Optimal Escape Theory Research Articles

Opposing effects of vigilance and foraging on escape behaviour in hooded crows

Abstract Escape represents an important component of animals’ antipredator behaviour entailing both benefits and costs dependent on a moment an animal flees upon predator’s approach. In this study, I examined how the level of vigilance and foraging activity affected escape decision in the urban hooded crow Corvus cornix, predicting that alert distance (AD) and flight initiation distance (FID) should be positively affected by the level of vigilance and negatively affected by foraging activity, whereas buffer distance (BD) should be negatively affected by the level of vigilance and positively affected by foraging activity. Using LMMs it was shown that percent of time crows allocated to vigilance was positively correlated with AD and FID, whereas foraging activity of crows had negative impact on AD and FID. In addition, both AD and FID were positively related to starting distance (SD), while AD was also positively influenced by tree coverage. BD was positively affected by foraging activity and AD. This study demonstrated that more vigilant birds detected predators earlier, which is in accordance with the major function of vigilance. Also, it was shown that foraging crows delayed their escape, once the predator has been detected, as benefits of delayed flight, such as feeding on a profitable food item or within a profitable patch, may outweigh costs, which is consistent with the optimal escape theory.

Heat stress increases risk taking in foraging shorebirds

Abstract Animals often face a trade‐off between food acquisition and predation/disturbance avoidance. Yet, the extent to which this trade‐off is affected by modulating factors such as thermal risk and foraging opportunities has been largely overlooked. Here, we examined the influence of temporal and environmental gradients on the flight initiation distance (FID, the distance at which animals flee from an approaching human‐simulated predator) and escape mode (flying/low risk versus running/low cost) in 16 species of shorebirds foraging on tidal flats of the Bijagós Archipelago, Guinea‐Bissau. We measured escape responses throughout the low tide period during wet and dry seasons and simultaneously recorded microclimate variables and occurrence of heat‐reduction behaviour (ptiloerection). Furthermore, we measured corticosterone metabolites (CORTm) from droppings in red knots Calidris canutus to assess whether ptiloerection is associated to a physiological stress response to hot conditions. Overall, birds tolerated a closer approach at higher environmental temperatures and when showing ptiloerection. They also had shorter FIDs during the dry season and towards the start/end of the low tide period. FIDs also increased with body mass and decreased in areas with more human presence. In red knots, individuals showing ptiloerection had higher levels of CORTm, demonstrating a link between physiological and behavioural stress coping responses to heat events. Our results suggest that heat‐stressed shorebirds take greater risks, supporting the idea of a thermoregulation–predation risk trade‐off. They also indicate that shorebirds adjust risk taking to tidal and seasonal cycles, generally reducing FIDs when the energetic costs of escape are expected to be large. Finally, they suggest that shorebirds habituate to non‐lethal human presence and respond to perceived predation risk in accordance with the predictions of optimal escape theory. These results are relevant to many animals that face a tight window for foraging activity while being exposed to predation/disturbance and heat during the day. We discuss management implications of our results in the context of global change. Read the free Plain Language Summary for this article on the Journal blog.

Should I stay or should I go: escape behaviour of Russell’s vipers, Daboia russelii (Shaw & Nodder, 1797) in India’s agricultural landscapes

Abstract Predation exerts a strong selective force on prey, and hence prey species have evolved a multitude of ways to escape predation. One strategy by which many mobile species escape predation is by fleeing when approached by predators. However, fleeing too early can have fitness costs. Thus, optimal escape theory suggests that escape behaviour in prey depends on the risk of being eaten and the fleeing costs. Several studies on mammals, birds and lizards lend support to this hypothesis. However, few studies have explored escape behaviour in snakes. Here, using radio telemetry to track snakes in the field, we study the escape behaviour in Russell’s vipers, a highly venomous and cryptic snake, responsible for the highest number of snakebite deaths in India. We show that escape response, i.e., the decision to stay or flee, was influenced by intrinsic factors such as the snake’s behaviour and body temperature. We also show that the flight initiation distance, the distance at which the snake flees, was mostly determined by habitat selection, i.e., the visibility of the snake and the distance to the nearest cover. Overall, we show that different factors could determine the decision to flee and when to flee. We also highlight how understanding escape response in such highly venomous, medically important yet secretive snakes could potentially help reduce human-snake encounters and mitigate the snakebite crisis.

