Mobbing Calls Research Articles

Location and caller familiarity influence mobbing behaviour and the likely ecological impact of noisy miners around colony edges

Mobbing is a widespread, vocally coordinated behaviour where species approach and harass a threat. The noisy miner (Manorina melanocephala) is a notorious native Australian honeyeater, well‐known for its hyperaggressive mobbing. Numerous studies have identified negative impacts of their mobbing behaviour, highlighting the exclusion of competitors from colony areas and the resulting loss of woodland‐bird biodiversity. Despite this, few studies have investigated mobbing itself, and our understanding of the factors which influence its expression remains limited. Here, we use a field‐based playback experiment to investigate whether mobbing responses vary in relation to colony borders and caller familiarity. Noisy miners were more likely to respond, reacted more quickly and responded more strongly to mobbing calls broadcast inside as opposed to outside the colony. These behavioural differences likely arise from variation in the relative costs and benefits of responding. When noisy miners did mob outside the colony, more individuals joined in response to unfamiliar as opposed to familiar callers. Our results reveal that noisy miner mobbing may not be as indiscriminate as often assumed, with caller familiarity and location influencing this behaviour. We suggest there are benefits to greater consideration of the factors impacting noisy miner mobbing behaviour.Keywords: Avian biodiversity, interspecific aggression, mobbing behaviour, noisy miner, vocal communication

Relative divergence of mobbing calls and songs structures in passerine birds

Divergence and convergence in acoustic signals may be driven by different processes. Uncertainty about which of these processes best explains the patterns of acoustic variation between species has fuelled a long-standing debate in evolutionary biology. In particular, the features of acoustic signals are expected to vary according to their functional support. To compare the relative divergence of vocalizations according to their function, we examine two types of signals within 23 bird species, whilst controlling for species size, phylogeny and within-species variation: (i) mobbing calls emitted to recruit both conspecifics and heterospecifics against a predator and (ii) territorial songs, a signal playing an important role in sexual and social communication. We found that divergence of acoustic features varies according to their function (mobbing calls vs. territorial songs). Furthermore, species size influences spectral features, while phylogeny explained acoustic variation in only one of the variables measured: note richness. Finally, our results reveal that the acoustic characteristics can vary greatly within species, regardless of the vocalization type, indicating that such variations cannot be ignored when performing comparative analyses across species.

Both learning and syntax recognition are used by great tits when answering to mobbing calls

Abstract Mobbing behavior, in addition to its complex cooperative aspects, is particularly suitable to study the mechanisms implicated in heterospecific communication. Indeed, various mechanisms ranging from pure learning to innate recognition have been proposed. One promising yet understudied mechanism could be syntax recognition, especially given the latest works published on syntax comprehension in birds. In this experiment, we test whether great tits use both learning and syntax recognition when responding to heterospecifics. In the first part of the experiment, we demonstrate that great tits show different responses to the same heterospecific calls depending on their sympatric status. In the second part, we explore the impact of reorganizing the notes of the heterospecific mobbing calls to fit the syntax of great tits. Great tits showed an increased mobbing response toward the heterospecific calls when they shared their own call organization. Our results corroborate the recent finding that syntactic rules in bird calls may have a strong impact on their communication systems and enlighten how various mechanisms can be used by the same species to respond to heterospecific calls.

The impact of high temperatures on bird responses to alarm calls

Given the current pace of climate change, high temperature events will become increasingly frequent in many parts of the world. Predicting how high temperatures will impact the behavior of songbirds—highly sensitive to temperature change due to their tendency to be small in size, and to have high metabolic rates and diurnal habits—is therefore crucial and urgent. However, the behavioral impacts of high temperatures on acoustic communication in birds have rarely been studied. Responsiveness to antipredator signals is an important behavior to consider because failure to detect such signals could be lethal. We investigated whether, in great tits (Parus major), high temperatures would affect behavioral responses to playback of conspecific mobbing calls. We found a significant lag effect of temperature on how closely great tits would approach the playback speaker, with birds approaching less closely at higher temperatures. Further, we found that the emission of mobbing calls by great tits was affected by the current ambient temperature, with birds calling less at higher temperatures. The results suggest that at high temperatures, great tits change their tactic from active defense to less active response. High temperatures can thus induce behavioral shifts in great tits. In the current context of increasing average temperatures, such effect of temperature on response to vital indicators such as antipredator signals could impact survival when inducing greater risk of depredation.Significance statementClimate change is causing heatwaves to increase in number and intensity. High temperatures can reduce the ability of birds to respond to vocalizations. Here, we test if high temperatures affect the ability of great tits (Parus major) to respond to conspecific mobbing calls—these calls generally serve to mob a predator and to recruit conspecifics and heterospecifics to join the caller. At higher temperatures, great tits produce fewer mobbing vocalizations and approach the loudspeaker broadcasting mobbing calls less often.

