Mixed-species flocks are formed on the basis of both positive and negative species interactions. We use foraging behavior in two different flock types to interpret the extent to which core species minimize niche overlap to reflect negative interactions. We also use the foraging behavior of alarm-calling species to infer whether their behavior is consistent with predictions for species that accrue benefits by associating with other flocking species. The foraging patterns of core species in tierra firme flocks show large differences with respect to foraging maneuvers and substrates, a finding that is consistent with niche theory. In igapó (a blackwater seasonally inundated forest), only the alarm-calling species show differences in foraging patterns among core flock members. We also show that alarm-calling species in different sites show different patterns of association with other flocking species: one species, Thamnomanes saturninus, shows no strong tendency to associate with any other species in the flock and the other, Thamnomanes schistogynus, perches close to and immediately below other species in the flock. These observations are consistent with the hypothesis that alarm-callers benefit from insects flushed from other flock members in igapó forest but not in tierra firme forest. In northeastern Peru, subtle variation in the foraging behaviors among alarm-calling species in tierra firme and igapó flocks may reflect differences in species interactions among key flock members.