Flight initiation distance, color and camouflage

Toward a unified framework for studying behavioural tolerance

Behavioural responses are widely held to allow animals to cope with human-induced environmental changes. Less often appreciated is that the absence of behavioural response can also be advantageous. This is particularly true when animals become tolerant to situations that may be perceived as risky, although the actual risk is nonexistent. We provide a framework to understand the causes and consequences of behavioural tolerance. Tolerance can emerge from genetic, epigenetic, or learning mechanisms, each exerting different degrees of influence on its speed of acquisition, reversibility, specificity, and duration. The ultimate impact on fitness hinges on the interplay between these mechanisms and the nature of the stressor. Mechanistic clarity is therefore essential to better understand and manage human-wildlife interactions in the Anthropocene.

Disentangling the "many-eyes", "dilution effect", "selfish herd", and "distracted prey" hypotheses in shaping alert and flight initiation distance in a colonial seabird

Group living is thought to have important antipredator benefits for animals, owing to the mechanisms of shared vigilance ("many-eyes" hypothesis), risk dilution ("dilution effect" hypothesis), and relative safety in the center of the group ("selfish herd" hypothesis). However, it can also incur costs since social stimuli, such as conspecific aggression, may distract individuals from anti-predator behavior ("distracted prey" hypothesis). We simultaneously evaluated how these four different hypotheses shape anti-predator behaviors of breeding king penguins (Aptenodytes patagonicus), which aggregate into large colonies, experience frequent aggressive social interactions, and are regularly exposed to predation by giant petrels (Macronectes sp.) and brown skuas (Catharacta loonbergi) when breeding on land. We approached 200 incubating penguins at four different periods of the breeding season across a range of overall increasing colony densities. We measured the distance at which focal birds detected the approaching threat (alert distance: AD), whether birds decided to flee or not, and the distance of flight initiation (flight initiation distance: FID, viz. the bird attempting to walk away with its egg on its feet). We quantified relative local neighbor density, centrality within the colony (rank), and the number of aggressions the focal bird emitted towards neighbors during the approach. We found that birds engaged in aggressive conflicts with neighbors were less likely to flee, and that increasing relative local neighbor density at low and medium overall colony density resulted in a decrease in bird AD, both supporting the "distracted prey" hypothesis. However, at maximal overall colony density, increasing relative local neighbor density resulted in longer AD, supporting the "many-eyes" hypothesis. We found no support for the "dilution effect" and "selfish herd" hypotheses, and no effects of any hypothesis on FID.

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

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.

Factors influencing the flush response and flight initiation distance of three owl species in the Andaman Islands

Effects of anthropogenic pressures on birds of the Andaman Islands have been documented to some extent, however studies on the effect of human activities on the behavioural response of these birds are limited. This study assessed the anti-predatory behaviour (flush response - FR and flight initiation distance - FID) of three owl species (Otus sunia, Otus balli, and Ninox obscura) in response to human stimuli and factors influencing it on the Andaman Islands. In total, 63 % of owls flushed from their roost sites in response to approaching human, and such a response varied between species. Similarly, FID varied widely among the species ranging from 4.23 to 6.73 m. The FR of N. obscura was influenced by the count of climbers, presence of spine, and branch status, while roost height, ambient temperature, and lower count of climbers contributed to a higher FID. For the two Otus species, camouflage and pairing were found to influence their FR while FID of O. balli was influenced by roost height, pairing, and presence of spines. Our results indicated that the anti-predatory behaviour of owls on the Andaman Islands was species- and site-specific and prolonged disturbance to their roost sites may affect the survival and reproductive rate of these owls.  

The anti-predation benefit of flash displays is related to the distance at which the prey initiates its escape

Flash behaviour is widespread in the animal kingdom and describes the exposure of a hidden conspicuous signal as an animal flees from predators. Recent studies have demonstrated that the signal can enhance survivorship by leading pursuing predators into assuming the flasher is also conspicuous at rest. Naturally, this illusion will work best if potential predators are ignorant of the flasher's resting appearance, which could be achieved if the prey flees while the predator is relatively far away. To test this hypothesis, we compared the survival of flashing and non-flashing computer-generated prey with different flight initiation distances (FIDs) using humans as model predators. This experiment found that flash displays confer a survivorship advantage only to those prey with a long FID. A complementary phylogenetic analysis of Australian bird species supports these results: after controlling for body size, species with putative flashing signals had longer FIDs than those without. Species with putative flashing signals also tended to be larger, as demonstrated in other taxa. The anti-predation benefit of flash displays is therefore related to the nature of escape behaviour. Since birds with hidden signals tend to flee at a distance, the flash display here is unlikely to function by startling would-be predators.

