EXPERIMENTAL EVIDENCE OF ALARM EAVESDROPPING ON AMAZONIAN MIXED-FLOCK FOLLOWERS BY TWO SOCIAL SENTINEL SPECIES: THE DUSKY-THROATED ANTSHRIKE THAMNOMANES ARDESIACUS AND THE CINEREOUS ANTSHRIKE T. SCHISTOGYNUS

Abstract

Mixed-species flocks of bird represent an ecological phenomena resulting from a diversity of complex interactions and evolutionary pressures. Paleotropical, Neotropical and temperate forests include many examples of passerine bird who interact by forming mixed-species flocks and by eavesdropping on the alarm calls produced by heterospecifics. In the mixed-flock systems of Amazonia, species of antshrike (Thamnomanes sp.) have been shown to provide other flock members valuable information regarding threats from predation through their frequent alarm calls. Several species of flock attendees respond to the alarms of antshrikes with predator-avoidance behaviors, but no study has yet shown a reciprocal relationship between flock members and flock-leading antshrikes. Apparently distinct from mixed-flock systems of other forests, there has been little evidence to support that flock-leading species of antshrike benefit from the alarm signals of other flock members. Using alarm signals recorded during in situ exposures of mixed-flock species to trained raptors, we conducted a playback experiment to 16 different mixed flocks to see to what degree antshrikes may be eavesdropping on the alarms of Myrmotherula antwrens. 8 Dusky-throated Antshrike Thamnomanes ardesiacus and 8 Cinereous Antshrike T. schistogynus were presented with i) conspecific alarms ii) White-flanked Antwren Myrmotherula axillaris alarms iii) Grey Antwren M. menetriesii alarms and iv) a control. The results of our experiment support that the alarms of the two species of antwren illicit a significant response from both species of antshrike. Our model also shows the responses of antshrikes to antwren alarms are indistinguishable from their response to conspecific alarms, yet different from a control (Marginal R²=.36, Conditional R²=.37). This suggests that highly social flock-leading species such as Thamnomanes antshrikes may benefit from the information provided by flock mates regarding predators. Further, these patterns are consistent with eavesdropping interactions observed within mixed-flocks in ecologically similar but geographically distant forests.