Collective Escape Research Articles

Fine-scale tracking reveals visual field use for predator detection and escape in collective foraging of pigeon flocks.

During collective vigilance, it is commonly assumed that individual animals compromise their feeding time to be vigilant against predators, benefiting the entire group. One notable issue with this assumption concerns the unclear nature of predator 'detection', particularly in terms of vision. It remains uncertain how a vigilant individual utilizes its high-acuity vision (such as the fovea) to detect a predator cue and subsequently guide individual and collective escape responses. Using fine-scale motion-capture technologies, we tracked the head and body orientations of pigeons (hence reconstructed their visual fields and foveal projections) foraging in a flock during simulated predator attacks. Pigeons used their fovea to inspect predator cues. Earlier foveation on a predator cue was linked to preceding behaviors related to vigilance and feeding, such as head-up or down positions, head-scanning, and food-pecking. Moreover, earlier foveation predicted earlier evasion flights at both the individual and collective levels. However, we also found that relatively long delay between their foveation and escape responses in individuals obscured the relationship between these two responses. While our results largely support the existing assumptions about vigilance, they also underscore the importance of considering vision and addressing the disparity between detection and escape responses in future research.

A robotic falcon induces similar collective escape responses in different bird species.

Patterns of collective escape of a bird flock from a predator are fascinating, but difficult to study under natural conditions because neither prey nor predator is under experimental control. We resolved this problem by using an artificial predator (RobotFalcon) resembling a peregrine falcon in morphology and behaviour. We imitated hunts by chasing flocks of corvids, gulls, starlings and lapwings with the RobotFalcon, and compared their patterns of collective escape to those when chased by a conventional drone and, in case of starlings, hunted by wild peregrine falcons. Active pursuit of flocks, rather than only flying nearby by either the RobotFalcon or the drone, made flocks collectively escape more often. The RobotFalcon elicited patterns of collective escape in flocks of all species more often than the drone. Attack altitude did not affect the frequency of collective escape. Starlings escaped collectively equally often when chased by the RobotFalcon or a wild peregrine falcon. Flocks of all species reacted most often by collective turns, second most often by compacting and third by splitting into subflocks. This study demonstrates the potential of an artificial aerial predator for studying the collective escape behaviour of free-living birds, opening exciting avenues in the empirical study of prey-predator interactions.

Escape response of Guiana dolphins to bottlenose dolphin playback

An individual reaction to natural threats, such as the presence of predators or aggressors, can shape the social behaviour of an entire population. In southern Brazil, two coastal Delphinid species, Guiana dolphin (Sotalia guianensis) and Lahille's bottlenose dolphin (Tursiops truncatus gephyreus) co-occur on a regional scale. However, within Baía Norte, there is a clear spatial segregation between these dolphins at a fine-scale that, combined with records of bottlenose dolphins displaying agonistic behaviours towards Guiana dolphins, raised the hypothesis that such aggressive behaviours may contribute to shape this segregated distribution. Here, to investigate this hypothesis, we carried out a controlled exposure experiment in which surface behavioural responses of Guiana dolphins were evaluated before, during and after acoustic playbacks of bottlenose dolphins. Through aerial footages, collected with an unmanned aerial vehicle, we accessed significant behavioural responses of Guiana dolphins in terms of group organization and behavioural states. During playbacks, we observed a decrease in the cohesion index between individual Guiana dolphins and an increase in group synchrony, indicating that individuals coordinate their movements to aggregate when exposed to playback. In addition, time spent in foraging behaviours decreased significantly during playbacks, suggesting that foraging activities are interrupted, and individuals start to travel. These results demonstrate that Guiana dolphins perform a collective escape behaviour when facing bottlenose dolphins, a potential aggressor, reinforcing the hypothesis that such defensive behaviour is mediating the spatial segregation between these dolphins' populations.

