Heterospecific Alarm Research Articles

Within the sound of trouble: Do humans use pitch to correctly assess emotional arousal across species?

Comments on an article by Jay W. Schwartz , Kayleigh H. Pierson, and Alexander K. Reece (see record 2024-19488-001). In this issue, Schwartz et al. (2024) tackle the pitch rule in humans by testing to what extent we use pitch alone to judge emotional arousal across closely and distantly related animal species. The findings of Schwartz et al. open a number of intriguing possibilities for future research: Notably important additional steps would include to further investigate the accuracy of the pitch rule across closely and distantly related species. Upon this, in order to study the evolutionary ancestry of the pitch rule, it will be necessary to study its applicability across nonhumans. Particularly interesting would be the inclusion of subject species that have been found to eavesdrop on heterospecific alarm calls. Previous research (see Hoeschele, 2017 for a review) as well as present findings on human ratings of macaque versus cricket calls also suggest that we should additionally focus on sound features that compliment emotional arousal rating beyond pitch such as spectral information. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Behavioral responses to predator and heterospecific alarm calls are habitat-specific in Eurasian tree sparrows

Acoustic communication plays a vital role in predator-prey interactions. Although habitat structure has been shown to affect anti-predator tactics, little is known about how animals vary their behaviors in response to predator calls or heterospecific alarm calls in different environments. Here we used sound playbacks to test the responses of Eurasian tree sparrows (Passer montanus) foraging in harvested/unharvested rice paddy and open residential area. In the first experiment, we tested their behavioral responses to dove calls, male common cuckoo (Cuculus canorus) calls, hawk-like calls mimicked by female common cuckoo, sparrowhawk (Accipiter nisus) calls, and human yell calls produced to scare birds (predator signal playbacks). In the second experiment, we tested their behavioral responses to the Japanese tit’s (Parus minor) territorial songs and alarm calls (heterospecific alarm signal playbacks). Results showed that the tree sparrows had less fleeing in unharvested ripe rice paddy than in harvested rice paddy and open residential area. In predator signal playbacks, call type affected the escape behavior of sparrows in unharvested rice paddy and open residential area but not harvested rice paddy. In alarm signal playbacks, tit alarm calls evoked more fleeing than territorial songs in harvested rice paddy and open residential area but not unharvested rice paddy. These results suggest that anthropogenic habitat changes may influence avian anti-predator tactics.

Auditory risk recognition is socially transmitted across territory borders in wild birds

Prey species commonly assess predation risk based on acoustic signals, such as predator vocalizations or heterospecific alarm calls. The resulting risk-sensitive decision-making affects not only the behavior and life-history of individual prey, but also has far-reaching ecological consequences for population, community, and ecosystem dynamics. Although auditory risk recognition is ubiquitous in animals, it remains unclear how individuals gain the ability to recognize specific sounds as cues of a threat. Here, it has been shown that free-living birds (Wood Warblers Phylloscopus sibilatrix) can learn to recognize unfamiliar, complex sounds (samples of punk rock songs) as cues of a threat from conspecifics holding adjacent territories during the spring breeding season. In a playback experiment, Wood Warblers initially ignored the unfamiliar sounds, but after repeatedly hearing that these sounds trigger alarm calling reaction of neighbors, most individuals showed an anti-predator response to them. Moreover, once learned soon after nestlings hatching, the anti-predator response of parents toward previously unfamiliar sounds was then retained over the entire nestlings rearing period. These results demonstrate that social learning via the association of unfamiliar sounds with known alarm signals enables the spread of anti-predator behavior across territory borders and provides a mechanism explaining the widespread abilities of animals to assess predation risk based on acoustic cues.

