Shorter Flight Initiation Distance Research Articles

Differences in the flight initiation distance near expressways and in open farmland in wintering Common Buzzards (Buteo buteo)

The development of road networks has given rise to man-made wildlife habitats. Birds of prey use roadsides and highway infrastructure as attractive foraging sites. In order to measure reactions to human disturbance, the flight initiation distance (FID) of Common Buzzards (Buteo buteo) foraging on expressways and in open farmland was investigated. The GLM model showed that FID was affected by habitat types, distance from the nearest building and distance from the nearest tree. Common Buzzards have a shorter FID on expressway than in farmland. The distance from the nearest building and the distance from the nearest tree were positively correlated with FID. Our results confirmed the behavioural plasticity of these birds and may provide a theoretical basis for measures aimed at reducing the risk of collisions with moving road vehicles.

Birds’ Flight Initiation Distance in Residential Areas of Beijing Are Lower than in Pristine Environments: Implications for the Conservation of Urban Bird Diversity

(1) Background: With rapid urbanization, birds are facing a variety of challenges. Evaluating bird behaviour changes in response to urbanization can help us understand how to make them coexist sustainably with humans. We aimed to investigate whether birds inhabiting residential areas differed in their escape behaviour and their influencing factors. (2) Methods: We used the flight initiation distance (FID), the horizontal distance between the observer and the target bird when it escapes, to measure the escape behaviour of birds. We chose 40 urban residential areas within the 5th ring road in Beijing and conducted surveys each month for one year. We applied Generalized Linear Mixed Models (GLMMs) separately to evaluate the response variable of FID for the total species, the most common species, and the other species. (3) Results: Birds that appear more frequently in residential areas or as ground foragers, insectivores, and omnivores are better adapted to human interference and have shorter FIDs. Individual initial conditions affect bird FID, and environmental characteristics can be used as predictors for the most common birds. Tree canopy coverage was found to positively affect FID, while floor area ratio (FAR) is negatively correlated with FID. (4) Conclusions: Our results demonstrated that birds in residential areas have been adapting to the human environment, and urban tree canopies can provide refuge for birds to avoid human interference. Our study focused on the response of bird FIDs to human interference and urban trees under high urbanization, which has substantial practical implications for urban managers to improve habitat quality to ensure that birds coexist with human beings.

Beyond simple habituation: Anthropogenic habitats influence the escape behaviour of spur-winged lapwings in response to both human and non-human threats.

Habitat development may affect wildlife behaviour, favouring individuals or behaviours that cope better with perceived threats (predators). Bolder behaviours in human-dominated habitats (HDH; e.g. urban and rural settlements) may represent habituation specifically to humans, or a general reduction in predator-avoidance response. However, such carry-over effects across threat types (i.e. beyond humans) and phases of the escape sequence have not been well studied to date. Here we investigated escape behaviours of a locally common wader species, the spur-winged lapwing Vanellus spinosus. We assayed their flight initiation distance (FID) and subsequent escape behaviours in agricultural areas and in HDH. We found that lapwings in HDH were bolder, and that the difference was manifested in several phases of the predator-avoidance sequence (shorter FIDs, shorter distances fled, and a higher probability of escape by running vs. flying). When re-approached (by an observer) after landing, lapwings in HDH were also more repetitive in their FID than those in other habitats. To determine whether this apparent bolder behaviour in HDH areas is merely a consequence of habituation to humans or represents a broader behavioural change, we introduced an additional threat type-a remotely-operated taxidermic jackal ('Jack-Truck'). Finding bolder responses in the HDH to the human threat alone (and not to the Jack-Truck) could have supported the habituation hypothesis. In contrast, however, we found a bolder response in the HDH to both threat types, as well as a correlation between their FIDs across different sites. These bolder behaviours suggest that HDH impose a broader behavioural change on lapwings, rather than just simple habituation. Overall, our findings demonstrate how FID trials can reveal strong behavioural carry-over effects of HDH following human and non-human threats, including effects on the subsequent phases of escaping the predator. Further, FID assays may reveal consistent behavioural types when assessed under field conditions, and offer a direct way to differentiate among the various poorly understood and non-mutually exclusive mechanisms that lead to behavioural differences among organisms in HDH. The mechanistic perspective is essential for understanding how rapid urbanization impacts wildlife behaviour, populations, and the range of behaviours within them, even in species apparently resilient to such environmental changes.