Stochasticity may generate coherent motion in bird flocks

Murmurations along with other forms of flocking have come to epitomize collective animal movements. Most studies into these stunning aerial displays have aimed to understand how coherent motion may emerge from simple behavioral rules and behavioral correlations. These studies may now need revision because recently it has been shown that flocking birds, like swarming insects, behave on the average as if they are trapped in elastic potential wells. Here I show, somewhat paradoxically, how coherent motion can be generated by variations in the intensity of multiplicative noise which causes the shape of a potential well to change, thereby shifting the positions and strengths of centres of attraction. Each bird, irrespective of its position in the flock will respond in a similar way to such changes, giving the impression that the flock behaves as one, and typically resulting in scale-free correlations. I thereby show how correlations can be an emergent property of noisy, confining potential wells. I also show how such wells can lead to high density borders, a characteristic of flocks, and I show how they can account for the complex patterns of collective escape patterns of starling flocks under predation. I suggest swarming and flocking do not constitute two distinctly different kinds of collective behavior but rather that insects are residing in relatively stable potential wells whilst birds are residing in unstable potential wells. It is shown how, dependent upon individual perceptual capabilities, bird flocks can be poised at criticality.

Collective escape and homoclinic bifurcation phenomena in a nonlinear oscillators chain

The phenomenon of deterministic collective escape of particles from the cubic on-site potential well in the presence of both uniform damping and a periodic force is studied. Using analytical techniques such as the separation of time and space as well as the Melnikov theorem, the condition on the periodic force for which a single particle exhibits an irregular motion induced by the homoclinic bifurcation (HB) is derived. Numerical simulation showed that this irregular motion can lead to a strong localization of energy on all the coupled particles allowing them to collectively cross the energy barrier. Moreover, the critical value of the driving force inducing collective escape increases as the potential energy barrier increases and decreases as its frequency increases. Depending on the frequency range of the driving frequency, the collective escape and HB can occur simultaneously; otherwise, the HB prevails.

Confinement and Collective Escape of Active Particles.

Active matter broadly covers the dynamics of self-propelled particles. While the onset of collective behavior in homogenous active systems is relatively well understood, the effect of inhomogeneities such as obstacles and traps lacks overall clarity. Here, we study how interacting, self-propelled particles become trapped and released from a trap. We have found that captured particles aggregate into an orbiting condensate with a crystalline structure. As more particles are added, the trapped condensates escape as a whole. Our results shed light on the effects of confinement and quenched disorder in active matter.

Emergence of splits and collective turns in pigeon flocks under predation.

Complex patterns of collective behaviour may emerge through self-organization, from local interactions among individuals in a group. To understand what behavioural rules underlie these patterns, computational models are often necessary. These rules have not yet been systematically studied for bird flocks under predation. Here, we study airborne flocks of homing pigeons attacked by a robotic falcon, combining empirical data with a species-specific computational model of collective escape. By analysing GPS trajectories of flocking individuals, we identify two new patterns of collective escape: early splits and collective turns, occurring even at large distances from the predator. To examine their formation, we extend an agent-based model of pigeons with a ‘discrete’ escape manoeuvre by a single initiator, namely a sudden turn interrupting the continuous coordinated motion of the group. Both splits and collective turns emerge from this rule. Their relative frequency depends on the angular velocity and position of the initiator in the flock: sharp turns by individuals at the periphery lead to more splits than collective turns. We confirm this association in the empirical data. Our study highlights the importance of discrete and uncoordinated manoeuvres in the collective escape of bird flocks and advocates the systematic study of their patterns across species.

Self-organization of collective escape in pigeon flocks.

Bird flocks under predation demonstrate complex patterns of collective escape. These patterns may emerge by self-organization from local interactions among group-members. Computational models have been shown to be valuable for identifying what behavioral rules may govern such interactions among individuals during collective motion. However, our knowledge of such rules for collective escape is limited by the lack of quantitative data on bird flocks under predation in the field. In the present study, we analyze the first GPS trajectories of pigeons in airborne flocks attacked by a robotic falcon in order to build a species-specific model of collective escape. We use our model to examine a recently identified distance-dependent pattern of collective behavior: the closer the prey is to the predator, the higher the frequency with which flock members turn away from it. We first extract from the empirical data of pigeon flocks the characteristics of their shape and internal structure (bearing angle and distance to nearest neighbors). Combining these with information on their coordination from the literature, we build an agent-based model adjusted to pigeons' collective escape. We show that the pattern of turning away from the predator with increased frequency when the predator is closer arises without prey prioritizing escape when the predator is near. Instead, it emerges through self-organization from a behavioral rule to avoid the predator independently of their distance to it. During this self-organization process, we show how flock members increase their consensus over which direction to escape and turn collectively as the predator gets closer. Our results suggest that coordination among flock members, combined with simple escape rules, reduces the cognitive costs of tracking the predator while flocking. Such escape rules that are independent of the distance to the predator can now be investigated in other species. Our study showcases the important role of computational models in the interpretation of empirical findings of collective behavior.