Domestic chicken hens can eavesdrop on the alarm calls of wild tits

Eavesdropping on heterospecific alarm calls is common in nature, and successful eavesdropping on heterospecific call information can provide benefits to animal survival. Previous studies have shown that free-ranging flocks of domestic chickens can respond to avian heterospecific alarm calls. However, there are limited studies on whether solitary active domestic chickens (Gallus gallus domesticus), especially hens with chicks eavesdrop on heterospecific alarm calls to detect different types of predators and adjust their response depending on whether the predators are a threat to themselves or their young. In the present study, we evaluated the behavioral responses of hens with chick activity to two alarm calls (test playback) and territorial calls (control playback) of wild Japanese tits (Parus minor). The results showed that hens were more vigilant toward the alarm calls than territorial calls and that anti-predator behavior differed significantly between the two alarm calls as predicted: they engage in self-preservation behaviors when the call informs about aerial predators, and in protective behaviors of their chicks when the calls inform about ground predators. In the later case, the number of chicks also has an impact on the behavior of hens (dilution effect). This suggests that hens can eavesdrop on the alarm call of Japanese tits, identify predator information in the alarm call, and adjust the anti-predation strategy based on the type of alarm and the number of chicks.

Noise constrains heterospecific eavesdropping more than conspecific reception of alarm calls.

Many vertebrates eavesdrop on alarm calls of other species, as well as responding to their own species' calls, but eavesdropping on heterospecific alarm calls might be harder than conspecific reception when environmental conditions make perception or recognition of calls difficult. This could occur because individuals lack hearing specializations for heterospecific calls, have less familiarity with them, or require more details of call structure to identify calls they have learned to recognize. We used a field playback experiment to provide a direct test of whether noise, as an environmental perceptual challenge, reduces response to heterospecific compared to conspecific alarm calls. We broadcast superb fairy-wren (Malurus cyaneus) and white-browed scrubwren (Sericornis frontalis) flee alarm calls to each species with or without simultaneous broadcast of ambient noise. Using two species allows isolation of the challenge of heterospecific eavesdropping independently of any effect of call structure on acoustic masking. As predicted, birds were less likely to flee to heterospecific than conspecific alarm calls during noise. We conclude that eavesdropping was harder in noise, which means that noise could disrupt information on danger in natural eavesdropping webs and so compromise survival. This is particularly significant in a world with increasing anthropogenic noise.

Tit alarm calls trigger anti-predator behavior in free-range domestic chickens

Interspecific call transmission is a common information-gathering behavior in birds. Unlike centralized large-scale chicken farming in urban areas, domestic chickens (Gallus gallus domesticus) in remote mountainous areas of China are often raised free-range by individual households. Free-range domestic chickens are exposed to many natural predators and can increase their chances of survival if they can respond to heterospecific alarm signals. To this end, we observed free-ranging domestic chickens in Miao village communities around Leishan County, Guizhou, southwestern China, and we performed comparisons on the behavioral responses of these chickens to the territorial song and alarm calls of wild Japanese tits (Parus minor). Our findings showed that the chickens were more vigilant to Japanese tit alarm calls than to territorial songs and exhibited significant anti-predator behaviors such as vigilance and runaway. Furthermore, there was significant difference in behavioral response to the two different alarm calls, with greater vigilance to tit alarm calls for Siberian chipmunks (Tamias sibiricus) than for sparrowhawks (Accipiter nisus). This suggests that domestic chickens can differentiate between territorial songs and alarm calls of wild Japanese tits, and there is a behavioral difference in the two types of alarm calls of Japanese tits.

The evolution of eavesdropping on heterospecific alarm calls: Relevance, reliability, and personal information.