Fear in urban landscapes: conspecific flock size drives escape decisions in tropical birds.

Human-induced disturbances affect animal behaviours such as anti-predatory responses. Animals in urban environments tend to exhibit a reduced escape response, measured as a shorter flight initiation distance (FID), compared to their rural counterparts. While FID has been evaluated in animals dwelling in contrasting habitats (e.g. urban versus rural), little is known about how this response varies within urban environments, especially in tropical cities. Here, we studied the FID of 15 resident bird species in Bogota, Colombia, at 22 sites grouped into four categories (natural sites, metropolitan parks, zonal parks and residential areas) that differed in landscape features and evaluated which factors affected the escape responses of birds. We showed that birds foraging in larger flocks are more tolerant when being approached but they do not seem to be influenced by other factors such as heterospecific flock size, noise levels, pedestrian density, predator density, natural cover or body length. Also, birds inhabiting residential areas and parks showed a shorter FID compared to birds in natural areas suggesting that they are more tolerant of human-related disturbances compared to their conspecifics that live in natural areas within the city. Our study shows important differences in bird anti-predatory responses within the city and suggests that social strategies (i.e. flocking patterns) may be a mechanism for adapting to human-induced disturbances in urban tropical environments.

Morph-specific differences in escape behavior in a color polymorphic lizard

Variation in color morph behavior is an important factor in the maintenance of color polymorphism. Alternative anti-predator behaviors are often associated with morphological traits such as coloration, possibly because predator-mediated viability selection favors certain combinations of anti-predator behavior and color. The Aegean wall lizard, Podarcis erhardii, is color polymorphic and populations can have up to three monochromatic morphs: orange, yellow, and white. We investigated whether escape behaviors differ among coexisting color morphs, and if morph behaviors are repeatable across different populations with the same predator species. Specifically, we assessed color morph flight initiation distance (FID), distance to the nearest refuge (DNR), and distance to chosen refuge (DR) in two populations of Aegean wall lizards from Naxos island. We also analyzed the type of refugia color morphs selected and their re-emergence behavior following a standardized approach. We found that orange morphs have different escape behaviors from white and yellow morphs, and these differences are consistent in both populations we sampled. Orange morphs have shorter FIDs, DNRs, and DRs; select different refuge types; and re-emerge less often after being approached compared to white and yellow morphs. Observed differences in color morph escape behaviors support the idea that morphs have evolved alternative behavioral strategies that may play a role in population-level morph maintenance and loss.Significance statementColor polymorphic species often differ in behaviors related to reproduction, but differences in other behaviors are relatively underexplored. In this study, we use an experimental approach in two natural populations of color populations of color polymorphic lizards to determine that color morphs have diverged in their escape behaviors. By conducting our experiments in two different populations with similar predator regimes, we show for the first time that behavioral differences among intra-specific color morphs are repeatable across populations, suggesting that alternative behavioral strategies have evolved in this species. Using this experimental approach, we demonstrate that the brightest orange morph stays closer to refuge than other morphs, uses a different refuge type (vegetation) more often than other morphs (wall crevices), and take much longer to emerge from refuge after a simulated predation event than other morphs. Thus, selective pressures from visual predators may differ between morphs and play a role in the evolution and maintenance of color polymorphisms in these types of systems. Our study species, Podarcis erhardii, belongs to a highly color polymorphic genus (19/23 spp. are color polymorphic) that contains the same three color morphs, thus we believe our results may be relevant to more than just P. erhardii.