The number of Great Tit mobbers influences the mobbing response of heterospecific birds

Many species mob predators to drive them away. Mobbing carries personal risk, but the risk of injury or death declines and the likelihood of repelling the predator increases in larger groups. The capacity to evaluate the number of mobbers before joining a mobbing group may be highly beneficial for individuals when deciding to join. Although recent studies have found that birds can use individual vocal discrimination to assess the number of conspecifics involved in initiating mobbing events, it is little known whether birds are able to evaluate the number of heterospecific mobbers. In this study, we investigated whether the number of heterospecifics responding to mobbing calls of Great Tits Parus major was influenced by the number of callers (a mix of Great Tit callers simulated by playbacks and live Great Tits attracted to playbacks, range one to seven callers). The total number of responding heterospecific individuals and species was positively influenced by the total number of callers, with heterospecifics responding more to larger than smaller Great Tit groups. However, these results may be driven by the total number of calls, call overlap and increased noise generated by the additional mobbers, and not the number per se. These findings therefore allow us to assess whether birds are more inclined to join a mobbing group when it consists of more heterospecific callers, but we cannot tell whether birds use individual vocal discrimination to assess the number of heterospecific callers.

Two common European songbirds elicit different community responses with their mobbing calls

Heterospecific communication is common for birds when mobbing a predator. However, joining the mob should depend on the number of callers already enrolled, as larger mobs imply lower individual risks for the newcomer. In addition, some ‘community informant’ species seem more reliable regarding the information transferred in mobbing calls. Birds should therefore rely on both the number of callers and the species identity of the caller(s) when mobbing. In the present study, we tested the potential interaction between two acoustic cues. In a playback experiment, we modified the number of callers (through an increased number of calling individuals correlated to an increased duty cycle) and the emitter species (crested tits versus coal tits). Overall, we found that soundtracks with three callers triggered more mobbing than soundtracks with one caller and that soundtracks with coal tits’ calls triggered more mobbing than soundtracks with crested tits’ calls. Our results therefore support the hypothesis that birds consider both the species and the number of callers when joining a mobbing chorus in winter. Finally, we replicated the experiment in spring and did not record the same responses from the bird community. Indeed, only soundtracks with three coal tits triggered a mobbing response, suggesting therefore that the seasonal context can affect the results of studies on heterospecific communication. The potential mechanisms implicated in the varying responses to different acoustic cues and different seasons are discussed and should deserve further investigations.

Antipredatory call behavior of lapwing species in an Afrotropical environment

Predation is an important source of natural selection on prey species and has resulted in adaptations such as antipredator vocal signals, which can alert others to the presence of predators and solicit cooperative attack. Although vocal alarm signals of birds have been well studied, they are poorly known in tropical African species. To address this lack of information, the antipredatory signals and responses of two lapwings (Wattled Lapwing Vanellus senegallus and Spur-winged Lapwing Vanellus spinosus) to potential predators were investigated using data collected from focal observation, distance measurements, focal recordings, and playback experiment. The lapwing calls elicited to predators were classified as alarm or mobbing calls based on whether the calls elicited alert behavior or attack from other lapwings. Discriminant linear analysis (DLA) was used to compare the time and frequency parameters of the call types measured in Raven PRO. Also, lapwings’ responses to intruders, alert and start distance, time of day, and latency, as well as the effects of flock size and distance to cover were examined. About 48% of all calls was correctly classified by DLA. The best predictors of call type for the lapwings were maximum frequency and high frequency. Both alarm and mobbing calls were elicited by African Wattled Lapwings to dogs and humans. Mobbing calls were elicited to intruders by the Spur-winged Lapwings. Alert distance was positively associated with start distance, and differed between morning and evening in both lapwings. With scarce information from tropical Africa, this study put in perspective vocal and antipredator behavior of lapwing species in Africa.