Interceptive eavesdropping on the alarm calls of heterospecifics provides crucial information about predators. Previous research suggests predator discrimination, call relevance, reliability, and reception explain when eavesdropping will evolve. However, there has been no quantitative analysis to scrutinize these principles, or how they interact. We develop a mathematical framework that formalizes the study of the key principles thought to select for eavesdropping. Interceptive eavesdropping appears to be greatly affected by the threat faced by caller and eavesdropper, as well as presence of informational noise affecting the detection of calls and predators. Accordingly, our model uses signal detection theory to examine when selection will favor alarm calling by a sender species and fleeing by an eavesdropping receiver species. We find eavesdropping is most strongly selected when (1) the receiver faces substantial threats, (2) species are ecologically similar, (3) senders often correctly discriminate threats, (4) receivers often correctly perceive calls, and (5) the receiver's personal discrimination of threats is poor. Furthermore, we find (6) that very high predation levels can select against eavesdropping because prey cannot continuously flee and must conserve energy. Reliability of heterospecific calls for identifying threats is thought to be important in selecting for eavesdropping. Consequently, we formally define reliability, showing its connection to specificity and sensitivity, clarifying how these quantities can be measured. We find that high call relevance, due to similar vulnerability to predators between species, strongly favors eavesdropping. This is because senders trade-off false alarms and missed predator detections in a way that is also favorable for the eavesdropper, by producing less of the costlier error. Unexpectedly, highly relevant calls increase the total number of combined errors and so have lower reliability. Expectedly, when noise greatly affects personally gathered cues to threats, but not heterospecific calls or detection of predators, eavesdropping is favored.

Comprehension of own and other species’ alarm calls in sooty mangabey vocal development

Primates understand the meaning of their own and other species’ alarm calls, but little is known about how they acquire such knowledge. Here, we combined direct behavioural observations with playback experiments to investigate two key processes underlying vocal development: comprehension and usage. Especifically, we studied the development of con- and heterospecific alarm call recognition in free-ranging sooty mangabeys, Cercocebus atys, across three age groups: young juveniles (1–2y), old juveniles (3–4y) and adults (> 5y). We observed that, during natural predator encounters, juveniles alarm called to a significantly wider range of species than adults, with evidence of refinement during the first four years of life. In the experiments, we exposed subjects to leopard, eagle and snake alarm calls given by other group members or sympatric Diana monkeys. We found that young juveniles’ locomotor and vocal responses were least appropriate and that they engaged in more social referencing (look at adults when hearing an alarm call) than older individuals, suggesting that vocal competence is obtained via social learning. In conclusion, our results suggest that alarm call comprehension is socially learned during the juvenile stage, with comprehension preceding appropriate usage but no difference between learning their own or other species’ alarm calls.Significance statementUnder natural conditions, animals do not just interact with members of their own species, but usually operate in a network of associated species. However, ontogenetic research on primate communication frequently ignores this significant element. We studied the development of con- and heterospecific alarm call recognition in wild sooty mangabeys. We found that communicative competence was acquired during the juvenile stages, with alarm call comprehension learning preceding appropriate vocal usage and with no clear difference in learning of con- and heterospecific signals. We also found that, during early stages of life, social referencing, a proactive form of social learning, was key in the acquisition of competent alarm call behaviour. Our results show that primates equally learn to interpret alarm calls from their own and other species during their early stages of life and that this learning process is refined as the animals mature.

Avian Alarm Calls Do Not Induce Anti-Predator Response in Three Anuran Species.

Many species produce alarm calls in response to predators, and the anti-predator signals are often used by other species. Eavesdropping on heterospecific alarm calls has been widely found in bird and mammal species. Other taxa, such as reptiles and amphibians, however, receive limited attention at present. Here, we selected three types of alarm calls of Japanese Tits (Parus minor) that were evoked by the Siberian Chipmunk (Eutamias sibiricus), Eurasian Sparrow Hawk (Accipiter nisus), and model snake (Elaphe spp.), respectively, and then carried out playback experiments to test whether three frog species changed their behaviors in response to the three treatments of Japanese Tit calls while the tit's territory song was used as a control. The results showed that Little Torrent Frogs (Amolops torrentis), Ornamented Pygmy Frogs (Microhyla fissipes) and Spot-legged Treefrogs (Polypedates megacephalus) did not jump off their positions in response to the same four acoustic signals. They also did not change their calling behaviors in response to the alarm calls of Japanese Tits. This study found no evidence that these anuran species can eavesdrop on heterospecific tits' alarm signals.