Escape behaviour varies with distance from safe refuge

Abstract Locomotor performance and behaviour are important for escape from predators, yet the intersection of these strategies is poorly studied. Escape behaviour is context dependent, and optimal escape theory predicts that animals that are farther from a safe refuge will generally use faster running speeds but might choose to use more variable escape paths. We studied locomotor performance and behaviour of six-lined racerunner lizards (Aspidoscelis sexlineata) escaping on natural surface runways that were varied experimentally to be either 5 or 10 m from a safe refuge. On the 5 m runway, lizards usually escaped directly towards the refuge, attained a slower maximal running speed (3.2 m s−1) at ~3 m from the start, and reached the target refuge in most of the trials (80%). On the 10 m runway, lizards used more variable behaviour, including reversals and turns, attained a faster maximal running speed (3.7 m s−1) at ~6 m from the start, and reached the final refuge in only 43% of trials. Free-ranging racerunners were rarely > 5 m from their nearest refuge and used escape paths that were typically < 5 m. Our findings align with predictions from optimal escape theory, in that the perceived risk of a predator–prey encounter can drive adjustments in locomotor behaviour and performance. Additionally, we show that the escape behaviour of free-ranging lizards closely matches their escape behaviour and performance during controlled escape trials.

Level of local human disturbance and feeding state determines escape behaviour in Eurasian Oystercatchers

AbstractHuman disturbances may constitute a significant stressor for wildlife, especially where human recreational activities overlap with fitness‐enhancing activities such as feeding. Disturbances cause an animal to flee from the perceived predation risk, using energy, rather than remaining and gaining energy. Such reactions are, however, context‐dependent, as predicted by optimal escape theory: animals should only flee when the perceived risk of predation of remaining exceeds the risk of starvation of leaving. Animals may also learn that humans do not generally represent a predation risk after repeated false alarms and then learn to only respond when approached deliberately and closely. We tested the Flight Initiation Distance (FID; distance at which an approached animal begins escape) of feeding or roosting Eurasian Oystercatchers Haematopus ostralegus (oystercatchers) to a human (as a simulated predator) in areas with or without humans on beaches in south‐east Scotland. We predicted roosting oystercatchers to respond earlier (greater FID) than feeding birds because the risk of starvation will be lower, and earlier on beaches where there were no humans at the start of the experiment because a human will then represent a more novel predator signal. As predicted, oystercatchers escaped at greater distances away from the perceived threat when feeding and in areas with humans. We found that escape distances decreased (greater tolerance of the perceived threat) as the number of people in an area increased, but people being present or not explained more of the variation in the data. The results show that oystercatchers habituate on a short timescale to non‐lethal human presence and generally respond to perceived predation risk in accordance with the predictions of optimal escape theory.

Comparison of Escape Behavior Between Solitary and Grouped Liolaemus leopardinus Lizards from the Central Chilean Andes

Optimal escape theory states that animal prey counterbalance costs and risks before fleeing from predators. The theory seeks to explain the variation in distance between the predator and prey when the prey initiates flight (flight initiation distance [FID]). Although group living is commonly observed in the animal kingdom, it is not the predominant social behavior in squamates. Thus, studies of flight responses of grouped or social lizard species are very scarce. The Chilean lizard Liolaemus leopardinus is a highly social species and individuals of different age classes and sexes form social groups on rock outcrops where it is also common to observe solitary conspecifics. The flexible social behavior of L. leopardinus makes it a good model species to compare FID in solitary and grouped individuals. We measured escape behavior in solitary and grouped juvenile and adult L. leopardinus. We conducted trials on 61 solitary lizards and 14 grouped lizards (one focal individual in each group; maximum group size = 3). We found that FID, with basking habitat temperature (BHT) as a covariate, was significantly shorter in grouped L. leopardinus. This is a novel finding for a social lizard. The correlation between FID and distance to nearest refuge (DNR) was not significant, and DNR and BHT were not different between solitary and grouped lizards. In total, 38 lizards entered a refuge (29 solitary and 9 grouped individuals), mostly rock refuges. In this subsample, the correlation between distance fled and DNR was highly significant. The FID of those individuals that entered a refuge was not different than that of individuals that did not enter a refuge (with BHT as a covariate). Our finding that FID was shorter in grouped lizards was consistent with (1) a dilution effect that decreases individual risk of predation and/or (2) coincidental cooperation for indirectly enhanced threat detection. In the second interpretation, grouped lizards may spend less time vigilant and more time thermoregulating, waiting for some signal of enhanced predation risk by other individuals in the group.