How great tits respond to urgency‐based information in allopatric Southern house wren mobbing calls

AbstractMany species of birds use alarm calls to signal information about predators, including the level of threat. Previous playback experiments suggest that the urgency response towards heterospecific calls is phylogenetically conserved, notably in the Paridae family. Using playback experiments conducted on European great tits (Parus major), we tested whether this species perceives information about urgency in mobbing calls produced by an allopatric non‐Paridae species, the Southern house wren (Troglodytes aedon bonariae), by broadcasting calls with high‐calling rate (high threat) and calls with low‐calling rate (low threat). We found that great tits tend to approach the loudspeaker during playbacks of calls with high‐calling rate more often than during playbacks of calls with low‐calling rate. Female great tits gave more calls during playbacks of calls with high‐calling rate than during playbacks of calls with low‐calling rate, whereas there was no significant difference in the number of calls given by males between playbacks of calls with high‐ and low‐calling rates. Thus, our results suggested that great tits perceived the urgency message encoded in calls given by an allopatric non‐Paridae species.

Discrimination of High- and Low-Threat Vocalizations: An Examination of Referential Communication in Black-Capped Chickadee (Poecile atricapillus) Mobbing Calls

Referential communication has been defined as the exchange of information regarding an object or event, but few studies have examined referential alarm calls in songbirds. In contrast, it has been well-supported that chickadees produce mobbing calls in response to predators that vary depending on the threat level posed, and the auditory brain areas of chickadees produce similar neural expression in response to predator calls and conspecific mobbing calls of the same threat level. This suggests that chickadees perceive these acoustically distinct vocalizations as similar, potentially as referent signals. In the current study, we trained 33 birds on an operant go/no-go discrimination task in which chickadees were presented with predator and mobbing calls of high- or low-threat. Following the first round of training, birds completed a second round with high- or low-threat calls and we predicted that birds would show transfer of training when contingencies (i.e., threat level regarding a species of predator) were the same between hetero- and conspecific vocalizations. However, high- and low-threat mobbing calls were not treated similarly to the corresponding predator’s calls. Our asymmetrical results may be due to the acoustic distinction between calls produced by two owl predator species, but we cannot make definitive conclusions about referential communication from this study. Nonetheless, we believe that this experiment was an important exploration of the perception of referential signal.

Anthropogenic noise alters the perception of a predator in a local community of great tits

Anthropogenic noise is increasing at an alarming rate in urban as well as rural ecosystems and detrimental effects have been reported on many animals relying on acoustic communication. Little is known regarding the impact of different types of noise on community level predator perception and breeding behaviour. Here, we presented a predator model alone or a predator combined with a specific noise type, such as traffic, a lawnmower or a chainsaw, to breeding pairs of great tits, Parus major , giving them the opportunity to perform mobbing behaviour that could provide public information to their neighbours. We then measured the provisioning behaviour of adjacent tit pairs, which were 50–150 m away from the focal pairs' nests. The adjacent breeding pairs decreased nest visits when focal birds were exposed to the sparrowhawk model or to the sparrowhawk model combined with the lawnmower noise. Neighbouring pairs did not reduce the rate of nest visits when focal birds were exposed to the hawk model combined with the traffic or the chainsaw noise. The distance from focal nests did not alter the noise effect on neighbouring pairs. We suggest that auditory masking is the most likely mechanism to explain why noise compromises public information. The chainsaw and traffic noise overlap with the frequency of mobbing calls and disrupt the transmission of auditory signals in the community. Further, when the hawk presentation was combined with the chainsaw noise, which was the type of noise that had the highest frequencies, the latency of mobbing by the focal birds was strongly increased, and the number of species involved in mobbing was reduced. This study demonstrates that the signalling function of mobbing is ineffective in noisy environments which ultimately impacts fitness via increased predation to adults or their offspring. • Focal great tits give mobbing calls in response to a sparrowhawk presentation. • Neighbouring pairs reduce nest visit rate when focal birds mob the hawk. • Neighbours do not respond to mobbing calls along with a chainsaw or traffic noise. • Acoustic masking by high-frequency noise disrupts the transmission of mobbing calls. • Low-frequency noise does not disrupt the transmission of public information.

