Foraging guild influences dependence on heterospecific alarm calls in Amazonian bird flocks

Interspecific eavesdropping on alarm calling has been considered evidence that species participating in mixed-species groups benefit from reduced risk of predation. Few studies, however, have examined interspecific variation in dependence on and ability to evaluate alarm signals in mixed-species groups. We conducted a playback experiment to evaluate how species in different foraging guilds varied in their response to alarm calls of birds that lead Amazonian mixed-species flocks in both upland and inundated forests. We predicted that species that search nearby substrates myopically would react more strongly to alarm calls (i.e., take longer to resume foraging) than flycatching species that search for insects at a greater distance from a perch. We used likelihood functions to model the latency response to resume foraging for both upland and inundated forests samples, and we were able to detect significant differences among different foraging guilds. Our results indicate that flycatching birds respond weakest in both forest types, but contrary to our predictions, live-leaf gleaners showed a stronger response to alarms than dead-leaf–gleaning insectivores in inundated forest and no difference in upland forest. These results suggest that foraging guild may underlie different levels of dependence on public versus private information and, thus, the dependence of different species on heterospecific informants. These different levels of dependence on alarm calls provide a potential mechanistic basis for understanding assembly rules of flocks.