Predator Detection is Limited in Microhabitats with High Light Intensity: An Experiment with Brown‐Headed Cowbirds

Variations in ambient light conditions across different microhabitats can modify the detectability of predators and prey. Prey have been shown to be more visible in sunlit than in shaded patches, leading to higher predation risk and more investment in vigilance (predation risk hypothesis). Additionally, prey have been hypothesized to take longer to detect predators in sunlit compared to shaded patches because of the excess of sunlight causing glare effects (disability glare hypothesis). We tested the predictions of these two non‐mutually exclusive hypotheses in a seminatural experiment with brown‐headed cowbirds by measuring vigilance behavior and detection of a ground predator in patches under the shade of vegetation and in the open. Light intensity and achromatic contrast were higher in the sunlit patches, which could enhance glare effects, but chromatic contrast was higher in the shaded patches. Brown‐headed cowbirds took longer to show alert reactions to and flee from a ground predator in sunlit compared to shaded patches. However, the two parameters associated with perceived predation risk (vigilance prior to the predator exposure and time to resume foraging after the attack) did not differ between sunlit and shaded patches. Our findings support to a greater extent the disability glare hypothesis than the predation risk hypothesis. Overall, ambient light conditions can affect two critical components of behavioral predator–prey interactions in terrestrial habitats: detection of and escape from predators. The effects of disability glare are expected to be more pronounced in bird species with wider visual fields or without sun‐shading structures; however, species may compensate through various behaviors (e.g. avoidance of sunlit patches and changes in head orientation).     

What determines probability of surviving predator attacks in bird migration?: the relative importance of vigilance and fuel load

Migrating birds must accumulate fuel during their journeys and this fuel load should incur an increased risk of predation. Migratory fuelling should increase individual mass-dependent predation risk for two reasons. First, acquisition costs are connected to the increased time a bird must spend foraging to accumulate the fuel loads and the reduced predator detection that accompanies foraging. Second, birds with large fuel loads have been shown to suffer from impaired predator evasion which makes them more vulnerable when actually attacked. Here, I investigate the relative importance of these two aspects of mass-dependent predation risk and I have used published data and a hypothetical situation for a foraging bird to investigate how much migratory fuelling in terms of escape performance and natural variation in predator detection contribute to individual risk during foraging. Results suggest that for birds foraging close to protective cover the negative impact of fuel load on flight performance is very small, whereas variation in time to predator detection is of great importance for a bird's survival. However, the importance of flight performance for predation risk increases as the distance to cover increases. Hence, variation in predator detection (and vigilance) probably influences individual survival much more than migratory fuel load and consequently, to understand risk management during migration studies that focus on vigilance and predator detection during fuelling are much needed.     

Costs and Benefits of Sociality Differ Between Female Guanacos Living in Contrasting Ecological Conditions

According to current theory, anti‐predator benefits promote group formation in open‐dwelling ungulates. An inverse relationship between vigilance effort and group size has been documented frequently and thought to reflect the consequent decrease in perceived predation risk as group size increases. In contrast, competition costs are supposed to set the upper limit to the number of individuals that can forage together. As anti‐predator behavior is no longer functional in the absence of predation and competition costs might be affected by resource distribution, the net benefit of aggregation will depend on the particular combination of predation risk and habitat structure experienced by the individual. To test this hypothesis, group‐size effects on female time allocation and within‐group aggression rate were compared between two guanaco populations exposed to contrasting levels of puma predation. Habitat structure within both sites consisted of mosaics of shrublands and grasslands, and group‐size effects were also compared between these habitat types. Females under predation risk showed a strong reduction in vigilance as the number of adults in the group increased, whereas females from the predator‐free population showed overall low levels of vigilance, regardless of group size. These results emphasize the anti‐predator significance of the group‐size effect on female vigilance, as well as guanaco plasticity to adjust time allocation to local conditions. On the other hand, within‐group aggression rate increased with the number of adults in the group. Aggression rate was almost null within groups located in grasslands but was significantly higher in shrublands, regardless of predation risk, suggesting that the more heterogeneous distribution of shrubs increases the interference competition level. These results strengthen the notion of predation pressure and habitat structure as major determinants of the balance between costs and benefits of group living, and highlight the potential of individual behavioral patterns to make qualitative predictions about group‐size variation within territorial ungulates.     

