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.     

A Comparison of Methods for Estimating Northern Bobwhite Covey Detection Probabilities

Abstract: We compared the time-of-detection and logistic regression methods of estimating probability of detection for northern bobwhite (Colinus virginianus) coveys. Both methods are unusual in that they allow estimation of the total probability of detection (i.e., the product of the probability that a covey is available for detection [i.e., that a covey vocalizes] and detection given availability). The logistic regression method produced an average detection probability of 0.596 (SE = 0.020) and the time-of-detection method produced a detection probability estimate of 0.540 (SE = 0.086), and the 2 estimates were not significantly different. This is the first evaluation of the time-of-detection method with empirical field data. Although the time-of-detection and logistic regression method each have advantages, both can be used under appropriate conditions to improve estimates of bobwhite abundance by allowing for the estimation of detection probabilities. Improved estimates of bobwhite abundance will allow land managers to make more informed management decisions.     

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.     

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.     

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.     

Group size elicits specific physiological response in herbivores

With increasing group size, individuals commonly spend less time standing head-up (scanning) and more time feeding. In small groups, a higher predation risk is likely to increase stress, which will be reflected by behavioural and endocrine responses. However, without any predator cues, we ask how the predation risk is actually processed by animals as group size decreases. We hypothesize that group size on its own acts as a stressor. We studied undisturbed groups of sheep under controlled pasture conditions, and measured in situ the cortisol and vigilance responses of identified individuals in groups ranging from 2 to 100 sheep. Both vigilance and average cortisol concentration decreased as group size increased. However, the cortisol response varied markedly among individuals in small groups, resulting in a lack of correlation between cortisol and vigilance responses. Further experiments are required to explore the mechanisms that underlie both the decay and the convergence of individual stress in larger groups, and whether these mechanisms promote adaptive anti-predator responses.     

Harbour seal grouping as an anti-predator strategy

Terrestrial grouping by western Atlantic harbour seals, Phoca vitulina concolor, was investigated to determine whether it has a functional significance and, if so, what the function might be. Two nofunctional hypotheses were tested: (1) groups arise from a shortage of haul-out sites or space on sites; and (2) terrestrial groups are extensions of groups formed in the water for reasons not related to hauling-out. Aerial surveys of haul-out site use by seals indicated that sites were not in short supply. Observations of the use of space on one haul-out site suggested that space on sites was not limiting. Arrivals and departures of seals at one study site showed that groups formed at the haul-out site and disbanded after the haul-out period. The hypothesis that grouping functions to increase the probability of predator detection was tested by measuring individual and group vigilance relative to group size, and simulating predator attacks in the field. Although individual vigilance decreased with increasing group size, group vigilance increased. Larger groups detected ‘attacks’ at greater distances and a solitary seal failed to detect an ‘attack’. It is proposed that an individual increases its probability of detecting a predator by joining a group.     

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.     

Vigilance in a Cooperatively Breeding Primate

Collective vigilance is considered a major advantage of group living in animals. We investigated vigilance behavior in wild mustached tamarins (Saguinus mystax), small, arboreal, cooperatively breeding New World primates that form stable mixed-species groups with saddleback tamarins (Saguinus fuscicollis). We aimed 1) to investigate whether vigilance patterns change according to individual activity and 2) to examine whether there is a social component of vigilance in their cooperative and nonaggressive society. We studied 11 factors that may influence vigilance and used this data to interpret the possible functions of vigilance. We observed 44 individuals in 3 mixed-species and 2 single-species groups of 2 populations that differed in population density and home range sizes. Vigilance changed greatly when individuals were engaged in different activities and individual vigilance was affected by different sets of factors depending on the activity. As vigilance decreased in proximity of conspecifics and heterospecifics when feeding, and in larger mixed-species groups when resting, we conclude that the predominant function of vigilance in mustached tamarins is predator related. However, the absence of the group size effect in very large single-species groups suggests that it may also function to maintain group cohesion. In the population with higher density and smaller home ranges individuals also increased their vigilance in home range overlap areas. We found no evidence that mustached tamarins monitor group mates to avoid food stealing or aggression. The effect of heterospecifics on individual vigilance suggests that collective vigilance might have been an important incentive in the evolution of tamarin mixed-species groups.     