Season does not influence the response of great tits (Parus major) to allopatric mobbing calls

Many species of birds emit mobbing calls to recruit prey to join mobbing events. This anti-predator strategy often involves several species and, therefore, implies heterospecific communication. Some species of tit exhibit a sensitivity to allopatric mobbing calls, suggesting that heterospecific recognition is based on an innate component. To date, however, we have no information on whether the perception of allopatric calls varies with season, despite seasonality playing an important role in the perception of heterospecific call in some species. In this study, I investigate the responses of European great tits (Parus major) to the calls of a North American bird species, the black-capped chickadee (Poecile atricapillus), during two seasons: spring and in autumn (breeding and non-breeding seasons, respectively). Great tits approached the sound source during both seasons, with no significant difference in response between seasons. These findings indicate that season does not influence the response of birds to allopatric calls, and will help to shed light on the evolution of interspecific communication.

Heterospecific eavesdropping of jays (Garrulus glandarius) on blackbird (Turdus merula) mobbing calls

Heterospecifics eavesdrop on mobbing calls and respond with appropriate behavior, but the functional aspects are less studied. Here, I studied whether jays (Garrulus glandarius) eavesdrop on blackbird (Turdus merula) mobbing calls in comparison to blackbird song. Furthermore, it was studied whether jays provided with extra information about predators differ in their response. Three different experimental designs were carried out: (1) control playback of blackbird song to control for the species’ presence, (2) experimental playback of different mobbing events of blackbirds towards different predators, (3) experimental playback similar to (2) but combined with different predator models. In the combined experiments, mobbing calls were tied to the respective visual stimuli. Comparing the experiments with and without predator presentation, a similar number of jays occurred during the playback-only experiment (n = 7) and the playback combined with model presentation (n = 6). However, during the playback-only experiment, jays approached the speaker closer and stayed for longer time in the nearer surrounding. These results show that jays need extra information to make an informed decision.

Mobbing behaviour in non-breeding flocks of a passerine bird in northern Patagonia

Abstract Animal cooperation occurs in both genetically related and unrelated groups of individuals, involving costs and benefits that have not been fully elucidated. For example, risky behaviour such as mobbing a predator would be selected if participants are genetic relatives because they share a fraction of their gene pool (i.e., kin selection or indirect benefits). However, in the absence of genetic relatedness, benefits can be achieved by direct benefits such as reciprocity or mutualism, among others. In this study we analyzed the cooperative mobbing behaviour in winter flocks of an endemic passerine of the austral temperate forests, the Thorn-tailed rayadito (Aphrastura spinicauda). We first tested whether the probability of and the latency to mobbing response differed depending on the acoustic stimulus perceived by the flock (i.e., conspecific mobbing calls vs. predator calls), and whether the intensity of the mobbing was related to the number of individuals and species participating. We found that flocks were more likely to approach the predator when the acoustic signal was a conspecific mobbing call than when it was the predator’s call, and that the intensity of mobbing increased with the number of participants. Secondly, we explored if the level of kinship within the group potentially played a role in the development of the mobbing behaviour. The proportion of close-relatives found in these flocks was low, and the within- and among-flock degree of kinship did not differ. This suggests that kin selection might not be related with the expression of mobbing behaviour in winter flocks.

Speciation by Sensory Drive in the Paragalago zanzibaricus Species Complex

Cryptic animal species are often distinguishable by nonvisual communication signals used in specific-mate recognition. The three cryptic taxa comprising the Paragalago zanzibaricus species complex use distinct loud calls that do not obviously reflect molecular phylogenetic relationships. Paragalago granti and P. cocos both have incremental calls but are not sister taxa; P. cocos is the sister taxon to P. zanzibaricus, which has a rolling (trilling) call like that of P. rondoensis, one of the outliers to the complex. To test current hypotheses of species delimitation, I conducted 378 playback trials, using conspecific and heterospecific loud calls and recording both vocal responses and movement toward the speaker. To investigate the sensory drive hypothesis (acoustic adaptation), I measured reverberation duration of 215 short “yap” mobbing calls and examined 282 sonotopes (local soundscapes). I tested the hypothesis that different species’ ear lengths allow the animals to filter out interfering frequencies in local background noise. On Zanzibar Island, P. zanzibaricus responded solely to conspecific calls (23/32). In South Africa, P. granti individuals responded to conspecific calls (12/22) but also sometimes to P. cocos calls (9/22). In Kenya, P. cocos individuals responded to conspecific calls (34/40) but also sometimes to P. zanzibaricus calls (8/40). The three habitats showed differences in reverberation duration, and soundscapes differed in background orthopteran frequencies. Incremental calls probably evolved in the ancestor of the complex, in response to high levels of reverberation and a continuous backdrop of katydid stridulation in dry forest. Secondary evolution of trilling and decreased ear size probably evolved as result of decreased reverberation and higher frequency katydid interference in the P. cocos–P. zanzibaricus ancestor. A loss of short units in P. zanzibaricus probably occurred as result of decreased reverberation in thicket. The study validates the three species of the complex and provides information supporting speciation by sensory drive.