Predator detection and avoidance by starlings under differing scenarios of predation risk

Practically all animals must find food while avoiding predators.An individual's perception of predation risk may depend on manyfactors, such as distance to refuge and group size, but it isunclear whether individuals respond to different factors ina similar manner. We tested whether flocks of foraging starlingsresponded in the same way to an increased perception of predationrisk by assessing three factors: (1) neighbor distances, (2)habitat obstruction, and (3) recent exposure to a predator.We found that in all three scenarios of increased risk, starlingsreduced their interscan intervals (food-searching bouts), whichincreased the frequency of their vigilance periods. We thenexamined how one of these factors, habitat obstruction, affectedescape speed by simulating an attack with a model predator.Starlings were slower to respond in visually obstructed habitats(long grass swards) and slower when they had their head downin obstructed habitats than when they had their head down inopen habitats. In addition, reaction times were quicker whenstarlings could employ their peripheral fields of vision. Ourresults demonstrate that different sources of increased riskcan generate similar behavioral responses within a species.The degree of visibility in the physical and social environmentaffects both the actual and perceived risk of predation.     

Parent birds assess nest predation risk: influence of cavity condition and avian nest predator activity

Skutch hypothesized that nest predators visually assess parental activities to locate a prey nest, whereas parents modify fitness‐related traits to reduce the probability of nest predation. We examined how cavity condition and parental activity interact with avian nest predators to shape the nest success of two coexisting parid species, marsh tits Poecile palustris and oriental tits Parus minor, breeding in nest‐boxes during the incubation period. Nest‐boxes were manipulated to create a prolonged risk of nest predation (entrance diameter 2.6 cm control vs 5.5 cm treatment) soon after clutch completion. To measure changes in parental behavior, we also simultaneously simulated a pulsed risk of nest predation, using sound playbacks of a coexisting control bird and an avian nest predator. We found that the parent tits merely responded the pulsed risk, presumably due to an environment with high avian nest predator encounters, compared to the prolonged risk. Instead, both species spent more time on vigilance at the nest, only under prolonged risk conditions. The activity of corvids near the nest‐box was higher in the marsh tit than that in oriental tits. This activity was also higher in the treatment nest box than that in the control nest‐box. Nest predation during the incubation period was higher in marsh tits than in oriental tits, presumably due to higher and more plastic vigilance in oriental tits, compared to marsh tits. Our results highlight that the differences in cavity condition and parental activities at the nests of two coexisting non‐excavators may contribute to differential nest predation by attracting avian nest predators.     

Vigilance and fitness in grey partridges Perdix perdix: the effects of group size and foraging-vigilance trade-offs on predation mortality

1. Vigilance increases fitness by improving predator detection but at the expense of increasing starvation risk. We related variation in vigilance among 122 radio-tagged overwintering grey partridges Perdix perdix (L.) across 20 independent farmland sites in England to predation risk (sparrowhawk Accipiter nisus L., kill rate), use of alternative antipredation behaviours (grouping and use of cover) and survival. 2. Vigilance was significantly higher when individuals fed in smaller groups and in taller vegetation. In the covey period (in early winter when partridges are in flocks), vigilance and use of taller vegetation was significantly higher at sites with higher sparrowhawk predation risk, but tall vegetation was used less by larger groups. Individuals were constrained in reducing individual vigilance by group size and habitat choice because maximum group size was determined by overall density in the area during the covey period and by the formation of pairs at the end of the winter (pair period), when there was also a significant twofold increase in the use of tall cover. 3. Over the whole winter individual survival was higher in larger groups and was lower in the pair period. However, when controlling for group size, mean survival decreased as vigilance increased in the covey period. This result, along with vigilance being higher at sites with increasing with raptor risk, suggests individual vigilance increases arose to reduce short-term predation risk from raptors but led to long-term fitness decreases probably because high individual vigilance increased starvation risk or indicated longer exposure to predation. The effect of raptors on survival was less when there were large groups in open habitats, where individual partridges can probably both detect predators and feed efficiently. 4. Our study suggests that increasing partridge density and modifying habitat to remove the need for high individual vigilance may decrease partridge mortality. It demonstrates the general principle that antipredation behaviours may reduce fitness long-term via their effects on the starvation-predation risk trade-off, even though they decrease predation risk short-term, and that it may be ecological constraints, such as poor habitat (that lead to an antipredation behaviour compromising foraging), that cause mortality, rather than the proximate effect of an antipredation behaviour such as vigilance.     