Free-range domestic chickens can distinguish between different alarm calls of Japanese tits.

Heterospecific eavesdropping of alarm information is an anti-predator adaptation in the wild as it improves the monitoring of shared predators and hence provides opportunities to avoid predators. Our current understanding of heterospecific eavesdropping information is based primarily on studies of bird, primate, or ungulate populations in the wild, whereas little is known regarding the ability of domesticated species to respond to alarm signals from wild individuals. Using sound playback experiments, we assessed the behavioral responses of free-range domestic chickens (Gallus gallus domesticus) in Hainan to alarm calls (test playback) and songs (control playback) of wild Japanese tits (Parus minor). The results revealed that free-range domestic chickens showed greater vigilance to the alarm calls of Japanese tits than to the songs of Japanese tits, especially their alarm calls for Siberian chipmunks (Tamias sibiricus). Furthermore, individual-foraging single chickens were significantly more vigilant than those foraging collectively in groups. Our findings suggest that free-range domestic chickens are as capable as wild birds in responding to heterospecific alarm calls and are likely to be able to distinguish between different types of heterospecific alarm calls.

Heterospecific eavesdropping on disturbance cues of a treefrog.

Alarm signals and cues are crucial to animal survival and vary greatly across species. Eavesdropping on heterospecific alarm signals and cues can provide eavesdroppers with information about potential threats. In addition to acoustic alarm signals, evidence has accumulated that chemical alarm cues and disturbance cues can also play a role in alerting conspecifics to potential danger in adult anurans (frogs and toads). However, there is very little known about whether disturbance cues are exploited by heterospecifics. In the present study, we conducted a binary choice experiment and a prey chemical discrimination experiment, respectively, to test the responses of a sympatric anuran species (red webbed treefrogs, Rhacophorus rhodopus) and a sympatric predator species (Chinese green tree vipers, Trimeresurus stejnegeri) to disturbance odors emitted by serrate-legged small treefrogs (Kurixalus odontotarsus). In the binary choice experiment, we found that the presence of disturbance odors did not significantly trigger the avoidance behavior of R. rhodopus. In the prey chemical discrimination experiment, compared with odors from undisturbed K. odontotarsus (control odors) and odorless control, T. stejnegeri showed a significantly higher tongue-flick rate in response to disturbance odors. This result implies that disturbance odor cues of K. odontotarsus can be exploited by eavesdropping predators to detect prey. Our study provides partial evidence for heterospecific eavesdropping on disturbance cues and has an important implication for understanding heterospecific eavesdropping on chemical cues of adult anurans.

The role of donor and receiver size in the response to public cues in Hart’s rivulus, Anablepsoides hartii

Abstract Aquatic prey use chemical alarm cues as public information sources to optimize behavioural decisions. Recent studies suggest that the contextual value of these cues is shaped by their source, the size of the donor relative to the receiver, and the size of the receiver itself. Here, we exposed Hart’s rivulus (Anablepsoides hartii) to conspecific or heterospecific alarm cues from donors that were either smaller or larger than the mean focal rivulus size. Smaller rivulus reduced foraging in response to conspecific and heterospecific cues, regardless of donor size. However, larger rivulus exhibited no reduction in foraging towards small conspecific cues and increased foraging towards small heterospecific cues. Additionally, while conspecific donors elicited strong predator avoidance, rivulus exhibited stronger responses to large vs. small heterospecific cues. Our results demonstrate that the value of alarm cues is shaped by the interacting effects of receiver size and the size and species of cue donors.

Anti-Predation Responses to Conspecific versus Heterospecific Alarm Calls by the Nestlings of Two Sympatric Birds.