Occurrence of Conspecific in the Neighboring Space Influence on Flight Initiation Distance in Cape Hare <i>Lepus capensis</i> under Human Stimuli

Optimal escape theory predicts that animals would balance the costs and benefits of flight. One cost of not fleeing is the ongoing cost of vigilance for upcoming environmental threats. Our results show that FID increases for vigilant hares with predator starting distance, due to the costs acquired by continuing to scan for ecological dangers. The presence of conspecifics within proximity distance for social hare was reduced FID due to collective vigilance, while a solitary animal had greater FID, due to less cooperative defense for predator detection. In both seasons, detection and flight initiation distance of the focal hare increased in open habitat due to a higher probability of detection for upcoming danger, while dense cover provided concealment but reduced the probability of detecting an incoming threat, reducing FID. Moreover, proximity to roads and the nearest refuge significantly influenced anti-predator risk by compensation energy to cope with approaching stimuli. In a landscape with heavy human hunting in retaliation to plantations damage has modified the natural behavior of the hare in the Shigar valley. The findings are discussed in the context of hare FID by humans and the suggestions for management and mitigation of human-wildlife conflict are also considered.

Managing human disturbance: factors influencing flight-initiation distance of birds in a West African nature reserve

Escape behaviour in response to perceived predators can be employed as a guide when designating protected areas around sensitive bird species to minimise the impact of human disturbance. A key measure of escape response is flight-initiation distance (FID), the distance at which a prey animal initiates its escape when approached by a potential predator. We tested the predictions of optimal escape theory by determining the factors that influence FID of bird species in a Nigerian reserved area and its surrounding habitats, and so the potential utility of FID in managing human disturbance on birds, for the first time within a West African context. We tested how FID varied with group size, proximity to vegetation acting as protective cover, levels of human use, and survival rate, and whether these relationships varied by species. We collected 504 FIDs for seven bird species in Amurum Forest Reserve and its surrounding habitats (Jos, Nigeria). The FID was lower in larger groups and when species were closer to protective cover. The FID was lower outside of the protected area because animals in sites with higher levels of human presence and use may become habituated. The FID was higher for species with higher survival, being consistent with predictions from life history theory. Overall, birds perceived humans as a potential threat and responded in accordance to the predictions of optimal escape theory, with FID increasing with increased cost of staying. Reserve managers in Africa could use species- and context-specific FIDs to designate buffer distances for the protection of wildlife from human disturbance.

Intrinsic factors are relatively more important than habitat features in modulating risk perception in a tropical lizard

Anti-predator responses in animals are dynamic and depend on multiple factors. However, most of our understanding about animal escape responses comes from studies which examine only a small set of factors at a time and are done over a short period of animal life spans. This limits our understanding of the dynamic nature of animal escape behaviour and the relative importance of individual factors in determining their escape behaviour. We used a repeated-measures study design to assess the anti-predator response of a wild population of a sexually dimorphic tropical lizard, Psammophilus dorsalis. We followed marked individuals throughout their breeding lifespan, repeatedly assayed their escape response and measured representative intrinsic and extrinsic factors that could modulate their escape response. Our findings suggest that intrinsic factors, such as sex and body size, influenced escape response relatively more than extrinsic factors did, such as distance to refuge and perch height. Although individual variables influenced escape behaviour, in a direction mostly consistent with predictions from optimal escape theory, the interaction between factors led to novel insights into how animals dynamically evaluate multiple and changing costs throughout their lifetime to evade predation. Fleeing from a potential predator is crucial to animals and depends on different factors, as there are various costs to escaping. Although several studies in the past have evaluated the role of various factors in determining escape response, the relative importance of factors and how it changes across the lifetime of individuals are not clear. By assessing individuals repeatedly over a substantial part of their lifespan, we could gain insight into how escape response changes as associated factors, intrinsic or extrinsic, change for an individual over time. This approach provided us a more robust and accurate representation of the dynamic nature of escape decisions. We also demonstrate that when multiple factors are considered simultaneously, their relative importance in determining timing of escape can considerably differ from theoretical predictions.