Impact of increased predation risk on vigilance behaviour in a gregarious waterfowl,the Egyptian goose Alopochen aegyptiaca

Vigilance is amongst the most universal of anti‐predator strategies and commonly declines with increasing group size. We experimentally manipulated predation risk in a system with a known relationship between group size and vigilance levels to explore whether this relationship changes in response to elevated predation risk. We investigated the vigilance levels of Egyptian geese Alopochen aegyptiaca at eight golf courses in the western Cape, South Africa, to assess the perception of and reaction to predation risk. We manipulated predation risk by introducing trained Harris's hawks Parabuteo unicintus where avian predation was otherwise low or absent. The study confirmed the typical reduction in vigilance with group size on control sites, where the risk of predation is low. However, at experimental sites with elevated predation risk, a positive relationship between vigilance and group size was observed. We hypothesize that the mechanism for this relationship might be linked to social information transfer via copying behaviour and manipulation to induce vigilance. Thus, larger groups will have a higher probability of containing individuals with experience of elevated predation risk and their increased vigilance behaviour is copied by naïve individuals. This prediction is based on the intended outcome of introducing avian predation to make the geese feel less safe and to eventually leave the site as a management tool for controlling nuisance geese.     

Relationship between Distance to Cover,Vigilance and Group Size in Staging Flocks of Semipalmated Sandpipers

Cover can be either a source of protection or a source of danger for foragers. Distance to cover creates a gradient in predation risk that allows examining adjustments in anti‐predator behaviour such as group size and vigilance. As distance to obstructive cover increases, both group size and vigilance are expected to decrease given that individuals have more time to react to a more distant source of danger. I provide an empirical test of these predictions in staging semipalmated sandpipers (Calidris pusilla) in a system controlling for many confounding factors that have marred earlier research. Controlling for food density, forager density and phenotypic attributes often correlated with distance to cover, I found that as distance to obstructive cover increased, sandpipers foraged in sparser groups, were less flighty and to some extent less vigilant. Such controlled studies are needed to re‐assess the relationship between distance to cover and anti‐predator behaviour.     

Vigilance and grouping in the southern African ground squirrel (Xerus inauris)

Animals may form groups in response to the foraging–vigilance trade‐off, through enhanced predator detection (collective detection hypothesis) or reduced predation risk to the individual (dilution hypothesis), allowing individuals to decrease vigilance levels. Both hypotheses predict decreasing individual vigilance levels with increasing group size; however, the collective detection hypothesis also predicts increasing overall group vigilance with increasing group size. However, in species in which vigilance and foraging are not mutually exclusive, where vigilance may not be as costly, neither of these hypotheses may apply. Here, we examine the relationship between group size and vigilance in the social Cape ground squirrel (Xerus inauris), a species that can combine foraging and vigilance behaviours. Ten groups were observed using scan sampling, measuring both group and individual vigilance and group size. A negative relationship existed between individual vigilance and group size and a positive relationship between group vigilance and group size. Therefore, in Cape ground squirrels, vigilance seems to be costly even though it can be combined with foraging behaviours. Furthermore, group vigilance behaviour gives support to the collective detection hypothesis, whilst individual vigilance gives support to both hypotheses.     

Many eyes or many ewes: vigilance tactics in female bighorn sheep Ovis canadensis vary according to reproductive status

In gregarious animals, there is usually a negative relationship between individual vigilance and group size. This effect of group size is generally explained by increasing probability of predator detection (the many-eyes hypothesis) and by the dilution of risk occurring in larger groups. Few studies have attempted to examine the specific implications of either hypothesis on the expected vigilance pattern of an animal. Here we examine whether reproductive status affects vigilance patterns in bighorn sheep Ovis canadensis ewes. We also test whether the observed vigilance patterns are consistent with predictions from dilution or detection models of vigilance. Although vigilance decreased with increasing group size, vigilance tactics differed between barren and lactating females. Lactating ewes relied solely on predator detection. In contrast, barren ewes benefited from both detection and dilution effects when group size increased and adjusted vigilance effort according to the proportion of lactating ewes in their group. It is generally assumed that gregariousness increases safety. Here we further show that reproductive status influenced how animals reduce predation risk and that some individuals take advantage of the vigilance effort provided by others.     

