Interspecific communication in mixed-species bird flocks of a Sri Lankan rain forest
Mixed-species bird flocks are ideal systems for investigating fundamental questions of community ecology. Traditionally, researchers have studied how flocking species avoid competition while foraging; other behavioral interactions have received less attention. I investigated the role information exchange plays in the organization of flocks in a rain forest in Sri Lanka. In particular, I focused on the two most frequent species: the highly gregarious leaf-gleaning Orange-billed Babbler Turdoides rufescens, and the less gregarious fly-catching Greater Racket-tailed Drongo Dicrurus paradiseus.
Alarm calls are an important type of information shared among species. Babblers make the most frequent and rapid calls to natural and artificial stimuli; however, their calls are often made to non-raptors. Drongo alarm calls are more reliable and sensitive to real threats. Though playback experiments I asked two questions about alarm calls and flock organization. First, why does more than one species alarm call? Hypotheses include (a) conspecific calls receive greater response, (b) information in heterospecific calls is imperfect, and (c) there are direct benefits to calling. Hypothesis (a) is unlikely because in a playback experiment at least one species responded as much to heterospecific as to conspecific alarms. A second question was whether species prefer to associate with babblers or with drongos. Heterospecific birds were attracted to drongo playback as much as to babbler playback, demonstrating that an alarm-calling species may be as important to flock organization as a highly gregarious species.
Another form of communication in flocks is practiced by drongos that vocally mimic other species. Drongo mimicry is contextual: drongos include other species' alarm calls in their own alarm vocalizations and other species' songs in their own songs. I hypothesized that drongos might reform flocks through mimicry, since they are well known to benefit by catching insects other species disturb. I found that twice as many heterospecific birds were attracted to playback of drongo mimicry than when mimicry was removed. Drongo mimicry is thus an example of unidirectional and perhaps manipulative information flow in flocks. Put together, the playback experiments demonstrate that birds use other species' vocalizations to find flocks—or drongos that sound like flocks.