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Studies of mixed-species bird flocks have found that ‘nuclear’ species, those important to flock coherence, are either intraspecifically gregarious or are ‘sentinel’ species highly sensitive to predators. Both types of species are present in flocks of a Sri Lankan rain forest: orange-billed babblers (Turdoides rufescens Blyth) are highly gregarious, whereas greater racket-tailed drongos (Dicrurus paradiseus Linnaeus) are less so, but more sensitive and reliable alarm-callers. We hypothesized that flock participants would be attracted to the playback of both species more than to the clearly non-nuclear yellow-fronted barbet (Megalaima flavifrons Cuvier). Further, we hypothesized that insectivores would prefer babbler vocalizations, as babblers could facilitate their foraging in several ways. We found that the response of insectivores was three times greater during babbler or drongo playback, and eight times greater during playback of these two species together, than during barbet playback or silence. Insectivores did not show, however, any difference in their response to babbler as compared to drongo playback; omnivores and frugivores responded relatively equally to all treatments. Our results show that birds with high propensity to flock, such as insectivores, use the vocalizations of nuclear species to locate flocks and that a sentinel species may be as attractive as a highly gregarious species.
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