Ecomorphological adaptations to bioluminescence in Porichthys notatus

Synopsis The midshipman fish, Porichthys notatus, is a visually active nocturnal predator. It must acquire exogenous sources of luciferin to remain luminescent and feeds on a variety of luminescent prey. Its ecomorphological adaptations for nocturnal predation were examined by observing predation on zooplankton illuminated by dinoflagellates. Its visual sensitivity is well adapted for detection of its own luminescence and its prey's luminescence. P. notatus can detect the luminescence of dinoflagellates and use this indirect light to increase predation on nonluminescent prey. Flash frequency and duration modulated predation rates. The interval between flash onset and strike initiation regulated strike success. High prey concentration decreased predation success due to increased optical and mechanosensory noise. Other ecomorphological adaptations of this predator to its special photic environment include a pigmented digestive tract, duplex retina with the capacity to discriminate emission spectra, contractible pupil, and the potential to counter illuminate. These morphological adaptations combined with an ambush predator style allow effective predation while minimizing exposure risk.     

Behavioural Responses of European Roe Deer to Temporal Variation in Predation Risk

In natural environments, predation risk varies over time. The risk allocation hypothesis predicts that prey is expected to adjust key anti‐predator behaviours such as vigilance to temporal variation in risk. We tested the predictions of the risk allocation hypothesis in a natural environment where both a species‐rich natural predator community and human hunters are abundant and where the differences in seasonal and circadian activity between natural and anthropogenic predators provided a unique opportunity to quantify the contributions of different predator classes to anti‐predator behaviour. Whereas natural predators were expected to show similar levels of activity throughout the seasons, hunter activity was high during the daytime during a clearly defined hunting season. According to the risk allocation hypothesis, vigilance should then be higher during the hunting season and during daytime hours than during the non‐hunting season and night‐time hours. Roe deer (Capreolus capreolus) on the edge of Bia?owie?a Primeval Forest in Eastern Poland displayed vigilance behaviour consistent with these predictions. The behavioural response of roe deer to temporarily varying predation risks emphasises the behavioural plasticity of this species and suggests that future studies of anti‐predator behaviour need to incorporate circadian variation in predation pressure as well as risk gradients of both natural and anthropogenic predators.     

Investigating Differences in Vigilance Tactic Use within and between the Sexes in Eastern Grey Kangaroos

Aggregation is thought to enhance an animal’s security through effective predator detection and the dilution of risk. A decline in individual vigilance as group size increases is commonly reported in the literature and called the group size effect. However, to date, most of the research has only been directed toward examining whether this effect occurs at the population level. Few studies have explored the specific contributions of predator detection and risk dilution and the basis of individual differences in the use of vigilance tactics. We tested whether male and female (non-reproductive or with young) eastern grey kangaroos (Macropus giganteus) adopted different vigilance tactics when in mixed-sex groups and varied in their reliance on predator detection and/or risk dilution as group size changed. This species exhibits pronounced sexual dimorphism with females being much smaller than males, making them differentially vulnerable toward predators. We combined field observations with vigilance models describing the effects of detection and dilution on scanning rates as group size increased. We found that females with and without juveniles relied on predator detection and risk dilution, but the latter adjusted their vigilance to the proportion of females with juveniles within their group. Two models appeared to equally support the data for males suggesting that males, similarly to females, relied on predator detection and risk dilution but may also have adjusted their vigilance according to the proportion of mothers within their group. Differential vulnerability may cause sex differences in vigilance tactic use in this species. The presence of males within a group that do not, or only partially, contribute to predator detection and are less at risk may cause additional security costs to females. Our results call for reexamination of the classical view of the safety advantages of grouping to provide a more detailed functional interpretation of gregariousness.     