Wolf Responses to Experimental Human Approaches Using High-Resolution Positioning Data

Humans pose a major mortality risk to wolves. Hence, similar to how prey respond to predators, wolves can be expected to show anti-predator responses to humans. When exposed to a threat, animals may show a fight, flight, freeze or hide response. The type of response and the circumstances (e.g., distance and speed) at which the animal flees are useful parameters to describe the responses of wild animals to approaching humans. Increasing knowledge about behavioral responses of wolves toward humans might improve appropriate management and decrease conflicts related to fear of wolves. We did a pilot study by conducting 21 approach trials on seven GPS-collared wolves in four territories to investigate their responses to experimental human approaches. We found that wolves predominantly showed a flight response (N= 18), in a few cases the wolf did not flee (N= 3), but no wolves were seen or heard during trials. When wolves were downwind of the observer the flight initiation distance was significantly larger than when upwind, consistent with the hypothesis that conditions facilitating early detection would result in an earlier flight. Our hypothesis that early detection would result in less intense flights was not supported, as we found no correlation between flight initiation distances and speed, distance or straightness of the flight. Wolves in more concealed habitat had a shorter flight initiation distance or did not flee at all, suggesting that perceived risk might have been affected by horizontal visibility. Contrary to our expectation, resettling positions were less concealed (larger horizontal visibility) than the wolves’ initial site. Although our small number of study animals and trials does not allow for generalizations, this pilot study illustrates how standardized human approach trials with high-resolution GPS-data can be used to describe wolf responses at a local scale. In continuation, this method can be applied at larger spatial scales to compare wolf flight responses within and between populations and across anthropogenic gradients, thus increasing the knowledge of wolf behavior toward humans, and potentially improving coexistence with wolves across their range.

Variation in escape response of Himalayan marmots (Marmota himalayana) under different human disturbances

AbstractWe measured flight initiation distance (FID) and distance fled (DF) of Himalayan marmots (Marmota himalayana) from three populations suffering different human disturbances when they are near (<5 m) and away from (>5 m) a potential refuge to analyze the effects of varied disturbance intensity on their FID, DF, and the interaction between the two parameters. We found that: (1) highly disturbed individuals have shorter FIDs and smaller standard deviation of FID relative to undisturbed individuals, but there was no difference between lowly disturbed and undisturbed individuals. The diversifications arise possibly because extensive disturbance enhances the risk perception threshold of highly disturbed marmots but their lowly disturbed congeners; (2) the DF of highly disturbed individuals was shorter than undisturbed individuals, possibly because heavy disturbances drive reproductive pairs to dig more burrows in the habitat, resulting in the reduced inter‐burrow distance and thus decreased DF. No similar diversifications were observed between lowly disturbed individuals and individuals in the other two populations, possibly because reproductive pairs from low disturbance population concentrate their burrows, resulting in shorter DF when they are near their burrows, but longer DF when they are farther away; and (3) unlike lowly and undisturbed marmots, FID of highly disturbed individuals under different DFs differed significantly, potentially because shorter inter‐burrow distance reduced their flight costs, resulting in more tolerance of human proximity. Nevertheless, the DF‐derived risk perception threshold of undisturbed and lowly disturbed individuals drives them to prioritize fleeing when threatened. Generally, the escape response of Himalayan marmots under different human disturbances is determined by the interaction between predation risk and flight costs.

Differences in flight initiation distances between African and Australian birds

Associations between escape behaviours of birds approached by humans and life history traits differ between continents. This suggests environment and history have shaped the evolution of escape at the continental scale. We compared the escape behaviour of birds between eastern Australia (relatively low density and shorter history of human presence) and eastern Africa (high density and substantially longer history of human presence), in relation to life and natural history traits known to influence escape behaviour. We examined associations between flight initiation distance (FID) and life and natural history variables, using phylogenetically controlled analysis and model selection procedures, for 9372 FIDs from 516 bird species. Overall, African birds had shorter FIDs than Australian birds; however, this difference was only observed among nonmigratory (resident) species. Migratory birds showed no continental level differences in FIDs. Across continents, birds in ‘wetland’ and ‘treed’ habitats both had significantly longer FIDs than birds in ‘scrub’ habitats. Diet and social traits (cooperative breeding and flocking) did not significantly influence FID. The relationship between body mass and FID was positive for Australian birds but not so for African birds. This study (1) suggests that resident eastern African birds are more tolerant of humans and (2) supports the contention that some principles regarding escape behaviour of birds might represent universal patterns (e.g. longer FIDs in open wetland habitats), others (e.g. longer FIDs in larger birds) vary geographically or with habituation, perhaps in relation to the history of cohabitation between humans (and other predators) and birds.