Simple SummaryBirds can adjust their behavior by reacting to sound cues. Parents can adjust the feeding rate of nestlings by their response to begging calls. Acoustics play an important role in parent–offspring communication. When adults perceive predators, they can make mobbing alarm calls to warn other individuals. Chicks can also respond to the alarm calls of parents and thus reduce their predation risk. We tested whether the chicks of two species could hear, and respond to, conspecific and heterospecific acoustic alarm signals. Chicks were found to eavesdrop on conspecific and heterospecific mobbing alarm calls. Acoustical similarities between the alarm calls may explain why chicks can recognize heterospecific calls.Predation is generally the main cause of bird mortality. Birds can use acoustic signals to increase their predation survival. Bird response to mobbing alarm calls is a form of anti-predation behavior. We used a playback technique and acoustic analysis to study the function of mobbing alarm calls in the parent–offspring communication of two sympatric birds, the vinous throated parrotbill (Sinosuthora webbianus) and oriental reed warbler (Acrocephalus orientalis). The chicks of these two species responded to conspecific and heterospecific mobbing alarm calls by suppressing their begging behavior. The mobbing alarm calls in these two species were similar. Mobbing alarm calls play an important role in parent–offspring communication, and chicks can eavesdrop on heterospecific alarm calls to increase their own survival. Eavesdropping behavior and the similarity of alarm call acoustics suggest that the evolution of alarm calls is conservative and favors sympatric birds that have coevolved to use the same calls to reduce predation risk.

Is the urgency message encoded in heterospecific alarm calls perceived by domestic chickens?

AbstractVertebrate alarm calls can signal danger, and often encode information about predator proximity. Some species of birds alter the number of notes of their alarm calls to reflect urgency, with high‐urgency alarm calls having more notes than low‐urgency calls. Not only do individuals perceive the urgency message encoded in the alarm calls of conspecifics, but also in the alarm calls given by heterospecifics (sympatric and allopatric species). Yet, although well studied in wild species, the capacity to use heterospecific graded alarm call systems has not been investigated in domestic animals. Domestication may have altered the perceptual and communicative abilities of domestic species because they are less threatened by predators. Using playbacks conducted on domestic naked neck chickens (Gallus gallus domesticus) in France, we tested whether this species is able to decode information about urgency in alarm calls produced by a sympatric species; the common chaffinch (Fringilla coelebs), and by an allopatric species; the Australian magpie (Cracticus tibicen). We found the latency to resume foraging behaviour was higher after the high‐urgency call playbacks than the low‐urgency call playbacks, suggesting that the urgency encoded in heterospecific calls was perceived by chickens. However, results seem to indicate that the chickens are not able to discriminate between alarm and control (songs) sounds, but simply respond to specific sound characteristics of each playback stimulus. Our results raise some questions about how animals perceive and attend to information provided by heterospecific alarms. We hope this study will encourage future work with domesticated and nondomesticated animals.

Innate responses to con‐ and heterospecific alarm chemicals in juveniles of a freshwater shrimp

AbstractReleased substances from injured conspecifics and heterospecifics (alarm chemicals) often trigger inducible defenses (e.g., behavioral, morphological, and life‐history antipredator defenses). Although there are good reasons to assume that the detection of damage‐release chemicals may be inherited, immediate behaviors to alarm chemicals remain to be assessed in many aquatic taxa. This study examines changes in locomotor activity in juveniles of a freshwater shrimp, Palaemon argentinus, in response to (1) distilled water; (2) macerated adult conspecifics, and (3) macerated adult heterospecifics (a closely related, non‐coexisting species). We tested three sets of juveniles coming from (1) alarm cues‐unexposed mothers (naïve juveniles), (2) mothers exposed to macerated conspecifics, and (3) mothers exposed to macerated heterospecifics. Juveniles were tested for changes in locomotor activity before and after the addition of a corresponding stimulus. Locomotor activity of naïve juveniles decreased in response to con‐ and heterospecific stimuli, while it did not change in response to distilled‐water controls. Effect size, as Cohen's dRM (the standard mean difference between the pre‐ and the post‐stimulus period using the pooled standard deviation) indicated that the reduction in locomotor activity was strong in response to conspecific stimulus (dRM = −1.521), but it was moderate in response to heterospecific stimulus (dRM = −0.682). While the responses of naïve juveniles indicate an innate recognition of alarm substances, the responses to heterospecifics likely arise from the recognition of chemicals with a molecular structure partly conserved between related species, as predicted by the phylogenetic relatedness hypothesis. Furthermore, juveniles from treated mothers behaved differently than the naïve ones, suggesting that juvenile behavioral traits were modified through embryonic experience. As the embryonic exposure to alarm cues occurs during embryogenesis, we assume that the mothers have perceived these cues also, which might have caused part of the observed developmental effects.

