10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



and green tanager and honeycreeper alliance are produced by some 

 social factors, in addition to (or instead of) the direct and immediate 

 stimulus of food. 



The effects of such special social factors are obvious in the case of 

 some associations. Birds of some species definitely do tend to join 

 and follow birds of certain other species in places (e.g., the top 

 branches of tall, dead, bare trees) where there is little or no food 

 available and/or when they are obviously not in the mood to feed (as 

 shown by the fact that they do not begin to feed immediately after 

 joining or following). 



The factors controlling the behavior of other species are more 

 obscure. It is evident, nevertheless, that the birds of the blue and 

 green tanager and honeycreeper alliance inhabiting any given area 

 are relatively seldom scattered randomly among the particular parts 

 of that area where food is most abundant at any given time. They 

 tend to occur in "clumps," first in one part and then in another. 



The minor movements of many mixed flocks are also extremely 

 suggestive. The behavior of birds feeding in Cecropia trees on Barro 

 Colorado Island may be cited as an example. These trees are common 

 along the edges of clearings, and many or most of the trees in any 

 particular area seem to bear fruit at almost exactly the same time. 

 Large mixed flocks of the f rugivorous members of the blue and green 

 tanager and honeycreeper alliance frequently feed on the ripe Cecropia 

 fruits. Such flocks are usually very restless. All the birds of a mixed 

 flock may be feeding on the fruits of one particular tree, when one 

 bird or part of the flock may suddenly fly over to an adjacent tree and 

 begin to feed there. In such cases, the remaining members of the flock, 

 left in the first tree, frequently (perhaps usually) fly over to the second 

 tree sooner or later. A mixed flock may visit a dozen different Cecropia 

 trees in fairly rapid succession in this way. It is also quite common 

 for a flock to fly back and forth, visiting and feeding in these same 

 trees again and again. In view of the frequency of such back-and- 

 forth alternation, and the fact that the fruits of almost all the trees 

 usually seem to be in essentially identical condition, it is difficult to 

 believe that all the members of such a mixed flock are really looking 

 for a different food when they move. It seems more likely that most 

 of the members of the flock are simply reacting to the movements of 

 the first birds to fly; and the first birds may have flown, or started 

 to fly, for any one of several different reasons, including pure "restless- 

 ness," slight alarm, or a brief outburst of aggressiveness within the 

 flock. Such reactions would at least suggest that some positive social 