How Subtle Protocol Choices Can Affect Biological Conclusions: Great Tits' Response to Allopatric Mobbing Calls

In the last ten years, numerous replicated studies showed divergent results from the original papers, leading to the recognition that science may be facing a replication crisis. Apart from fraud or natural population variability, different results may emerge from flexibility in the protocol and/or restricted sample sizes. Replication studies are therefore fundamental to assess how robust a finding can be. However, while the original authors may be prone to p-hacking (to collect data, select data or use statistical analyses until nonsignificant results become significant), the replication-authors are, on the contrary, probably unwittingly prone to show opposite results (i.e., null-hacking). In this study, we face the unique opportunity to compare replicated studies with no null-hacking bias. Two teams independently investigated the response of great tits (Parus major) to mobbing calls of an allopatric species, in their natural and reversed order, on the same population. We first discuss how subtle protocol choices, especially regarding the soundtrack preparation and playback methodology, can explain variation in the results. In addition, we show that, although the effect sizes of the differences of interest are similar, biological conclusions solely based on the p-value would be different. Thus, we note the pitfall of reliance on p values, especially with small samples.

Mobbing responses of great tits (Parus major) do not depend on the number of heterospecific callers

AbstractNumerical competence—the ability to estimate and process the number of objects and events—is important during antipredator strategies because it influences individual's survival chances. Animals can benefit from numerical competence during mobbing—when a prey moves toward and vocalizes at or harasses a predator. Mobbing responses of some species are related to the number of conspecific callers, and responses are stronger during playbacks simulating multiple callers. Here, we tested heterospecific individual vocal recognition (IVR) in combination with numerical competence in great tits (Parus major). Since previous work showed that great tits respond to mobbing calls of chaffinches (Fringilla coelebs), we predict that great tits showed a stronger response when two or four different individual chaffinches were played back versus only one individual by keeping the duty cycle (number of total calls/min) identical. Individuals were exposed to playbacks simulating calling by one, two or four heterospecific callers, or singing by one caller (control song). Great tits approached the speaker closer and produced more calls during the mobbing calls treatments than during the control treatment (song). However, in contrast to our prediction, great tits responded similarly to mobbing calls from several heterospecific callers as they did toward mobbing calls from one heterospecific caller. Maybe the great tits still are capable of determining the identity of the callers, but what might matter most for them in this very specific situation might be the mobbing intensity. We assume that the duty cycle itself has the stronger impact on the response.

Number of callers may affect the response to conspecific mobbing calls in great tits (Parus major)

Numerical competence—the ability to represent, discriminate, and process numerical quantity information—is a widespread cognitive ability in animals that influences survival and reproductive success. Little is known about the role of numerical competence during predator mobbing—when a prey moves toward and harasses a predator. Since being in a larger group dilutes the risk of injury or death during a mobbing event and large groups are more efficient than small groups at repelling predators, the capacity to evaluate the number of mobbers before joining the mobbing flock may be highly beneficial for individuals. We tested whether the strength of the mobbing response of great tits (Parus major), a songbird that frequently mobs predators, is related to the number of callers. The minimum distance to the loudspeaker tended to be lower, and the number of calls produced by great tits was higher during playbacks simulating several callers than during the playbacks of one caller. These results suggest that numerical competence plays a central role during mobbing and that great tits reduce uncertainty of information by collating information from several individuals. We suggest further studies testing whether birds use individual vocal discrimination to assess the number of heterospecifics during mobbing. Can animals count? Although historically the ability to count has distinguished humans apart from the rest of the animal kingdom, studies in the last decades have shown that numerical competence, the ability to represent, discriminate, and process numerical quantity information, is a widespread cognitive ability in animals. While this competence influences an individual’s survival success, little is known about the role of numerical competence during predator mobbing. Using a field-based playback experiment on a population of wild great tits (Parus major), we demonstrate that great tit responses to mobbing calls were affected by the number of individuals calling. The minimum distance to the loudspeaker tended to be lower and the number of calls produced by great tits tended to be higher during playbacks simulating multiple callers than during the playbacks of one caller. Thus, numerical assessments are used to decide whether or not to participate in mobbing responses.

