68 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



the following four main categories: (i) "nucleus" species, (2) other 

 "regular" species, (3) "regular accidental" species, and (4) "acci- 

 dental" species. He also suggested that a fifth category might be 

 recognized : "accidental nucleus" species. 



This last scheme can be used as the basis for a classification of the 

 roles of different species in mixed blue and green tanager and honey- 

 creeper flocks ; but it needs to be revised in several ways for this pur- 

 pose. The criteria for distinguishing the various categories may also 

 be defined in more strictly behavioral terms in order to eliminate the 

 factors that are essentially extrinsic to the mixed flocks. 



An initial distinction may be made between "nuclear" and "attend- 

 ant" species. Nuclear species can be defined as species whose behavior 

 contributes appreciably to stimulate the formation and/or maintain 

 the cohesion of mixed flocks. Individuals of such species approach 

 individuals of other species relatively frequently and/or behave in 

 such a way that they are particularly attractive to individuals of other 

 species. In any case, they contribute something more than their mere 

 presence. Nuclear species, in this sense, may or may not be regular 

 members of the mixed flocks in which they are sometimes nuclear. 

 They may also occur at any place in mixed flocks. Some nuclear 

 species usually occur at the front and/or center of mixed flocks but 

 others do not. Attendant species do much less to stimulate the forma- 

 tion and/or maintain the cohesion of mixed flocks. They contribute 

 little or nothing except their presence. 



Nuclear species can be divided into two main types, which may be 

 called "active" and "passive." These may be defined as follows. Indi- 

 viduals of active nuclear species usually join and/or follow individuals 

 of other species much more often than they are joined and/or fol- 

 lowed by individuals of other species. (Many or most of the species 

 called "other regular species" by Winterbottom would probably be 

 classified as active nuclear species according to this definition.) Indi- 

 viduals of passive nuclear species are usually joined and/or followed 

 by individuals of other species much more often than they join and/or 

 follow individuals of other species. (All or most of Winterbottom's 

 "nucleus" species would probably be classified as passive nuclear 

 species according to this definition. ) 



It may also be convenient to divide the species occurring in any par- 

 ticular type of mixed flock into "regular" and "occasional" members 

 of such flocks. The characteristics of regular members have already 

 been mentioned. Regular members of a particular type of mixed flock 

 usually or always approach and/or are approached by one or more of 



