Concordance of movements and songs enhances receiver responses to multimodal display in the starling.

Many animals produce signals that consist of vocalizations and movements to attract mates or deter rivals. We usually consider them as components of a single multimodal signal because they are temporally coordinated. Sometimes, however, this relationship takes on a more complex spatiotemporal character, resembling choreographed music. Timing is important for audio-visual integration, but choreographic concordance requires even more skill and competence from the signaller. Concordance should therefore have a strong impact on receivers; however, little is known about its role in audio-visual perception during natural interactions. We studied the effects of movement and song type concordance in audio-visual displays of the starling, Sturnus vulgaris. Starlings produce two types of movements that naturally appear in specific phrases of songs with a similar temporal structure and amplitude. In an experiment with a taxidermic robotic model, males responded more to concordant audio-visual displays, which are also naturally preferred, than to discordant displays. In contrast, the effect of concordance was independent of the specific combination of movement and song types in a display. Our results indicate that the concordance of movements and songs was critical to the efficacy of the display and suggest that the information that birds gained from concordance could not be obtained by adding information from movements and songs.

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

Interactions among groups are often mediated through signals, including coordinated calls such as duets, and the degree of temporal coordination within a group can affect signal efficacy. However, in addition to intrinsic duet quality, the spatial arrangement of callers also affects the timing of calls. So, can listeners discriminate temporal effects caused by intrinsic duet quality compared to spatial arrangement? Such discrimination would allow assessment of quality of duets produced by a pair, as distinct from transient extrinsic spatial effects. To address this issue, we studied experimentally the influence of intrinsic duet quality and spatial arrangement on the efficacy of Australian magpie-lark (Grallina cyanoleuca) vocal duets. Breeding pairs duet at varying distances from each other and to multiple neighbours. Coordinated duets are more effective territorial signals than uncoordinated duets, but it remains unclear whether listeners can discriminate the effects of quality and spatial arrangement. Our playback experiment showed that any deviation from perfect regularity of partners' notes reduced duet efficacy, but that lack of coordination due to spatial separation (slower tempo and offset of notes) had a lower effect on efficacy than effects due to intrinsic quality (irregularity). Our results therefore provide experimental evidence that the temporal organisation of group vocalisations could signal coalition quality independently of spatial effects.

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

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.

Deceptive vocal duets and multimodal display in a songbird.

Many group-living animals cooperatively signal to defend resources, but what stops deceptive signalling to competitors about coalition strength? Cooperative-signalling species include mated pairs of birds that sing duets to defend their territory. Individuals of these species sometimes sing 'pseudo-duets' by mimicking their partner's contribution, but it is unknown if these songs are deceptive, or why duets are normally reliable. We studied pseudo-duets in Australian magpie-larks, Grallina cyanoleuca, and tested whether multimodal signalling constrains deception. Magpie-larks give antiphonal duets coordinated with a visual display, with each sex typically choosing a different song type within the duet. Individuals produced pseudo-duets almost exclusively during nesting when partners were apart, but the two song types were used in sequence rather than antiphonally. Strikingly, birds hid and gave no visual displays, implying deceptive suppression of information. Acoustic playbacks showed that pseudo-duets provoked the same response from residents as true duets, regardless of whether they were sequential or antiphonal, and stronger response than that to true duets consisting of a single song type. By contrast, experiments with robot models showed that songs accompanied by movements of two birds prompted stronger responses than songs accompanied by movements of one bird, irrespective of the number of song types or singers. We conclude that magpie-larks used deceptive pseudo-duets when partners were apart, and suppressed the visual display to maintain the subterfuge. We suggest that the visual component of many species' duets provides the most reliable information about the number of signallers and may have evolved to maintain honesty in duet communication.

Acoustic location of conspecifics in a nocturnal bird: the corncrake Crex crex.

Although the use of sounds in spatial orientation is widespread among animals, only a few groups advanced such specific adaptations as echolocation. In contrast, practically all animals and night-active species in particular, must occasionally orient themselves relative to invisible but audible objects such as a hidden rival or predator. In this study, I would like to determine the impact of locating which involves the use of acoustic parameters of sender's vocalisations by receivers and changes of positions and triangulation of sender's vocalisations by receivers in estimating the distance to the sender during night-time territorial interactions of the corncrake (Crex crex). Males were subjected to two kinds of stimuli: approaching one, imitating the change of the distance of the calling intruder toward the focal male while keeping the direction constant, or stationary stimuli, involving acoustic stimulation with no motion. Although males subjected to approaching stimulation moved longer distances, in both stimuli groups, males moved predominantly toward or out of the playback speaker, and only occasionally made sideway movements. However, the results gave no evidence of corncrakes moving specifically in order to locate the source of the sound; they suggest that males moved toward or away from the already located sound. The fact that males moved longer distances in response to approaching than stationary stimuli indicates that they were able to perceive the change of the distance to the playback speaker based only on structural parameters or amplitude of the calls played.