Changes in escape behaviour of black iguanas (Ctenosaura similis) over an anthropogenic disturbance gradient

Abstract Studies on the impact of human activity on animal behaviour are critical for understanding the extent to which humans affect ecological dynamics. Previous studies have found that human presence alters antipredator behaviours, which can be measured by flight initiation distance (FID). We investigated escape behaviour of 96 black iguanas (Ctenosaura similis) across a gradient of human disturbance in six sites inside a protected area in Costa Rica. We used a field experiment to test for effect of human disturbance on FID. We found that individuals from higher disturbance sites had shorter FIDs, meaning that black iguanas from disturbed areas allow closer approaches. This finding is consistent with the prediction that some animals become more habituated to human presence as the degree of human disturbance increases. We propose that black iguanas’ ability to alter their behaviour in response to humans’ presence could make them especially adept at invading new environments.

Alert time reflects the negative impacts of human disturbance on an endangered bird species in Changbai Mountain, China

Human proximity often has negative consequences for wildlife. In response to human disturbance, animals may display a variety of potentially costly behavioural responses, such as vigilance or flight, which were often used as measures of fearfulness and to indicate varying habituation levels. With rapid human development, the globally endangered Scaly-sided Merganser (Mergus squamatus) may be vulnerable to anthropogenic interference. However, human-induced disturbances to a local breeding population have not been quantified. In this study, we tested how flight initiation distance (FID) and behavioural budget (alert time (AT) in particular) changes in relation to human presence in this species in Changbai Mountain, China. The results showed that individuals who experienced greater levels of human disturbance had shorter FIDs, which indicate that the local population may be habituated to human approach. However, this species spent more time being vigilant in its behavioral budget, which suggested that some other human-induced stimuli may impose negative effects on the species. This study will allow managers to better understand the response of this endangered species to different human-induced disturbances and provided scientific basis for local wildlife managers to establish buffer distance for SSME in Changbai Mountain.

Effects of human activity on the habitat utilization of Himalayan marmot (Marmota himalayana) in Zoige wetland.

Human activity is increasingly and persistently disturbing nature and wild animals. Affected wildlife adopts multiple strategies to deal with different human influences. To explore the effect of human activity on habitat utilization of Himalayan marmot (Marmota himalayana), habitat utilization patterns of three neighboring marmot populations in habitats affected differently by human activities were recorded and compared. We found that (a) distance between reproductive burrows (a represent of reproductive pairs) becomes shorter under the influence of human activities, and more burrows were dug as temporary shelters, resulting in shorter distance between those shelters and shorter distance flee to those shelters and, consequently, shorter flight initiation distance when threatened. More burrows that are closer to the disturbed habitats improve the ability to escape from threats. (b) Reproductive burrow site selection of the species is determined by the availability of mounds in the habitat, and breeding pairs selectively build reproductive (also the hibernation) burrows on mounds, potentially to improve surveillance when basking and the drainage of burrows. Human activities generally drive breeding pairs away from the road to dig their reproductive burrows likely to reduce disturbance from vehicles. However, even heavy human activity exerts no pressure on the distance of reproductive burrows from the road or the mound volume of the high disturbance population, potentially because mounds are the best burrowing site to reproduce and hibernate in the habitat. Marmots deal with disturbance by digging more burrows in the habitat to flee more effectively and building reproductive burrows on mounds to gain better vigilance and drainage efficiency.

Behavioural and demographic changes in impala populations after 15 years of improved conservation management

Many protected areas (PAs) have been created globally in response to the dramatic human-induced decline in biodiversity. However, not all PAs successfully preserve their biodiversity and continued monitoring is vital to assess the effectiveness of conservation efforts. Here, we repeated a study done in 2003 to assess whether investments in anti-poaching in two partially PAs (PPAs), Grumeti Game Reserve (GGR) and Ikona Wildlife Management Area (IWMA), have had quantifiable impacts on impala (Aepyceros melampus) populations, compared to the adjacent strictly protected Serengeti National Park. Since 2003, management of the PPAs has changed dramatically under the non-profit organisation Grumeti Fund. We predicted that if conservation management had improved, impala populations in 2018 would have higher local density, less female-skewed sex ratio, larger group sizes, and shorter flight initiation distances (FID). To assess these parameters of the impala populations, we conducted the same road-based surveys in 2003 and 2018. We found that compared to 2003, impala population density doubled in GGR, group sizes were larger in IWMA and nearly doubled in GGR, and that impala had shorter FID in IWMA in 2018. Our results suggest that the changes in conservation management could have led to the observed positive impacts for impala populations. Our study is encouraging for managers and reveals a conservation success story: if proper action is taken to increase the protection of wildlife within PPAs, wildlife populations may respond quickly and positively.