Opposite companion effect on flight initiation distance in sympatric species: plateau pika (Ochotona curzoniae) and White-rumped Snowfinch (Onychostruthus taczanowskii)

There are many factors influencing prey’s risk perception and escape decision during predator–prey encounters. The distance at which animals move away from perceived danger (often quantified as flight initiation distance or FID) has been used by behavioral ecologists to understand the economics of antipredator behavior. Using general linear models, we investigated escape decision-making processes in plateau pika (Ochotona curzoniae (Hodgson, 1858)) and White-rumped Snowfinch (Onychostruthus taczanowskii (Prjevalsky, 1876), formerly known as Montifringilla taczanowskii Przewalski, 1876) together and we found that (i) there are significant positive correlations between starting distance and FID in both species; (ii) pika escapes at a longer distance from an approaching intruder when it is far from its burrow; (iii) foraging animals tolerate closer distances than watching ones, both in pikas and in Snowfinches; (iv) conspecifics seem to have no effect on pikas’ escaping behavior, while the appearance of Snowfinches dramatically decreases the FID of pikas. On the contrary, conspecifics significantly decrease the FID of Snowfinches, while the presence of pikas has no effects. These findings provide new evidence, which are consistent with optimal escape theory. These novel results in multispecies interactions prompt us to pay attention to the potential relationship between pikas and Snowfinches, as well as the “bird–pika in one hole” phenomenon.

Birds flush early and avoid the rush: an interspecific study.

Since 1986, studies about the escape decisions made by prey are grounded in optimal escape theory (OET) which states that prey will initiate escape when the risk of remaining and the costs of leaving are equal. However, a recent hypothesis, Flush Early and Avoid the Rush (FEAR), acknowledged that the cost of monitoring approaching predators might be a ubiquitous cost. The FEAR hypothesis predicts that prey will generally flee soon after they detect a predator so as to minimize the costs incurred by monitoring the predator. Knowing whether animals flee to reduce monitoring costs is of applied interest because wildlife managers use escape behavior to create set-back zones to reduce human-wildlife conflict. Here we provide the most comprehensive assessment of the FEAR hypothesis using data collected from 178 bird species representing 67 families from two continents. The FEAR hypothesis explains escape behavior in 79% of studied species. Because the FEAR hypothesis is a widespread phenomenon that drives escape behavior in birds, alert distance must be systematically incorporated into the design of set-back zones to protect vulnerable species.

Fifty years of chasing lizards: new insights advance optimal escape theory

Systematic reviews and meta-analyses often examine data from diverse taxa to identify general patterns of effect sizes. Meta-analyses that focus on identifying generalisations in a single taxon are also valuable because species in a taxon are more likely to share similar unique constraints. We conducted a comprehensive phylogenetic meta-analysis of flight initiation distance in lizards. Flight initiation distance (FID) is a common metric used to quantify risk-taking and has previously been shown to reflect adaptive decision-making. The past decade has seen an explosion of studies focused on quantifying FID in lizards, and, because lizards occur in a wide range of habitats, are ecologically diverse, and are typically smaller and differ physiologically from the better studied mammals and birds, they are worthy of detailed examination. We found that variables that reflect the costs or benefits of flight (being engaged in social interactions, having food available) as well as certain predator effects (predator size and approach speed) had large effects on FID in the directions predicted by optimal escape theory. Variables that were associated with morphology (with the exception of crypsis) and physiology had relatively small effects, whereas habitat selection factors typically had moderate to large effect sizes. Lizards, like other taxa, are very sensitive to the costs of flight.

Phi index: a new metric to test the flush early and avoid the rush hypothesis.

Optimal escape theory states that animals should counterbalance the costs and benefits of flight when escaping from a potential predator. However, in apparent contradiction with this well-established optimality model, birds and mammals generally initiate escape soon after beginning to monitor an approaching threat, a phenomena codified as the “Flush Early and Avoid the Rush” (FEAR) hypothesis. Typically, the FEAR hypothesis is tested using correlational statistics and is supported when there is a strong relationship between the distance at which an individual first responds behaviorally to an approaching predator (alert distance, AD), and its flight initiation distance (the distance at which it flees the approaching predator, FID). However, such correlational statistics are both inadequate to analyze relationships constrained by an envelope (such as that in the AD-FID relationship) and are sensitive to outliers with high leverage, which can lead one to erroneous conclusions. To overcome these statistical concerns we develop the phi index (Φ), a distribution-free metric to evaluate the goodness of fit of a 1∶1 relationship in a constraint envelope (the prediction of the FEAR hypothesis). Using both simulation and empirical data, we conclude that Φ is superior to traditional correlational analyses because it explicitly tests the FEAR prediction, is robust to outliers, and it controls for the disproportionate influence of observations from large predictor values (caused by the constrained envelope in AD-FID relationship). Importantly, by analyzing the empirical data we corroborate the strong effect that alertness has on flight as stated by the FEAR hypothesis.