Predator occurrence and perceived predation risk determine grouping behavior in guanaco (Lama guanicoe)

Grouping behavior of social ungulates may depend on both predator occurrence and perceived predation risk associated with habitat structure, reproductive state, and density of conspecifics. Over 3 years, we studied grouping behavior of guanaco (Lama guanicoe) families in Chilean Patagonia during the birthing season and determined their response to variation in predator occurrence and perceived predation risk (habitat structure, calf/adult rate, and density of conspecifics). We considered the effect of two predators, puma (Puma concolor) and culpeo fox (Lycalopex culpaeus). We measured two common (family group size and vigilance) and one novel (family group cohesion) behavioral responses of guanaco. Our results show that guanaco family groups adapted their grouping behavior to both predator occurrence and perceived predation risk. Larger family groups were found in open habitats and areas with high puma occurrence, while guanacos stayed in small family groups in areas with high shrub cover or low visibility. Group cohesion increased in areas with higher occurrence of pumas and culpeo foxes, and also increased in smaller family groups and in areas with low guanaco density. Vigilance (number of vigilant adults) was mainly related to group size and visibility, increasing in areas with low visibility, while residual vigilance (vigilance after removing the group‐size effect) did not vary with the explanatory variables examined. Our results suggest that a mix of predator occurrence and perceived predation risk influences guanaco grouping behavior and highlights the importance of evaluating different antipredator responses together and considering all predator species in studies aimed at understanding ungulate behavior.     

An Experimental Test of Predator Detection Rates Using Groups of Free-living Emus

Abstract Improved predator detection is often stated to be one of the principal benefits of social foraging. However, actual field evidence supporting this assumption is scarce. This may be the result of the fact that most observations are conducted on social animals acting in the absence of an acute predation threat, yet the benefits of grouping come to the fore in that brief moment when an individual's life is at risk. As predation attempts are typically rare in nature, experimental manipulations are necessary to further explore the costs and benefits of social foraging. This study utilizes simple predator simulations (by humans) to experimentally test the predator‐detecting abilities and escape strategies of groups of free‐living emus Dromaius novaehollandiae. Emus in larger groups spent less time in vigilance and more time foraging. Nonetheless, the combined vigilance of group members ensured that emus detected the ‘simulated predator’ sooner as group size increased. After detecting the ‘predator’, larger groups waited longer until opting to flee, and then spent less time and energy doing so. Thus, the results of this study provide experimental evidence that emus benefit from grouping in terms of both the ‘many‐eyes effect’ and the ‘dilution effect’.     

The effect of group size on vigilance in Ruddy Turnstones Arenaria interpres varies with foraging habitat

Foraging birds can manage time spent vigilant for predators by forming groups of various sizes. However, group size alone will not always reliably determine the optimal level of vigilance. For example, variation in predation risk or food quality between patches may also be influential. In a field setting, we assessed how simultaneous variation in predation risk and intake rate affects the relationship between vigilance and group size in foraging Ruddy Turnstones Arenaria interpres. We compared vigilance, measured as the number of ‘head‐ups’ per unit time, in habitat types that differed greatly in prey energy content and proximity to cover from which predators could launch surprise attacks. Habitats closer to predator cover provided foragers with much higher potential net energy intake rates than habitats further from cover. Foragers formed larger and denser flocks on habitats closer to cover. Individual vigilance of foragers in all habitats declined with increasing flock size and increased with flock density. However, vigilance by foragers on habitats closer to cover was always higher for a given flock size than vigilance by foragers on habitats further from cover, and habitat remained an important predictor of vigilance in models including a range of potential confounding variables. Our results suggest that foraging Ruddy Turnstones can simultaneously assess information on group size and the general likelihood of predator attack when determining their vigilance contribution.     

Dilution games: use of protective cover can cause a reduction in vigilance for prey in groups

Antipredatory vigilance usually decreases in groups. The generallyaccepted "collective detection" explanation implies that becausethere are more eyes to scan the surroundings for predators,individuals in a group can lower their personal investment invigilance without increasing their predation risk. The roleof other factors, such as numerical risk dilution caused bythe mere presence of companions, has been neglected. In a model,we explore a dilution game when foragers in groups have accessto protective cover. We show that foragers can take advantageof risk dilution and that this leads to changes in vigilancewith group size without the need to invoke collective detection.We identify a cost to maintaining high levels of vigilance asless vigilant foragers gather food faster and so depart thegroup sooner (to reach cover) leaving more vulnerable stragglersbehind. In groups, there is a scramble to reach safe sites thatcan induce a reduction in vigilance levels. Such a mechanismoperates less forcefully in large groups because individualsin these groups are less vulnerable to the departure of an individual.We also demonstrate that individuals should adopt lower levelsof vigilance, to reach safe sites sooner, when predator evasionis compromised or when the rate of food intake is high. Themodel provides new insights into the mechanisms underlying changesin vigilance with group size in animals.     

