120 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



desia. They do, however, seem to be very effective in the mixed flocks 

 in Panama. Alarm notes by one bird will at least put the other mem- 

 bers of a flock in a state of alert.) 



Birds that are not very shy by themselves will often retreat from 

 a potential predator sooner when they are associated with shyer species 

 in a mixed flock than when they are not in a mixed flock. Thus, for 

 instance, the comparatively tame and unsuspicious plain-colored tana- 

 gers usually flee from an approaching human being sooner when they 

 are in mixed flocks than when they are in similar flocks (of similar 

 size) of their own species alone. 



Predators attempting to attack a member of a flock may be dis- 

 tracted by the other members of the flock and hesitate for a moment, 

 thus allowing all the members of the flock to escape. It is even possible 

 that predators are more reluctant to try to attack a bird in a group 

 than a solitary bird, because a group is slightly more intimidating. 



Although protection may be the most important advantage obtained 

 by many members of the blue and green tanager and honey creeper al- 

 liance and the montane bush alliances in mixed flocks in ordinary cir- 

 cumstances, this does not necessarily mean that it is always the most 

 important advantage. Even in the case of the frugivorous and/or 

 nectarivorous birds, the discovery of food may be the most important 

 advantage obtained by association with mixed flocks in certain special 

 circumstances. Thus, for instance, a bird moving into a new and un- 

 familiar area may associate with individuals of other species in order 

 to find the sources of food in this new area ; but it may then continue 

 to associate with the other individuals after it has discovered the food 

 sources, and may then derive other advantages, such as added protec- 

 tion from predators, or no advantages at all. 



It will be noted from the above account that the advantages that 

 may be obtained by membership in mixed flocks, including both the 

 discovery of food and protection from predators, could also be ob- 

 tained by membership in flocks of a single species. This raises an in- 

 teresting question. Granted that gregariousness is often advantageous 

 in one way or another (as it obviously is), why do more species occur 

 in mixed flocks than in unmixed flocks of their own species alone 

 (apart from family groups) ? Unmixed flocks might be easier to 

 form and maintain, and might provide certain other social advantages 

 that are lacking in mixed flocks. The answer to this question is prob- 

 ably that the members of an unmixed flock often compete with one 

 another too strongly. Birds of the same species tend to compete with 

 one another more strongly than birds of different species. Associa- 