Exploration of aversive bioacoustics for the effective management of invasive green anoles (Anolis carolinensis)

The green anole (Anolis carolinensis) is an invasive lizard on the Ogasawara Islands, Japan. Green anoles have negatively impacted the native fauna, and thus, green anole eradication measures, such as the use of PTFE-sheet fencing to restrict their movement have been implemented. However, the effectiveness of fencing appears inadequate; therefore, new methods are needed to deter this species. In this study, we explored the use of aversive bioacoustics by testing whether green anoles froze or stayed away on being exposed to predatory sounds and heterospecific alarm calls. Green anoles showed longer freezing times on hearing bird calls than on hearing no sound or single tones. In addition, they stayed far from the audio sources playing the calls of the red-tailed hawk (Buteo jamaicensis) and alarm calls of the warbling white-eye (Zosterops japonicus). Our results suggest that green anoles avoid certain bioacoustics, especially the calls of B. jamaicensis and the alarm call of Z. japonicus. Hence, these bioacoustics can be used as an effective control method to restrict the invasion and dispersion of green anoles.

Male superb lyrebirds mimic functionally distinct heterospecific vocalizations during different modes of sexual display

Mimicry has long been a focus of research, but little is known about how and why many species of bird incorporate imitations of heterospecific sounds into their vocal displays. Crucial to understanding mimetic song is determining what sounds are mimicked and in what contexts such mimicry is produced. The superb lyrebird, Menura novaehollandiae, is a large oscine passerine with a lek-like mating system. Both sexes are accurate and versatile vocal mimics of the vocalizations of other species, but little is known about how males deploy their repertoire of mimicked sounds across contexts. Using extended focal watches, we recorded adult males displaying during the breeding season. We found that males mimicked heterospecific songs and nonalarm calls during ‘recital’ displays usually performed while they were perched and visually inconspicuous. In contrast, during visually conspicuous ‘dance’ displays, commonly performed on display mounds, males only mimicked heterospecific alarm calls. While much rarer than recital displays, dance displays were associated with the final stages of mate choice preceding copulation. These results provide the first evidence of any species varying its repertoire of mimicked sounds with different sexual contexts. Previous work suggests that mimicry in dance displays functions deceptively to manipulate the antipredator responses of females during the final stages of courtship. However, the structure and context of recital mimicry closely resembles the sexual advertisement song performed by nonmimicking songbirds. Given the importance of mimicry in the acoustic ecology of lyrebirds, our results suggest that with recital song males advertise the quality of their mimicry as it likely benefits both male and female offspring. Our finding that male superb lyrebirds mimic functionally distinct heterospecific vocalizations during different modes of courtship suggests that the evolution and maintenance of avian vocal displays are more complex than previously thought.

