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 birds represent an ecological phenomenon resulting from a diversity of complex interactions and evolu-tionary pressures. Tropical and temperate forests include many examples of passerine birds that interact by forming mixed-species flocks and by eavesdropping on the alarm calls produced by heterospecifics. In the mixed-flock systems of the Amazonia, species of antshrikes (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 antshrikes 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 experi-ment on 16 different mixed flocks to see to what degree antshrikes may be eavesdropping on the alarms of Myrmotherula antwrens. Eight Dusky-throated Antshrikes (Thamnomanes ardesiacus) and eight Cinereous Antshrikes (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 antwrens elicit a significant response from both species of antshrikes. Our model also shows that the responses of antshrikes to antwrens' alarms are indistinguishable from their response to conspecific alarms, yet different from a control (Marginal R2 = 0.36, Conditional R2 = 0.37). This suggests that highly social flock-leading species such as Tham-nomanes antshrikes may benefit from the information provided by flock mates regarding predators. Furthermore, these patterns are con-sistent with eavesdropping interactions observed within mixed-flocks in ecologically similar but geographically distant forests.