Reality and illusion: the assessment of angular separation of multi-modal signallers in a duetting bird

Abstract

The spatial distribution of cooperating individuals plays a strategic role in territorial interactions of many group-living animals, and can indicate group cohesion. Vocalizations are commonly used to judge the distribution of signallers, but the spatial resolution of sounds is poor. Many species therefore accompany calls with movement; however, little is known about the role of audio-visual perception in natural interactions. We studied the effect of angular separation on the efficacy of multimodal duets in the Australian magpie-lark, Grallina cyanoleuca . We tested specifically whether conspicuous wing movements, which typically accompany duets, affect responses to auditory angular separation. Multimodal playbacks of duets using robotic models and speakers showed that birds relied primarily on acoustic cues when visual and auditory angular separations were congruent, but used both modalities to judge separation between the signallers when modalities were spatially incongruent. The visual component modified the effect of acoustic separation: robotic models that were apart weakened the response when speakers were together, while models that were together strengthened responses when speakers were apart. Our results show that responses are stronger when signallers are together, and suggest that males were are able to bind information cross-modally on the senders' spatial location, which is consistent with a multisensory illusion.

The quality of avian vocal duets can be assessed independently of the spatial separation of signallers

“…Spatial effects on duet tempo and timing did affect responses to playbacks, potentially because the current spatial separation of callers is relevant to territorial defence. Consistent with our previous results 16 , 17 , there was a weakened response to duets with slow tempo or offset, which are both consequences spatial separation. The treatments in previous experiments, however, combined the vocal duet with visual display movements of taxidermic models.…”

“…Coordinated signalling can play a bonding role among related individuals or in mated pairs, such as in paired siamangs ( Hylobates syndactylus ) 7 , but it can also be strategically important during interactions with other groups 8 , 9 . As examples, sounds allow for a numerical assessment at a distance in lions ( Panthera leo ) 10 , spotted hyenas ( Crocuta crocuta ) 11 and subdesert mesites ( Monias benschi ) 12 ; and evaluation of the spatial distribution of callers in túngara frogs ( Physalaemus pustulosus ) 13 , black-capped chickadees ( Poecile atricapillus ) 14 , eastern towhees ( Pipilo erythrophthalmus ) 15 , Australian magpie-larks ( Grallina cyanoleuca ) 16 , 17 and orcas ( Orcinus orca ) 18 . The precision of timing may also allow the quality of the group or pair be assessed, as shown for duetting birds 19 , 20 .…”

“…In extreme cases, when the difference in distance is large, the offset may cause notes to overlap. Information on spatial arrangement of callers is useful in indicating readiness for immediate, joint cooperation 17 , 22 , 23 . However, given that the spatial arrangement of callers and listeners is not an intrinsic property of duet quality, listeners may prioritise temporal features of duets that are under immediate control by callers, and so more likely related to coalition quality.…”

“…However, in previous experimental work, coordinated and uncoordinated playbacks differed in multiple ways, so it was not possible to tell what features of temporal organisation affected response. In subsequent studies, birds responded less to offset duets than to regular ones, but the experiments were designed to examine audio-visual communication, so that the treatments also included robotic taxidermic models 16 , 17 . Here, we test experimentally the impact of temporal coordination of vocal duets related to both spatial separation and intrinsic duet quality.…”

The relative contribution of acoustic signals versus movement cues in group coordination and collective decision-making

“…On the other hand, vision has a greater angular resolution, making it more effective for determining the number of objects, direction and dynamic cues in groups. These attributes can supplement each other, and thus, enhance signal and communication efficiency between senders and receivers [144,145]. For example, when attempting to initiate a collective movement, white-faced capuchins display various behaviours, such as emitting ‘trill’ vocalizations, giving back-glances and/or moving at a slow speed, to increase the probability of a successful departure [40,146].…”