Behavioural responses to con- and heterospecific alarm cues by an alien and a coexisting native fish

The monkey goby Neogobius fluviatilis is an invasive Ponto-Caspian fish that enters habitats of the native gudgeon Gobio gobio in European freshwaters, likely belonging to the same prey guild. Their abilities to detect and avoid predation have been poorly understood, although these traits may contribute to the competitive advantage and drive the invasion success of the goby. We tested intra- and interspecific responses of fish from sympatric populations to damage-released alarm cues (skin extract) in laboratory. Both species of fish responded to conspecific and heterospecific cues, but their responses to conspecifics were more diverse (changed social distancing among individuals, reduced vertical and horizontal movement) than those elicited by heterospecifics (changed social distancing only). Moreover, the fish differed in their anti-predation behaviour: the gudgeon increased whilst the monkey goby decreased inter-individual distances and only the gudgeon exhibited thigmotaxis and reduced mobility in response to the conspecific cues. Interspecific differences show that the species exhibit distant anti-predation strategies. This might be associated with their phylogenetic distance and/or higher differentiation of their ecological niches than commonly assumed. Besides, our results suggest that alien species be included in the interspecific exchange of information in local fish assemblages.

Living in mixed species groups promotes predator learning in degraded habitats

Living in mix-species aggregations provides animals with substantive anti-predator, foraging and locomotory advantages while simultaneously exposing them to costs, including increased competition and pathogen exposure. Given each species possess unique morphology, competitive ability, parasite vulnerability and predator defences, we can surmise that each species in mixed groups will experience a unique set of trade-offs. In addition to this unique balance, each species must also contend with anthropogenic changes, a relatively new, and rapidly increasing phenomenon, that adds further complexity to any system. This complex balance of biotic and abiotic factors is on full display in the exceptionally diverse, yet anthropogenically degraded, Great Barrier Reef of Australia. One such example within this intricate ecosystem is the inability of some damselfish to utilize their own chemical alarm cues within degraded habitats, leaving them exposed to increased predation risk. These cues, which are released when the skin is damaged, warn nearby individuals of increased predation risk and act as a crucial associative learning tool. Normally, a single exposure of alarm cues paired with an unknown predator odour facilitates learning of that new odour as dangerous. Here, we show that Ambon damselfish, Pomacentrus amboinensis, a species with impaired alarm responses in degraded habitats, failed to learn a novel predator odour as risky when associated with chemical alarm cues. However, in the same degraded habitats, the same species learned to recognize a novel predator as risky when the predator odour was paired with alarm cues of the closely related, and co-occurring, whitetail damselfish, Pomacentrus chrysurus. The importance of this learning opportunity was underscored in a survival experiment which demonstrated that fish in degraded habitats trained with heterospecific alarm cues, had higher survival than those we tried to train with conspecific alarm cues. From these data, we conclude that redundancy in learning mechanisms among prey guild members may lead to increased stability in rapidly changing environments.

Visual obstruction, but not moderate traffic noise, increases reliance on heterospecific alarm calls

AbstractAnimals rely on both personal and social information about danger to minimize risk, yet environmental conditions constrain information. Both visual obstructions and background noise can reduce detectability of predators, which may increase reliance on social information, such as from alarm calls. Furthermore, a combination of visual and auditory constraints might greatly increase reliance on social information, because the loss of information from one source cannot be compensated by the other. Testing these possibilities requires manipulating personal information while broadcasting alarm calls. We therefore experimentally tested the effects of a visual barrier, traffic noise, and their combination on the response of Australian magpies, Cracticus tibicen, to heterospecific alarm calls. The barrier blocked only visual cues, while playback of moderate traffic noise could mask subtle acoustic cues of danger, such as of a predator’s movement, but not the alarm-call playback. We predicted that response to alarm calls would increase with either visual or acoustic constraint, and that there would be a disproportionate response when both were present. As predicted, individuals responded more strongly to alarm calls when there was a visual barrier. However, moderate traffic noise did not affect responses, and the effect of the visual barrier was not greater during traffic-noise playback. We conclude that a reduction of personal, visual information led to a greater reliance on social information from alarm calls, confirming indirect evidence from other species. The absence of a traffic-noise effect could be because in Australian magpies hearing subtle cues is less important than vision in detecting predators.