NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN I IQ 



territories or ranges. 17 They probably encounter more competition 

 when they associate with individuals of other species of more or less 

 similar feeding habits than they would if they always fed by them- 

 selves alone. It seems likely, therefore, that protection from enemies 

 is the most important advantage obtained by many members of the 

 blue and green tanager and honeycreeper alliance and the montane 

 bush alliances by most of their associations in mixed flocks in ordinary 

 circumstances. 



The importance of the protection factor has been doubted by some 

 observers of mixed flocks, largely because mixed flocks are so con- 

 spicuous that predators probably notice the members of such flocks 

 more frequently than they would notice the same birds apart from 

 mixed flocks. This does not, however, mean that birds in mixed flocks 

 are actually preyed upon more frequently than birds of the same 

 species apart from mixed flocks. (As far as I am aware, there have 

 been no quantitative studies comparing the amount of predation upon 

 birds in mixed flocks with the amount of predation upon birds of the 

 same species in the same environment but not in mixed flocks.) There 

 is, in fact, some actual evidence that birds in mixed flocks are particu- 

 larly efficient at discovering and/or discouraging potential predators ; 

 and various theoretical considerations would suggest that the habit of 

 forming mixed flocks may help to protect birds from predators in 

 several other ways. 



Birds in mixed flocks are usually particularly efficient at mobbing 

 predators. Because they usually stay rather close together, the mem- 

 bers of a flock usually respond relatively rapidly when one member of 

 the flock begins mobbing. 



Birds in mixed flocks probably notice predators more frequently 

 and sooner, on the average, than they would if they were not in flocks. 

 As soon as one member notices a predator, it will usually warn all the 

 others. (Winterbottom, 1943, did not think that such warnings pro- 

 duced much effect in the mixed flocks he observed in Northern Rho- 



17 In an earlier paper (Moynihan, i960) it was mentioned that gregariousness 

 may be particularly advantageous to birds that feed on fruits that occur in irregu- 

 larly scattered masses, e.g., on scattered fruit trees far apart from one another. 

 It may be necessary for such birds to make extensive searches over wide areas 

 for their food, and they may tend to get more food, on the average, if they go 

 searching in groups. The situation of most of the frugivorous members of the 

 blue and green tanager and honeycreeper alliance and the montane bush alliances 

 is usually quite different. They tend to feed on fruits and other vegetable mate- 

 rials that are fairly evenly distributed and common in the environments in which 

 they live; and their searching for food is usually intensive rather than extensive. 



