Mixed-species assemblages can involve positive and negative interactions, but uncertainty about high-value patchy resources can increase the value of information sharing among heterospecific co-foragers. I sampled species composition of bird-flocks attending army-ant raids in three adjacent elevation zones in Costa Rica, across multiple years, to test for positive and negative associations among raid-attending bird species. My goal was to test whether the most frequent and specialized raid-attending species showed evidence of facilitating or excluding other bird species. I quantified elevational variation in avian community composition at raids, then asked whether species composition was associated with variation in flock characteristics (flock size and species richness). I identified the most frequent raid-attending species (those that attended raids most frequently relative to their mist-net capture rates), and bird species that performed specialized army ant-following behavior (bivouac-checking, which allows birds to memorize and track mobile army-ant colonies). There was significant turnover of bird species among zones (including the frequent and specialized attendants); patterns of species overlap suggested a gradual transition from a Pacific-slope to an Atlantic-slope raid-attending bird fauna. Raid-attendance frequency was positively correlated with bivouac-checking behavior. With few exceptions, the most frequent raid-attending bird species, and the bivouac-checking species, also participated in the most species-rich flocks. High species-gregariousness suggests many of the frequently attending and/or bivouac-checking species functioned as core flock members. However, some bird species pairs were significantly negatively associated at raids. Despite species turnover, per-flock numbers of birds at raids did not differ among geographic zones, but flocks on the Pacific-slope were heavier because larger bodied bird species attended raids. Previous studies showed that the size (biomass) of bird-flocks corresponds to the amount of food the birds kleptoparasitize from ant raids, and the heavier Pacific-slope bird-flocks could have greater negative kleptoparasitic impacts.