La luna nelle tradizioni di Lama dei Peligni

Lama dei Peligni is a village in Abruzzo (province of Chieti) that has undergone various economic and socio-cultural transformations. In this work, after brief general considerations concerning the geography, general mythology and traditions of the moon, we proceed to the listing of all the traditions, anecdotes, proverbs and beliefs concerning the moon in the context for the purpose to preserve their memory and then to analyze and interpret their functions and symbolic meanings. The set of facts collected document that the moon had a great importance in the traditions of the context of investigation since it touches many spheres of human action. In the current situation it feeds: the beliefs according to which besides the tides it would influence other earthly events; the magical thought that has not completely dissolved; the romance of poets, singers and simple lovers who entrust their dreams and love messages to her; various traditions and beliefs recovered and inserted into the mechanisms that ensure the show and collective escape.

Absence of “selfish herd” dynamics in bird flocks under threat

The "selfish herd" hypothesis1 provides a potential mechanism to explain a ubiquitous phenomenon in nature: that of non-kin aggregations. Individuals in selfish herds are thought to benefit by reducing their own risk at the expense of conspecifics by attracting toward their neighbors' positions1,2 or central locations in the aggregation.3-5 Alternatively, increased alignment with their neighbors' orientation could reduce the chance of predation through information sharing6-8 or collective escape.6 Using both small and large flocks of homing pigeons (Columba livia; n= 8-10 or n= 27-34 individuals) tagged with 5-Hz GPS loggers and a GPS-tagged, remote-controlled model peregrine falcon (Falco peregrinus), we tested whether individuals increase their use of attraction over alignment when under perceived threat. We conducted n= 27 flights in treatment conditions, chased by the robotic "predator," and n= 16 flights in control conditions (not chased). Despite responding strongly to the RobotFalcon-by turning away from its flight direction-individuals in treatment flocks demonstrated no increased attraction compared with control flocks, and this result held across both flock sizes. We suggest that mutualistic alignment is more advantageous than selfish attraction in groups with a high coincidence of individual and collective interests (adaptive hypothesis). However, we also explore alternative explanations, such as high cognitive demand under threat and collision avoidance (mechanistic hypotheses). We conclude that selfish herd may not be an appropriate paradigm for understanding the function of highly synchronous collective motion, as observed in bird flocks and perhaps also fish shoals and highly aligned mammal aggregations, such as moving herds.

Ionic loss from Venus upper ionosphere via plasma wake

Recent observations have detected significant escape of ions from the Venusian ionosphere. The measurements indicate that the composition of the escaping mix of ions may reach a critical ratio of H + : O + ≈ 2 : 1 , suggesting that it originated in water molecules, in line with the observed dryness of the planet. In this work, we suggest an additional mechanism that can explain the escape of ions from Venus by invoking a test charge approach. For this purpose, an electrostatic wakefield potential, accompanying a moving test charge, is examined in a plasma consisting of two positive ions ( H + and O + ), as well as electrons. It is found that the wakefield potential can be attractive between like charges; it may thus be responsible for gluing charges to an escaping test charge of the same sign, facilitating the collective escape. This attractive potential is strongly influenced by the test charge speed and the plasma density at different altitudes. Our results are consistent with the observation of Venus Express space probe.

Ordering leads to multiple fast tracks in simulated collective escape of human crowds.

