A description of the composition and behavior of mixed-species flocks of birds in the forest of Horton Plains, Sri Lanka [Ceylon], is presented to examine the selective advantage of flocking behavior. Previous hypotheses advanced to explain the selective advantage of flocking for birds involve either reduced predation on or increased food intake by the birds. The birds probably do not benefit from the flushing of insects by other species in the flock. The birds span such a large horizontal distance that the rear birds frequently do not follow the same path as the front birds, suggesting that they do not encounter or see any of the insects disturbed by the front birds. As a result of being in flock, birds probably do research areas that were previously occupied by other individuals. The birds flew back and forth a great deal, and frequently fed out of sight of one another. Members of mixed flocks do not derive any substantial advantage in feeding as a result of the presence of other species which take different foods. A consistent horizontal species order in these flocks, with the species high in the canopy leading, was not described in other flocks. In the Sri Lanka flocks observed, some birds were left behind at times and would subsequently stop foraging and catch up with the other birds. The possibility that different species in flocks have different optimum speeds of movement through the forest when foraging suggests that there must be strong positive advantages in flocking that compensate for loss in efficiency as a result of adapting their foraging speed to that of the rest of the flock, as well as the loss in time and energy involved in maintaining contact with the other flock members. Low-level foragers with restricted vision may benefit from traveling with canopy species which can see further and, therefore, presumably detect approaching aerial predators sooner. The followers could warn of predators approaching from the rear and below. The canopy species may act as leader species and the others as followers when the flock moves, which might involve no advantage or disadvantage to the leader species from their presence in the mixed flocks.