Factors modulating the behavioral and physiological stress responses: Do they modify the relationship between flight initiation distance and corticosterone reactivity?

Understanding how vulnerable species are to new stressors, such as anthropogenic changes, is crucial for mitigating their potential negative consequences. Many studies have investigated species sensitivity to human disturbance by focusing on single behavioral or physiological parameters, such as flight initiation distance and glucocorticoid levels. However, little is known about the differential effect that modulating factors might have on behavioral versus physiological stress responses across species. This lack of knowledge make difficult to understand the relationship between both types of reactions, and thus to assess to what extent a behavioral reaction is representative of an internal physiological stress response or vice versa. We collected published data on bird flight initiation distances (FID) and corticosterone (CORT) responses, the two most frequently used indicators of stress reaction. We then investigated how spatio-temporal factors or species-specific characteristics relate to these behavioral and physiological stress responses, and potentially modify the relationship between them. Additionally, we evaluated the strength of the correlation between the two stress responses (behavioral and physiological). Our findings showed that FID and CORT responses were poorly correlated across species, and the lack of correlation was attributable to modulating factors (e.g. latitude and body mass) which influence behavior and physiology differently. These modulating factors, therefore, should be taken into consideration to better interpret FID and CORT responses in the context of species vulnerability to stress.

Disturbance and predation risk influence vigilance synchrony of black-necked cranes Grus nigricollis, but not as strongly as expected

Animals monitor surrounding dangers independently or cooperatively (synchronized and coordinated vigilance), with independent and synchronized scanning being prevalent. Coordinated vigilance, including unique sentinel behavior, is rare in nature, since it is time-consuming and limited in terms of benefits. No evidence showed animals adopt alternative vigilance strategies during antipredation scanning yet. Considering the nonindependent nature of both synchronization and coordination, we assessed whether group members could keep alert synchronously or in a coordinated fashion under different circumstance. We studied how human behavior and species-specific variables impacted individual and collective vigilance of globally threatened black-necked cranes (Grus nigricollis) and explored behavior-based wildlife management. We tested both predation risk (number of juveniles in group) and human disturbance (level and distance) effects on individual and collective antipredation vigilance of black-necked crane families. Adults spent significantly more time (proportion and duration) on scanning than juveniles, and parents with juveniles behaved more vigilant. Both adults and juveniles increased time allocation and duration on vigilance with observer proximity. Deviation between observed and expected collective vigilance varied with disturbance and predation risk from zero, but not significantly so, indicating that an independent vigilance strategy was adopted by black-necked crane couples. The birds showed synchronized vigilance in low disturbance areas, with fewer juveniles and far from observers; otherwise, they scanned in coordinated fashion. The collective vigilance, from synchronized to coordinated pattern, varied as a function of observer distance that helped us determine a safe distance of 403.75 m for the most vulnerable family groups with two juveniles. We argue that vigilance could constitute a prime indicator in behavior-based species conservation, and we suggesting a safe distance of at least 400 m should be considered in future tourist management.

Sex-dependent risk-taking behaviour towards different predatory stimuli in the collared flycatcher

Prey animals may react differently to predators, which can thus raise plasticity in risk-taking behaviour. We assessed the behavioural responses of nestling-feeding collared flycatcher (Ficedula albicollis) parents towards different avian predator species (Eurasian sparrowhawk, long-eared owl) and a non-threatening songbird (song thrush) by measuring the latency to resume feeding activity. We found that the sexes differed in their responses towards the different stimuli, as males resumed nestling-provisioning sooner after the songbird than after the predator stimuli, while latency of females was not affected by the type of stimulus. Parents breeding later in the season took less risk than early breeders, and mean response also varied across the study years. We detected a considerable repeatability at the within-brood level across stimuli, and a correlation between the latency of parents attending the same nest, implying that they may adjust similarly their risk-taking behaviour to the brood value. Repeated measurements at the same brood suggested that risk-taking behaviour of flycatcher parents is a plastic trait, and sex-specific effects might be the result of sex-specific adjustments of behaviour to the perceived environmental challenge as exerted by different predators. Furthermore, the nest-specific effects highlighted that environmental effects can render consistently similar responses between the parents.