Complex Relationships amongst Parasite Load and Escape Behaviour in an Insular Lizard

Economic escape models predict escape decisions of prey which are approached by predators. Flight initiation distance (FID, predator–prey distance when prey begins to flee) and distance fled (DF) are major variables used to characterize escape responses. In optimal escape theory, FID increases as cost of not fleeing also increases. Moreover, FID decreases as cost of fleeing increases, due to lost opportunities to perform activities that may increase fitness. Finally, FID further increases as the prey's fitness increases. Some factors, including parasitism, may affect more than one of these predictors of FID. Initially, parasitized prey may have lower fitness as well as impaired locomotor ability, which would avoid predation and/or reduce their foraging ability, further decreasing the opportunity of fleeing. For example, if parasites decrease body condition, prey fitness is reduced and escape ability may be impaired. Hence, the overall influence of parasitism on FID is difficult to predict. We examined relationships between escape decisions and different traits: parasite load, body size and body condition in the Balearic lizard, Podarcis lilfordi. Lizards that showed higher haemogregarines load had longer FID and shorter DF. Although results did not confirm our initial predictions made on the basis of optimal escape theory, our findings suggest that parasites can alter several aspects of escape behaviour in a complex way.

Responses to Predators Differ Between Native and Invasive Freshwater Turtles: Environmental Context and its Implications for Competition

AbstractOptimal escape theory predicts that animals facing a predation threat should react in a manner that balances perceived threats with the costs of escape. Flight distance, the distance at which animals flee in response to an approaching predator, is often used as a quantitative measure of animal risk assessment and is thus useful to examine predictions of optimal escape theory. However, applying this theory to a broader diversity of animals under more complex environmental conditions has been hampered by a paucity of data. In this study, I measure the flight distances of two freshwater turtles, the invasive Trachemys scripta elegans, and the native Emys marmorata, in an urban California water body to examine how flight distance is affected by time of day, initial visibility to the approaching predator, and the presence of conspecifics. I found that E. marmorata was more sensitive to predation risk than T. scripta and would flee earlier (at a greater distance). Time of day and the presence of conspecifics had no effect on E. marmorata's flight distance although visibility and the interaction between time and visibility did. Emys marmorata would flee earlier when visible, and in the afternoon visible turtles would flee earlier than non‐visible turtles, but in the morning, visibility did not affect turtle flight distance. In contrast, T. scripta's flight distance was not affected by any measured variables, although this may be due to a lack of statistical power given the low sample size. This study demonstrates that multiple environmental characteristics influence risk assessment in turtles, and implies that interspecific differences in responses to predation threats may lead to increased competition for thermal resources between native and invasive ectotherms. This is a likely mechanism contributing to the negative impacts of T. scripta on native turtle populations, especially in urban environments.

Optimal Predator Risk Assessment by the Sonar-Jamming Arctiine Moth Bertholdia trigona

Nearly all animals face a tradeoff between seeking food and mates and avoiding predation. Optimal escape theory holds that an animal confronted with a predator should only flee when benefits of flight (increased survival) outweigh the costs (energetic costs, lost foraging time, etc.). We propose a model for prey risk assessment based on the predator's stage of attack. Risk level should increase rapidly from when the predator detects the prey to when it commits to the attack. We tested this hypothesis using a predator – the echolocating bat – whose active biosonar reveals its stage of attack. We used a prey defense – clicking used for sonar jamming by the tiger moth Bertholdia trigona– that can be readily studied in the field and laboratory and is enacted simultaneously with evasive flight. We predicted that prey employ defenses soon after being detected and targeted, and that prey defensive thresholds discriminate between legitimate predatory threats and false threats where a nearby prey is attacked. Laboratory and field experiments using playbacks of ultrasound signals and naturally behaving bats, respectively, confirmed our predictions. Moths clicked soon after bats detected and targeted them. Also, B. trigona clicking thresholds closely matched predicted optimal thresholds for discriminating legitimate and false predator threats for bats using search and approach phase echolocation – the period when bats are searching for and assessing prey. To our knowledge, this is the first quantitative study to correlate the sensory stimuli that trigger defensive behaviors with measurements of signals provided by predators during natural attacks in the field. We propose theoretical models for explaining prey risk assessment depending on the availability of cues that reveal a predator's stage of attack.