A physiological cost to behavioural tolerance

Few studies of animal escape behaviour simultaneously investigate behavioural and physiological responses. Differences between these response types, however, have consequences for the way in which habituation or tolerance is interpreted – behavioural habituation may incur physiological costs. We simultaneously measured heart rate (HR) and behavioural responses during standardised approaches to incubating Masked Lapwings Vanellus miles, an urban-frequenting ground-nesting bird. We describe the existence of a distinct Physiological-Initiation Distance (PID) that precedes Flight-Initiation Distance (FID) but does not necessarily precede Alert Distance (AD). Two distinct response types occurred: ‘startle’, where a behavioural or physiological response coincided with the appearance of a person (always the investigator; 75.9 % of 58 birds) and ‘non-startle’ responses, where a behavioural or physiological response occurred after the appearance of, and commencement of the approach by, the person (24.1 % or 14 birds). For birds which were not startled, the interval between the initial heart rate increase and heart rate peak increased with clutch age. For birds which were startled, longer durations of post-peak HR elevation were associated with shorter FIDs and older clutches. Thus, reduced FIDs (generally interpreted as a sign of habituation or tolerance) are associated with greater physiological costs through longer durations of elevated HR. Additionally, the existence of, often long and undetectable, PIDs suggests: 1) that behavioural measures of response underestimate responses in general, and 2) that the methodological assumption when collecting FIDs, that starting distances exceed response distances, may often be incorrect yet are visually undetectable. Further studies of a variety of taxa are warranted to determine the associations between behavioural and physiological responses, and should these associations prove reliable, they would ideally generate general predictions of PID from readily measurable behavioural metrics (FID or AD), thus enabling prescriptions to manage the consequences of human interactions with wildlife.

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.

Flight initiation distance changes across landscapes and habitats in a successful urban coloniser

Urbanisation is a major land use change that introduces novel sources of disturbance and risk into an ecosystem. Successful urban species modify their fear behaviour in response to the new conditions, as evolutionary mismatches between fear responses and environmental conditions are likely to have negative fitness consequences. Here we tested the effect of urbanisation on the fear response of a successful urban coloniser, the Australian Brush-turkey (Alectura lathami), using flight initiation distance (FID) as a measure of boldness. We predicted that Brush-turkeys in areas of natural vegetation would have longer FIDs than birds in more urbanised environments, and that males would have shorter FIDs than females. We recorded the FIDs of 80 Brush-turkeys across the Sydney region. We found that Brush-turkeys in natural vegetation and urban bushland reserves had longer FIDs than birds in urban streets and lawns. However, there was no difference in FID between sites in natural vegetation and urban bushland reserves. There was no difference in FID between male and female Brush-turkeys, between birds engaged in different behaviours, or between birds approached in the breeding and non-breeding seasons. Our results identified that Brush-turkeys displayed reduced fear behaviour in response to humans in more urban environments.

Sex, personality and conspecific density influence natal dispersal with lifetime fitness consequences in urban and rural burrowing owls.

There is a growing need to understand how species respond to habitat changes and the potential key role played by natal dispersal in population dynamics, structure and gene flow. However, few studies have explored differences in this process between conspecifics living in natural habitats and those inhabiting landscapes highly transformed by humans, such as cities. Here, we investigate how individual traits and social characteristics can influence the natal dispersal decisions of burrowing owls (Athene cunicularia) living in urban and rural areas, as well as the consequences in terms of reproductive success and apparent survival. We found short dispersal movements among individuals, with differences between urban and rural birds (i.e., the former covering shorter distances than the latter), maybe because of the higher conspecific density of urban compared to rural areas. Moreover, we found that urban and rural females as well as bold individuals (i.e., individuals with shorter flight initiation distance) exhibited longer dispersal distances than their counterparts. These dispersal decisions have effects on individual fitness. Individuals traveling longer distances increased their reproductive prospects (productivity during the first breeding attempt, and long term productivity). However, the apparent survival of females decreased when they dispersed farther from their natal territory. Although further research is needed to properly understand the ecological and evolutionary consequences of dispersal patterns in transformed habitats, our results provide information about the drivers and the consequences of the restricted natal movements of this species, which may explain its population structuring through restricted gene flow between and within urban and rural areas.