Soft calls and broadcast calls in the corncrake as adaptations to short and long range communication.

Because birds' acoustic signals function in antagonistic interactions between males and in female attraction, a majority of vocalisations are loud. In contrast, some birds, additionally produce soft vocalisations in escalated agonistic and sexual contexts. Nevertheless, the relationship between the acoustic parameters of such signals and their function is not clear. Here I investigate the sound transmission degradation properties of soft and broadcast (loud) calls in the corncrake using calls with natural and changed amplitude. I show that, if played at the same amplitude, the maximum limit for communication distance with soft calls was significantly shorter than that of broadcast calls, indicating that frequency structure is important in determining the range of both signals independently of their amplitude. At the same time, the values of excess attenuation were lower for soft calls than for broadcast calls at most distances, which suggests that the short transmission of soft calls is achieved mostly due to their low and narrow frequency ranges, promoting their masking by ambient noise. Finally, contrary to soft calls, changes in the energy of tails of echoes in broadcast calls were associated with the distance of propagation, which might be useful in assessing the distance to senders. I suggest that the acoustic structure of soft vocalisations can be used to limit the range of the signal, which might be helpful in eavesdropping avoidance, whereas broadcast calls are designed for long-range transmission.

Does mating experience of male house crickets affect their behavior to subsequent females and female choice?

Male mating experience was shown to play an important role in settling conflicts between males; however, little is known about whether and how prior access to females influences male behavior during intersexual interactions and female choice itself. Here, I experimentally test this relationship in the house cricket (Acheta domesticus) by combining one-on-one interaction between the male and female with direct comparison of males by the female, but precluding aggression between males. I found that solitary males were more active during subsequent courtship displays than paired males, suggesting the detrimental effect of mating on courtship performance. At the same time, females spent significantly more time close to solitary males or playbacks of male's natural courtship songs, and responded positively to the condition of males, ignoring body size of males. In contrast, females responded similarly to computer-modified playbacks of courtship songs of solitary and paired males with standardized rate of phrases and amplitudes; however, when females were additionally allowed to contact with anesthetized males they spent more time close to bigger males, irrespective of the acoustic parameters of courtship songs. These results show that although females were able to differentiate between many behavioral and morphological characteristics of males, including voluntary and intrinsic ones, they preferred traits conditional upon the costliness of male's displays. In addition, mating experience appeared to be a crucial factor in the choice of a particular costly mating strategy by males.

Functionally similar acoustic signals in the corncrake (Crex crex) transmit information about different states of the sender during aggressive interactions.

We combined playback experiments with hormonal manipulations to study the information content of acoustic signals during aggressive interactions between male corncrakes. During territorial conflicts, fights are uncommon, but the intensity of signaling usually increases. Such signals can be temporally and contextually associated with many aggressive behaviors and most likely function as threats or as indicators of the sender's quality or motivation. However, such correlational data are unsatisfactory for the proper interpretation of the function and information content of signals. Experimental tests are required to determine whether signals and aggressive behaviors are controlled by common or independent mechanisms. In our experiment, we assigned subjects to four groups: testosterone-implanted birds, flutamide-implanted birds, birds with empty implants, and non-captured control birds. Males produced two types of calls (quiet soft calls and loud broadcast calls), both of which are known to be reliable predictors of aggressive escalation. When testosterone action was blocked with flutamide, males significantly limited the amount of time spent close to the playback speaker and stopped responding to playback with soft calls. Broadcast calling was unaffected by the blockage of testosterone. Conversely, increased levels of testosterone neither affected calling nor the time spent near the speaker, indicating a permissive, rather than a graded effect of androgens. We concluded that, despite the seemingly similar function, both signals may transmit information about different states of the sender; soft calls seem to imply a threat of force, while broadcast calls appear to be more similar to an announcement, which is only indirectly associated with a male's aggressive behavior.

No male identity information loss during call propagation through dense vegetation: the case of the corncrake.

Individually specific acoustic signals in birds are used in territorial defence. These signals enable a reduction of energy expenditure due to individual recognition between rivals and the associated threat levels. Mechanisms and acoustic cues used for individual recognition seem to be versatile among birds. However, most studies so far have been conducted on oscine species. Few studies have focused on exactly how the potential for individual recognition changes with distance between the signaller and receiver. We studied a nocturnally active rail species, the corncrake, which utters a seemingly simple disyllabic call. The inner call structure, however, is quite complex and expressed as intervals between maximal amplitude peaks, called pulse-to-pulse durations (PPD). The inner call is characterized by very low within-individual variation and high between-individuals difference. These variations and differences enable recognition of individuals. We conducted our propagation experiments in a natural corncrake habitat. We found that PPD was not affected by transmission. Correct individual identification was possible regardless of the distance and position of the microphone which was above the ground. The results for sounds from the extreme distance propagated through the vegetation compared to those transmitted above the vegetation were even better. These results support the idea that PPD structure has evolved under selection favouring individual recognition in a species signalling at night, in a dense environment and close to the ground.