Elucidating emergent regularities in intriguing crowd dynamics is a fundamental scientific problem arising in multiple fields. In this work, based on the social force model, we simulate the typical scenario of collective escape towards a single exit and reveal the striking analogy of crowd dynamics and crystallisation. With the outflow of the pedestrians, crystalline order emerges in the compact crowd. In this process, the local misalignment and global rearrangement of pedestrians are well rationalized in terms of the characteristic motions of topological defects in the crystal. Exploiting the notions from the physics of crystallisation further reveals the emergence of multiple fast tracks in the collective escape.

Collective escape and supratransmission phenomena in a nonlinear oscillators chain.

In this study, the collective escape and supratransmission phenomena along a nonlinear chain of coupled particles subjected to a cubic on-site potential are considered. It is shown that the minimum initial on-site amplitude for which there is a collective escape increases with the nonlinear coupling. When the chain is forced at one end by a periodical excitation, the system exhibits supratransmission phenomenon in both lower and upper forbidden bandgaps, and, subsequently, it appears that the driving amplitude threshold for supratransmission in the upper forbidden bandgap frequency decreases with the nonlinear coupling. Depending on the frequency range of the gap frequency, the collective escape and supratransmission can occur simultaneously; otherwise, the supratransmission prevails.

Bacterial Route Finding and Collective Escape in Mazes and Fractals

Experiments show that E. coli bacteria employ a number of strategies to explore complex environments, navigating mazes and fractals much more efficiently than would be expected for a random walk.

‘Rationality’ in Collective Escape Behaviour: Identifying Reference Points of Measurement at Micro and Macro Levels

Background. Evacuation behaviour of human crowds is often characterised by the notion of ‘irrational behaviour’. While the term has been frequently used in the literature, clear definitions and methods for measuring rationality do not exist. Objective. Here, we suggest that rationality, in this context, can alternatively and more effectively be formulated as a question of ‘optimal behaviour’. Decision optimality can potentially be measured and quantified. The main challenges, however, include (i) distinctly identifying the level at which we measure optimality, and (ii) identifying proper reference points at each level. Methods. We differentiate between optimality at the individual (i.e., micro) and the system (i.e., macro/aggregate) levels and illustrate how certain reference points can be established at each level. We suggest that, at the micro level, optimality of individual decisions can be quantified by comparing the outcome of each individual’s decision to those of their ‘nearly equal peers’. At the macro level, optimality can be measured by simulating the system using parametric numerical models and measuring the system performance while altering the behavioural parameters compared to their empirical estimates. Results. Having applied these methods, we observed that variation in micro level decision optimality rises rapidly as the space becomes more heavily crowded. As crowdedness increases in the environment, the difference between ‘good’ and ‘bad’ decisions becomes more distinct; and suboptimal decisions become more frequent. In other words, optimality at individual level seems to be moderated by the level of crowdedness. At the macro level, numerical simulations showed that, for certain exit attributes (like exit congestion), extreme marginal valuations (or preferences) were optimal, whereas for certain other attributes (like exit visibility), intermediate levels of valuation were closer to the optimal. In most cases, the natural observed (or estimated) tendency of evacuees (at the aggregate level) was not quite at the optimum level, meaning that the system could improve by modifying individuals’ marginal valuations of exit attributes. Applications and Recommendations. These results highlight the importance of guiding evacuation decisions particularly in heavily crowded spaces. They also theoretically illustrate the potential benefit of influencing/modifying people’s evacuation strategies, so they make decisions that are collectively more efficient. A crucial step to this end, however, is to identify what optimum strategy is and under what circumstances people are likelier to make suboptimal decisions.