Flight initiation distance, color and camouflage.

Camouflage is widespread throughout the animal kingdom allowing individuals to avoid detection and hence save time and energy rather than escape from an approaching predator. Thus, camouflage is likely to have co-evolved with antipredator behavior. Here, we propose that camouflage results in dichotomous escape behavior within and among species with classes of individuals and species with cryptic coloration having shorter flight initiation distances (FIDs; the distance at which an individual takes flight when approached by a human). We report the results of 2 tests of this hypothesis. First, bird species with cryptically colored plumage have consistently shorter FID than closely related species without such color. Within species with sexually dimorphic plumage, brightly colored adult male common pheasants Phasianus colchicus and golden pheasants Chrysolophus pictus have long and variable FID, whereas cryptically colored juveniles and adult females have short and invariable FID. Second, FID in females was predicted by presence or absence of cryptic color, FID in males and their interaction. These findings are consistent with the hypothesis that risk-taking behavior has been attuned to camouflage, and that species with different levels of camouflage differ consistently in their FID.

Antipredator behaviour of old-world tropical lizard, common agama Agama agama, in an urban environment

Abstract Escape behaviour is a common antipredator strategy of lizards. Here, we studied the effect of several variables predicted to have a potential effect on escape behaviour of lizards. Specifically, we measured the effects of starting distance (SD), distance to cover, sex–age and the observer’s head orientation on flight initiation distance (FID) in the common agama Agama agama. Agamas were approached in urban localities in Limbe city, Cameroon, where they were habituated to the presence of humans. We found no association between FID and SD, but individuals closer to potential cover had shorter FID than individuals farther from a refuge. Juveniles escaped later than adults, but no significant differences were found in the FID between adult males and females. Head direction of the approaching observer had no effect on FID. This is, to the best of our knowledge, the first study investigating factors affecting FID in common agamas, extending our knowledge of risk-related behaviour in lizards of the Old World tropics.

Differences in flight initiation distance (FID) between rural and urban populations of two species of Australian birds

Abstract Although flight initiation distance (FID) has been shown to be shorter in urban compared to rural populations of birds, less is known about how the characteristics of the urban environment, such as the population size and age of the city influences the FID and other aspects of anti-predator behaviour. Urban willie wagtails and magpie larks in a relatively small and new town had shorter FID than rural conspecifics. Both species were more likely to show a short, rather than long, escape flight if the experimenter started walking towards the bird from further away. There was some indication that urban birds may be more likely to show a short escape flight than rural birds. We conclude that anti-predator responses of birds can be influenced by a relatively small, recently established and sparsely-populated town. Additionally, the possibility of the characteristics of the urban centre influencing variation in the FID response is discussed.

Effect of human recreation on bird anti-predatory response

Wildlife perceive humans as predators, and therefore normally flushes. Flight initiation distance (FID) is the distance a human can approach an animal at a steady pace until it flushes. Recently, several studies showed differences in within-species FID according to human presence by comparing urban and rural habitats, with urban birds showing reduced FIDs. However, urban and rural habitats also differ in structure, which might affect FID. Therefore, in order to understand the real effect of human presence, we investigated whether differences in FID are also present in natural habitats (forests), differing only in the intensity of human use for recreation. We found that human frequentation had a distinct effect on bird escape responses, with shorter FIDs in forests more-heavily frequented by humans than in forests rarely visited by humans. Whether this finding is driven by non-random spatial distribution of personalities (shy vs. bold) or phenotypic plasticity (habituation to humans) cannot be assessed with our data. Studies relying on FIDs should also incorporate human recreation intensity, as this affects the measurements strongly.