Has Predation Shaped the Social Systems of Arboreal Primates?

I studied antipredator behavior in two species of monkeys to elucidate the role of predation in shaping the social systems of arboreal primates. I compared the responses of monkeys to auditory and visual contact with predators to response elicited by sound playback experiments using the recorded calls of predators. Changes in vigilance and aggregation persisting up to 30 min after predator encounter occurred in both cases. Measures of vigilance shed light on individual perceptions of risk, while aggregation measures—intragroup spatial cohesion and polyspecific associations—permit direct inference about the protective benefits of grouping for the monkeys. They responded to real predator encounters and simulations in similar ways. Thus, sound playbacks of predator vocalizations are effective to simulate predator proximity. Contrary to predictions, predator encounters did not lead invariably to increased cohesion within groups or to increased time spent vigilant. Moreover, behavior in polyspecific associations was no different from that in single-species groups. Only red colobus encountering chimpanzees behaved as predicted by increasing vigilance and intragroup cohesion. The red colobus social system may have developed to protect against chimpanzee attack. In contrast, red-tailed monkey encounters with raptors and chimpanzees involved no change in time spent vigilant, coupled with decreases in intragroup cohesion. I conclude that predation is not a uniform selective pressure and patterns of social behavior within groups do not predict antipredator behavior.     

Does an opportunistic predator preferentially attack nonvigilant prey?

The dilution effect as an antipredation behaviour is the main theoretical reason for grouping in animals and states that all individuals in a group have an equal risk of being predated if equally spaced from each other and the predator. Stalking predators, however, increase their chance of attack success by preferentially targeting nonvigilant individuals, potentially making relative vigilance rates in a group relatively important in determining predation compared with the dilution effect. Many predators, however, attack opportunistically without stalking, when targeting of nonvigilant individuals may be less likely, so that the dilution effect will then be a relatively more important antipredation reason for grouping. We tested whether an opportunistically hunting predator, the sparrowhawk, Accipiter nisus, preferentially attacked vigilant or feeding prey models presented in pairs. We found that sparrowhawks attacked vigilant and feeding mounts at similar frequencies. Our results suggest that individuals should prioritize maximizing group size or individual vigilance dependent on the type of predator from which they are at risk. When the most likely predator is a stalker, individuals should aim to have the highest vigilance levels in a group, and there may be relatively little selective advantage to being in the largest group. In contrast, if the most likely predator is an opportunist, then individuals should simply aim to be in the largest group and can also spend more time foraging without compromising predation risk. For most natural systems this will mean a trade-off between the two strategies dependent on the frequency of attack of each predator type. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.     

The Effect of Simulated African Wild Dog Presence on Anti‐predator Behaviour of Kudu and Impala

In this study, we examined the behavioural, temporal and spatial effects of simulated African wild dog (Lycaon pictus) presence on its two main prey species: kudu (Tragelaphus strepsiceros) and impala (Aepyceros melampus). We spread African wild dog faeces around waterholes and played African wild dog sounds at different intervals to mimic immediate and non‐immediate predation pressure. We looked at anti‐predator behaviour at both a herd and individual level and distinguished between high‐quality (detracts from all other activities), high‐cost vigilance and low‐quality (used to monitor the surrounding in spare time), low‐cost vigilance to determine costs involved. We found that simulated African wild dog presence had little effect on anti‐predator behaviour of their free‐ranging prey. Only when immediate predation risk was mimicked did kudu invest in (additional) high‐quality vigilance, whereas impala showed no response. Regardless of direct cues of African wild dog presence, behavioural adjustments to reduce predation risk were primarily based on environmental factors such as time of the day and broad‐scale habitat structure. Predators have been shown to utilize waterholes to hunt, and prey species are therefore likely to maximize anti‐predator behaviour in this high‐risk environment based on environmental variables affecting predation risk, the main predator within the system, and water requirements, leaving little flexibility to respond to (simulated) African wild dog presence.