Do animals generally flush early and avoid the rush? A meta-analysis

Optimal escape theory predicts that animals should balance the costs and benefits of flight. One cost of not fleeing is the ongoing cost of monitoring an approaching predator. We used a phylogenetic meta-analysis to test the general hypothesis that animals should initiate flight soon after they detect a predator-the 'flush early and avoid the rush' hypothesis. We found a large, significant overall relationship between the distance at which animals were approached or first detected a threat and the distance at which they fled. While these results are the first general test of the flush early and avoid the rush hypothesis, future work will be required to determine whether animals flush early to reduce ongoing attentional costs, or if they flush early as a form of risk reduction.

To Run or Hide?: Escape Behaviour in a Cryptic African snake

Optimal escape theory predicts that escape behaviour of an organism is best understood in terms of costs and benefits of escaping relative to risk of predation. However, risk of predation facing an organism is dependent on various biotic and abiotic factors. In order to better understand escape behaviour of an African snake, the Namaqua dwarf adder (Bitis schneideri), I simulated predatory attacks on 51 free-ranging snakes, quantifying whether or not they fled. Additionally I measured body size, sex, body condition, and the surface temperature of the location of each snake, as well as the situation (buried or active on the surface) of the snake. I used generalized logistic regression to assess which variables predicted whether the snake fled during/following the simulated attack or not. Surface temperature was significantly associated with a flight-response, with warmer snakes more likely to flee than cooler snakes. Buried snakes were less likely to flee than snakes that were active on the surface. These findings generally support optimal escape theory in that both factors are strongly linked to risk of predation: temperature is likely to influence sprint performance, while situation is likely to influence the detectability of the snake to predators.

To run or hide?: escape behaviour in a cryptic African snake

Optimal escape theory predicts that escape behaviour of an organism is best understood in terms of costs and benefits of escaping relative to risk of predation. However, risk of predation facing an organism is dependent on various biotic and abiotic factors. In order to better understand escape behaviour of an African snake, the Namaqua dwarf adder (Bitis schneideri), I simulated predatory attacks on 51 free-ranging snakes, quantifying whether or not they fled. Additionally I measured body size, sex, body condition, and the surface temperature of the location of each snake, as well as the situation (buried or active on the surface) of the snake. I used generalized logistic regression to assess which variables predicted whether the snake fled during/following the simulated attack or not. Surface temperature was significantly associated with a flight-response, with warmer snakes more likely to flee than cooler snakes. Buried snakes were less likely to flee than snakes that were active on the surface. These findings generally support optimal escape theory in that both factors are strongly linked to risk of predation: temperature is likely to influence sprint performance, while situation is likely to influence the detectability of the snake to predators.

Who are you looking at? Hadeda ibises use direction of gaze, head orientation and approach speed in their risk assessment of a potential predator

AbstractAnimals may update their assessment of predation risk according to how a potential predator approaches them. For example, the predator's head and gaze orientation (direction of attention) may reveal its intentions, and faster‐approaching predators are likely to represent greater risk. We examined the reactions of hadeda ibises Bostrychia hagedash. These large birds demonstrate a wide repertoire of responses to being approached (e.g. continuing to forage, slow walking, rapid escape walking, flight and alarm calling). Birds were approached tangentially 112 times by a human who either had the head and eyes directed towards (65 approaches) or directed away from (47 approaches) the birds to test the hypothesis that the direction of the observer's attention informs alert distance (AD) and flight initiation distance (FID) in these birds. Direction of attention had a significant effect on AD and FID as well as the likelihood of taking flight and alarm calling by hadedas, with birds appearing to associate attention directed towards them as an indication of increased risk. Hadedas were able to differentiate between the direction of attention of an approaching human, whether or not there were multiple other humans in the near vicinity. We also examined whether the observer's approach speed altered the birds' responses. Approach speed affected the birds' FID, suggesting that they perceive greater danger in a faster‐approaching intruder compared with a slower‐walking one. These results support the predictions of optimal escape theory and emphasize the high resolution of anti‐predatory awareness in these birds. The marked success of hadeda ibises in urban environments may be due to their ability to become habituated to human presence and to modify their antipredator behaviour in response to subtle cues. These may be common traits of bird species that successfully adapt to urban environments.