Amazonian mixed-species flocks of birds are remarkable by virtue of their strength of interspecific association. In these flocks, individuals of different species associate with each other for the duration of their lives, i.e. during all daylight hours, 365 days of the year across many years. These obligate relationships provide a unique opportunity to study the behavioral ecology of dependent interspecific relationships. I break my thesis into three chapters. First, I ask whether the relationship between two obligatorily flocking species (nuclear antshrike and antwren species) is symmetrical and whether transient species (non-obligate flocking species) are equally attracted to the nuclear species. Are the transient species equally attracted to the two nuclear species? And are the two nuclear species equally attracted to each other? This first question was necessary to ask before my other questions because I needed to know which species are following other species and which species are being followed. Then, I ask whether the behavior of a single species (the same antshrike species I determined was mostly being followed in the first question) can predict the space use of the whole flock and what environmental and behavioral variables explain the space use. Finally, I ask whether a particular context-dependent vocalization of the antshrike might serve as a mechanism of interspecific cohesion of the flock. After having spent hundreds of hours studying the space use of flocks in order to define the spatial distribution of flocks within their territories, this last question followed naturally since the mechanism of interspecific cohesion was central to the coordination of the movement of the flock through space. In resident Amazonian mixed-species flocks, the pattern of attraction of transient species to nuclear antshrikes and antwrens, and the pattern of heterospecific attraction between the nuclear species, are undocumented. Patterns of attraction can help elucidate the nature of interspecific relationships, i.e. whether they are mutualistic, commensal, or parasitic. We played antshrike and antwren vocalizations to flocks and observed how many transient species approached the playbacks, whether antshrikes and antwrens were attracted to each others' playbacks, and how strongly they responded to the playbacks. More transient species were attracted to antshrike vocalizations than antwren vocalizations and more transient species were attracted to antwren than control vocalizations. Antshrikes and antwrens approached each others' playback significantly more often than they approached control playbacks; antwrens responded significantly more strongly to antshrike vocalizations than antshrikes did to antwren vocalizations. The primary reason transients are attracted to antshrikes may be the reliable alarm calling of the antshrikes, and the primary reason transient species are attracted to antwrens may be that antwrens serve as an indicator of a flock. Finally, antshrikes and antwrens likely confer benefits to each other, but those benefits may not be symmetrical. Investigating the drivers of space use is critical for understanding the ecology of a species. The drivers of space use patterns of multi-species groups have been studied seldom, although many avian species participate in mixed-species flocks throughout the world. We characterized the space use patterns of mixed-species flocks in Amazonian Peru and investigated the drivers of those patterns. We predicted that foraging and anti-predation behavior, i.e. attack rate, vigilance rate, and surrounding vegetation density of the "leader" Dusky-throated Antshrike (Thamnomanes ardesiacus) would explain the space use patterns of the whole flock. We found that low surrounding vegetation density, but not attack rate or vigilance rate, of the Dusky-throated Antshrike (Thamnomanes ardesiacus) explained a substantial amount of the space use patterns of the flock. The results are consistent with the hypothesis that a single species in these mixed-species flocks has a disproportionately large influence on space use decisions. The results may apply to other flocks worldwide that are led by species that behave similarly to the Dusky-throated Antshrike, for example Asian flocks led by drongos (Dicrurusspp.). Cohesion of interspecific associations requires communication among heterospecifics, although the mechanisms of communication are often unexplored. In many monospecific groups, cohesion among individuals is maintained with contact calls given in a certain context. We investigated whether such context-specific calls are responsible for the cohesion of mixed-species flocks of antshrikes and antwrens in Amazonian Peru. Dusky-throated Antshrikes (Thamnomanes ardesiacus) vocalize while in flight, and the number of times they vocalize covaries with the distance of the flight. Antwrens (Myrmotherula spp.) fly toward an antshrike sooner after the flight call than after a randomized control and another common vocalization not associated with flight. In addition, antwrens fly toward an antshrike sooner after a longer series of flight calls than after a shorter series. It remains unclear whether the antshrikes are deliberately signaling the antwrens or the antwrens are eavesdropping on the vocalizations, although signaling is possible since antshrikes may benefit from associating with antwrens through the dilution effect, confusion effect, and the many-eyes effect. The results suggest that the Dusky-throated Antshrike plays a critical role in maintaining Amazonian biodiversity through positive interactions with many species. Future conservation efforts should aim to conserve the Dusky-throated Antshrike, which likely has a disproportionately large positive effect on many avian species.
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