Urban invaders are not bold risk-takers: a study of 3 invasive lizards in Southern California

Biological invasions threaten biodiversity worldwide, and therefore, understanding the traits of successful invaders could mitigate their spread. Many commonly invasive species do well in disturbed habitats, such as urban environments, and their abilities to effectively respond to disturbances could contribute to their invasiveness. Yet, there are noninvasive species that also do well in disturbed habitats. The question remains whether urban invaders behave differently in urban environments than noninvaders, which could suggest an "urban-exploiting" phenotype. In Southern California, the co-occurrence of invasive Italian wall lizards Podarcis siculus, brown anoles Anolis sagrei, and green anoles A. carolinensis, and native western fence lizards Sceloporus occidentalis offers an opportunity to test whether invasives exhibit consistent differences in risk-taking within human-altered habitats compared with a native species. We predicted that invasive lizards would exhibit more bold behavior by having shorter flight-initiation distances (FIDs) and by being found farther from a refuge (behaviors that would presumably maximize foraging in low-risk environments). Invasive populations had similar or longer FIDs, but were consistently found at distances closer to a refuge. Collectively, invasive lizards in urban habitats were not bolder than a native species. Reliance on nearby refuges might help species successfully invade urban habitats, and if a general pattern, may pose an added challenge in detecting or eliminating them.

Face masks matter: Eurasian tree sparrows show reduced fear responses to people wearing face masks during the COVID-19 pandemic

Continuous exposure to human activity has led to considerable behavioural changes in some wildlife populations. Animals are more likely to survive in a changing environment by adjusting their behaviour to repeatedly occurring but harmless stimulations. During the COVID-19 pandemic, starting in late 2019, face masks were recommended to the public to prevent the spread of pathogens. In this context, we compared the flight initiation distance (FID) of the Eurasian tree sparrow (Passer montanus), a commonly seen bird across China, in Yibin and Dazhou, Sichuan, in response to people with or without face masks. After continuous exposure to people wearing face masks for nearly six months, sparrows evidently became adapted to people wearing face masks, and correspondingly showed shorter FIDs in response to people wearing masks. To our knowledge, this is the first study showing that birds show reduced fear responses to people wearing face masks during the COVID-19 pandemic. Our results suggest a novel aspect of short-term adaptation of wildlife to human behaviour, and that the learning ability of sparrows allows them to adjust their behaviours to adapt to such subtle changes in the environment.

Anti-predator behavior along elevational and latitudinal gradients in dark-eyed juncos

Flight-initiation distance (FID), the distance between an individual and experimenter when it begins to flee, can be used to quantify risk-assessment. Among other factors, prior studies have shown that latitude explains significant variation in avian FID: at lower latitudes, individuals and species have longer FIDs than those living at higher latitudes. No prior studies have focused on the effect of elevation on FID. Given the similar patterns of seasonality, climate, and potentially predator density, that covary between latitude and elevation, birds at higher elevations might tolerate closer approaches. We asked whether elevation or latitude would explain more variation in the FID of a common passerine bird species, dark-eyed juncos (Junco hyemalis). Juncos live in a variety of habitats along both latitudinal and elevational gradients. We found that statistical models containing elevation as a variable explained more of the variation in FID than did models containing latitude. We also found, unexpectedly, that birds at higher elevation fled at greater distances. While more predators were sighted per hour at higher elevations than at lower elevations, the frequency of predator sightings did not explain a significant amount of variation in FID. This result questions whether predator density is the main driver of risk perception along elevational gradients. Nonetheless, because elevation explains more variation in FID than latitude in at least one species, these findings have direct implications on how human impacts on birds are managed. Specifically, those designing set-back zones to reduce human impact on birds may consider modifying them based on both latitude and elevation.