Damping of waves of agitation in starling flocks

When a predator attacks a flock of starlings (Sturnus vulgaris), involving thousands of individuals, a typical collective escape response is the so-called agitation wave, consisting of one or more dark bands (pulses) propagating through the flock and moving away from the predator (usually a Peregrine falcon, Falco peregrinus). The mechanism underlying this collective behavior remains debated. A theoretical study has suggested that the individual motion underlying a pulse could be a skitter (in the form of a zigzag), that is copied by nearby neighbors, and causes us to temporarily observe a larger surface of the wing because the bird is banking during turning while zigzagging. It is not known, however, whether pulses during a wave event weaken over time. This is of interest, because whereas during the usual turning by an undisturbed flock the motion is copied completely without weakening, we may expect that pulses dampen during a wave event because individuals that are further away from a predator react less because of reduced fear. In the present paper, we show in empirical data that pulses during a wave event weaken over time. Our computational model, StarDisplay, reveals that this is most likely a consequence of a reduction of the maximum banking angle during the zigzag escape maneuver rather than by a reduced tendency to copy this maneuver with time. The response seems adaptive because of lowered danger at a larger distance to the location of attack.Significance statementHuge flocks of starlings display amazing patterns of collective escape when attacked by an avian predator, such as a Peregrine falcon. One of them is the “agitation wave” in which dark bands move away from the predator. Dark bands arise probably from the temporarily larger wing area, which is observed when birds perform a skitter escape motion (zigzag) while temporarily banking sideward. Whereas during regular flock turns birds copy each other’s motion completely, it is unknown whether this happens during agitation waves, because individuals further away from the attack may be less frightened. Studying this both empirically at the group level only and in a computational model at both the level of the individual and the group, we show that pulses of waves fade out with time and that this is probably due to a reduced maximum banking angle during the zigzag maneuver rather than a lower tendency of copying. This seems an adaptive response.

‘Herding’ in direction choice-making during collective escape of crowds: How likely is it and what moderates it?

The most likely responses of humans in emergency escape scenarios is not perfectly understood and many of the current behavioural assumptions are speculative. Because of the insufficiency of empirical evidence, certain assumptions and terminologies in this area have been derived from analogical experiments with non-human crowds (e.g. ants or mice, as models of humans) or from purely numerical analyses. Some have suggested that stressful collective escape situations trigger an increased tendency to imitate the decision of the majority, the so-called ‘herd-behaviour’ assumption. Here, we empirically test this assumption using a series of novel experiments with human crowds. Individual-level observations of direction choice were gathered and analysed using econometric modelling methods. Results showed that humans do not tend to imitate direction choices of the majority. To the contrary, they tend to avoid the direction chosen by the majority, and the bigger the majority is, the less likely they are to follow it. The high-urgency treatment (assumed to be associated with higher degrees of stress) did not reverse, nor did it decrease this avoid-the-majority tendency. If anything, it even amplified it in certain choice situations. We also found out that the general level of crowding (i.e. the total number of people in the choice-maker’s vicinity) is another factor that can moderate the reaction to peers’ decision. Higher levels of crowding also amplified the avoid-the-crowd tendency in certain direction choice scenarios. The results overall suggested that escaping humans, when not facing substantial degrees of information uncertainty, tend to avoid the direction chosen by the majority (opposite the ‘herding’ assumption); and they do so more distinctly when they perceive higher urgency or greater number of people in their vicinity. An implication of our finding is that strong parallels between escape behaviour of humans and animals/insects may not exist. Such analogies (particularly on decision-making aspects) need to be drawn with great caution. They might misguide modelling assumptions and lead to unrealistic predictions.

Imitative (herd) behaviour in direction decision-making hinders efficiency of crowd evacuation processes

Humans use social information to improve their decisions in scenarios that involve uncertainty. Previous research has shown that this social influence can have positive or negative outcomes depending on the context of decision and characteristics of the system. A particular scenario where the social information matters is the collective escape where behaviour of others could be used as an extra source of information and thus, influence the decision-making. The question is whether there could be any benefit for escaping human crowds to display imitative tendencies (or the so-called ‘herd’ behaviour) in their decision making. Here, we investigate this question in relation to exit direction decision-making through extensive computer simulation analyses. Using parameteric models of decision making, we increased the imitative tendency of (behaviourally) homogenous crowds on a continuum, simulated the escape process and outputted measures of system efficiency. We examined a broad range of simulated scenarios in terms of the shape of the environment, exit visibility and spatial distribution of exit capacities. Results showed that for none of the scenarios we tested, the escaping crowd could benefit from amplified tendencies to imitate peers. Our numerical analyses suggested that, at least when familiar with escape routes, there is no rationale for blindly following the direction choices of the crowd in emergency scenarios. Strong tendency to copy direction choices of the crowd could substantially hinder the process of evacuations and increase the risk of casualty. These findings have potential implications for evacuation training guidlines. They provide insights into how individuals should be guided/instructed to conduct themselves in case of an emergency in order to facilitate and optimise the evacuation process. A question for future research is how imitation (herding) in other aspects of escape behaviour (such as movement initiation or adapting (changing) direction choices) can influence the system.