Wood warblers learn to recognize mobbing calls of an unfamiliar species from heterospecific tutors

Mobbing, a collective harassment of a predator by multiple prey species, is ubiquitous in animals. In birds, it is often prompted by specific vocalizations (mobbing calls) produced by an individual that detects a predator; this attracts nearby conspecifics and heterospecifics in a joint assault on the predator. Mutual recognition of mobbing calls among heterospecifics, even those distantly related, is widespread in birds and learning appears important for its acquisition and maintenance. Yet, the evidence for learned recognition is mostly indirect and the specific learning mechanisms remain largely unknown. Here, I provide experimental evidence that free-living birds (wood warblers, Phylloscopus sibilatrix) can learn to recognize novel heterospecific mobbing calls through interspecific social learning, by associating mobbing calls of a previously unknown species with those of the already recognized heterospecifics during joint mobbing events. Moreover, individuals retained the learned recognition of novel mobbing calls over at least 2 weeks. These results illustrate interspecific social learning as one of the mechanisms underlying the widespread mutual recognition of mobbing calls among bird species. This mechanism of learning is likely to be common in the wild, enabling naïve individuals to utilize a wide set of possible information sources, and result in a rapid cultural transmission of novel antipredator signal recognition within avian communities.

The function of avian mobbing: an experimental test of ‘attract the mightier’ hypothesis

Various avian species give mobbing calls that draw conspecific and heterospecific individuals to react against the potential predator. According to the ‘attract the mightier’ hypothesis, when predators appear, prey will produce mobbing calls to attract superior predators to repel or kill the primary predator. To test this hypothesis, we broadcast calls from light-vented bulbuls, Pycnonotus sinensis (the prey) in the vicinity of a taxidermy model of a collared scops owl, Otus lettia (the primary predator). Three call types were used: the typical call (TC, the control treatment), a mobbing call to a collared scops owl (the MtO treatment) and a mobbing call to a crested goshawk, Accipiter trivirgatus (the superior predator; the MtH treatment). Our results showed that crested goshawks appeared and attacked the taxidermy model during the MtO treatment, but not during the control or MtH treatment. The difference between the MtO and other treatments was significant. Our results, therefore, provide strong evidence for the hypothesis that mobbing calls to primary predators can successfully attract superior predators to harm primary predators.

Great tits (Parus major) adequately respond to both allopatric combinatorial mobbing calls and their isolated parts

AbstractWhen mobbing a predator, birds often produce specific mobbing calls that are efficient in recruiting both conspecifics and heterospecifics. Recent studies on Parids have demonstrated that these mobbing calls are in fact a combination of two distinct calls—first, introductory notes eliciting vigilance in the receiver, then broadband frequency notes (D notes) triggering approach. Debates on a parallel between human syntax and this form of combination have emerged. The degree to which this combinatoriality is perceived in heterospecific communication may shed light onto the relative complexity of such combinatoriality. In this study, our aim was to determine whether European great tits (Parus major) appropriately responded to mobbing calls (and their isolated parts) of an allopatric species, the black‐capped chickadee (Poecile atricapillus), a North‐American species which produces similar combinatorial mobbing calls. In addition, we tested whether the behavioural response to complete mobbing sequences was different than the simple sum of its two constituents. As we hypothesized, great tits behaved differently when hearing the two isolated calls or the complete mobbing sequence: they produced calls and displayed excitement signs only towards the complete mobbing sequence. Moreover, great tits responded to the introductory and D notes by respectively scanning and approaching, and to the complete sequence by mobbing. Our results altogether support the emerging hypothesis of semantic compositionality in Parids, although the present study does not definitively demonstrate the existence of this cognitive process in the great tit.