Many animals use cues from other species to gather information about foraging opportunities, but engaging in such behavior requires balancing the potential benefits with the risks of increased competition. Neotropical birds that follow army ants are an appropriate system within which to study such eavesdropping behavior. Mixed-species groups of these understory birds congregate around ant swarms, where they forage on insects flushed by the ants. Obligate ant-followers have specialized behaviors to track ant swarms and may serve as information sources for facultative ant-followers. Owing to strong dominance hierarchies among species, however, the value of the information likely depends on the competitive relationship between the information provider and the eavesdropper. I investigated whether ant-following birds preferentially followed the songs of some antbird species over others, testing the prediction that the vocalizations of small (subordinate) ant-following species would attract more individuals than the vocalizations of large (dominant) species. I conducted playback experiments that broadcast songs of the small White-cheeked Antbird (Gymnopithys leucaspis), the larger Reddish-winged Bare-eye (Phlegopsis erythoptera), and a control that does not attend ant swarms (the Peruvian Warbling-Antbird [Hypocnemis peruviana]). I also observed birds at swarms to investigate additional factors that may affect information utility. Although overall response rates to playback were low (as expected in this low-density setting), heterospecific individuals responded significantly more often to playback of songs of the White-cheeked Antbird compared with those of both the Reddish-winged Bare-eye and the control. Observations at swarms did not show significant differences in abundance or vocalization rate between species, so this difference in response rate may be explained by the White-cheeked Antbird's favorable combination of subordinate status and far-carrying song. These 2 factors (competition and cue prominence) are also important determinants of information utility in analogous systems involving other taxa. These dynamics of interspecific information use by ant-following birds may provide insights into the general mechanisms involved in the formation and maintenance of mixed-species foraging aggregations.
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Vol. 134 • No. 1