Heterogeneity of decision strategy in collective escape of human crowds: On identifying the optimum composition

IntroductionWe raise the question of whether an escaping crowd can benefit from a mixture (or heterogeneity) of direction-choice-making strategies, and if so, how would an optimal composition of strategies be like. MethodsWe perform extensive computer simulation analyses and examine crowds of evacuees composed of two different behavioural classes in terms of their direction-choice strategies. We examine three possible strategies: (I) follow the majority (II) follow the minority without the possibility of decision update and (III) follow the minority with the possibility of decision update. FindingsResults showed that the types of escape decision strategy and their composition in the crowd vastly impacted on the efficiency of the evacuation process. The numerical analyses showed the followings: (1) The highest degree of optimality was achieved from a homogenous crowd of minority followers with the possibility of decision update. (2) The magnitude of the detriment resulted from greater concentrations of the follow-the-majority strategy in the crowd was much larger than the benefit of having more of the follow-the-minority strategy. (3) The marginal benefit of increasing the concentration of the follow-the-minority strategy largely diminished once this strategy became dominant in the crowd. A substantial portion of this benefit was realised at about 50% concentration level. (4) The system's efficiency was more sensitive to the behavioural strategy when there was only a one-off chance of direction-choice making. ApplicationsThese findings have implications for evacuation management. They offer quantitative evidence on how training or modifying individuals’ decision-making behaviour can potentially facilitate evacuation processes. They provide insight into ideal (or optimum) forms of behaviour.

Complex patterns of collective escape in starling flocks under predation

Collective behaviour of animals has been a main focus of recent research, yet few empirical studies deal with this issue in the context of predation, a major driver of social complexity in many animal species. When starling (Sturnus vulgaris) flocks are under attack by a raptor, such as a peregrine falcon (Falco peregrinus), they show a great diversity of patterns of collective escape. The corresponding structural complexity concerns rapid variation in density and shape of the flock over time. Here, we present a first step towards unravelling this complexity. We apply a time series analysis to video footage of 182 sequences of hunting by falcons on flocks of thousands of starlings close to two urban roosts during winter. We distinguish several types of collective escape by determining the position and movement of individuals relative to each other (which determines darkness and shape of the flock over time) as well as relative to the predator, namely ‘flash expansion’, ‘blackening’, ‘wave event’, ‘vacuole’, ‘cordon’ and ‘split’. We show that the specific type of collective escape depends on the collective pattern that precedes it and on the level of threat posed by the raptor. A wave event was most likely to occur when the predator attacked at medium speed. Flash expansion occurred more frequently when the predator approached the flock at faster rather than slower speed and attacked from above rather than from the side or below. Flash expansion was often followed by split, but in many cases, the flock showed resilience by remaining intact. During a hunting sequence, the frequencies of different patterns of collective escape increased when the frequency of attack by the raptor was higher. Despite their complexity, we show that patterns of collective escape depend on the predatory threat, which resembles findings in fish.Significance statementPatterns of collective escape in flocks of starlings have always intrigued laymen and scientists. A detailed analysis of their complex dynamics has been lacking so far, and is the focus of our present study: we analysed video footage of hunting by falcons on flocks of thousands of starlings and show how patterns of collective escape (namely flash expansion, blackening, wave event, vacuole, cordon and split) depend on the preceding pattern and on details of attack. A higher frequency of attack during a hunting sequence resulted in a higher frequency of collective escape events. Flash expansion happened most often when the predator attacks at greater speed. A wave event was most likely when the raptor attacks at medium (rather than high or low) speed. These results provide a first quantitative approach to social complexity in collective avoidance of a predator.