

 breeding season). Thus, a rare species that is a regular member of a 

 certain type of mixed flock may actually occur in flocks of that type 

 less frequently than a common species that is an occasional member, if 

 the common species has many more opportunities to become associated 

 with mixed flocks of that particular type. The term "occasional," in 

 this sense, is probably largely synonymous with the term "accidental," 

 as used by Davis and Winterbottom. The latter term is not used in 

 this paper because it might convey a misleading impression. The 

 associations between occasional members of mixed flocks and their 

 companions in such flocks are not really fortuitous, or accidental, in 

 the ordinary sense of the word. 



The characters used to define the categories listed above are essen- 

 tially relative. It is perfectly conceivable that some species might play 

 intermediate roles in certain types of mixed flocks. This may be true 

 of some of the species of montane bush flocks (see below). The roles 

 of most species in mixed blue and green tanager and honeycreeper 

 flocks, however, are so clear-cut and one-sided that they can be 

 assigned to particular categories without any difficulty. It is possible, 

 therefore, to summarize the roles of the various species in mixed blue 

 and green tanager and honeycreeper flocks as follows : 



The plain-colored tanager is always a regular and passive nuclear 

 species. The red-legged blue honeycreeper is always regular and 

 nuclear and probably always passive. 



The blue tanager and the green honeycreeper are always regular 

 and active nuclear species. The golden-masked tanager is probably 

 similar. The shining honeycreeper may be a regular or an occasional 

 active nuclear species. 



The palm tanager is always an active nuclear species and always 

 regular except in some environments in the breeding season. 



The bananaquit is always regular. Sometimes it is a passive nuclear 

 species. At other times it is an attendant species. 



The summer tanager is always an attendant, and usually or always 

 occasional. 



The role of the crimson-backed tanager is more obviously compli- 

 cated than those of all or most of the other species. It is a very 