Human Activity Dampens the Benefits of Group Size on Vigilance in Khulan (Equus hemionus) in Western China

Animals receive anti-predator benefits from social behavior. As part of a group, individuals spend less time being vigilant, and vigilance decreases with increasing group size. This phenomenon, called “the many-eyes effect”, together with the “encounter dilution effect”, is considered among the most important factors determining individual vigilance behavior. However, in addition to group size, other social and environmental factors also influence the degree of vigilance, including disturbance from human activities. In our study, we examined vigilance behavior of Khulans (Equus hemionus) in the Xinjiang Province in western China to test whether and how human disturbance and group size affect vigilance. According to our results, Khulan showed a negative correlation between group size and the percentage time spent vigilant, although this negative correlation depended on the groups’ disturbance level. Khulan in the more disturbed area had a dampened benefit from increases in group size, compared to those in the undisturbed core areas. Provision of continuous areas of high-quality habitat for Khulans will allow them to form larger undisturbed aggregations and to gain foraging benefits through reduced individual vigilance, as well as anti-predator benefits through increased probability of predator detection.     

Vigilance and group size in ostriches

Wild ostriches were observed while feeding alone or in groups of up to four birds, and their vigilance (proportion of time with the head up) recorded. Individual vigilance declined as group size increased, mainly through a decrease in the frequency with which the head was raised. Males were more vigilant than females, mainly because they kept their heads up for longer. Vigilance was influenced more by the presence than by the vigilance of a companion. When a head would stay down for a long time was impossible for a predator to predict. It is concluded that grouping by ostriches when feeding results in only a slight reduction in the group's vulnerability to successful predator attack, but in a considerable decrease in individual vulnerability.     

Home range and habitat selection of northern bobwhite coveys in an agricultural landscape

Northern bobwhites thrive in fine-grained landscapes with a diversity of early succession woodland, grassland, and agriculture-associated habitat types. Bobwhite conservation has proved challenging in the increasingly coarse-grained Midwestern landscape as simplified agricultural cropping systems are implemented at larger spatial scales. Regardless, managing agricultural landscapes on private lands is the primary opportunity to restore bobwhite populations in the Midwestern United States. Although bobwhite habitat requirements are well understood, habitat selection in contemporary Midwestern landscapes is not well understood, especially on private lands where populations are declining. We used compositional analysis to investigate second- (study area) and third- (home range) order habitat selection by radiomarked bobwhite coveys on 4 private land study areas in southwestern Ohio. Mean covey home range size was 26.1 ± 2.2 ha (n = 48). Although home ranges were established in areas with more grassland cover, bobwhites most strongly selected early succession woody habitat (e.g., fencerows and ditches) at all scales, and selection for grassland diminished between the study area and home range scales. Grassland selection varied among sites and was strongest on sites with more row crop area. Woodlots were avoided at the study area scale, but were selected within home ranges. Grassland cover, like that provided by contemporary conservation programs, is an essential component of bobwhite habitat in the Midwest, but our results suggest more emphasis should be placed on early succession woody cover. Woody cover associated with fencerows, ditches, and woodlots adjacent to food sources and breeding habitat will likely improve non-breeding season survival, which is an influential vital rate in northern populations. © 2012 The Wildlife Society.     

Group‐Size Effect on Vigilance and Foraging in a Predator‐Free Population of Feral Goats (Capra hircus) on the Isle of Rum,NW Scotland

Many previous studies have found that as group size increases, individual vigilance levels decrease and forage intake increases (group‐size effect), but few such studies have considered the impact of within‐group interactions and other confounding factors on the direction of group‐size effects. A free‐ranging population of feral goats (Capra hircus), with little predation threat, was studied on the Isle of Rum (northwest Scotland), from Jun. to Nov. 2000, to investigate the effects of group size on individual vigilance levels and foraging efficiency after taking into account the effect of confounding factors (e.g. sex, season, time of day, habitat, predation risk) and within‐group interactions (via changes in movement rates while feeding). Our results show that, while group size exerted a negative influence on individual vigilance levels and a positive effect on movement rate, foraging efficiency never increased with group size and even decreased at certain times of day. There was no sex difference in individual vigilance in feral goats, but foraging efficiency was higher in females than in males. Goats were more vigilant in fall than in summer. The results imply that the benefits for foraging obtained from the reduced vigilance level in larger groups may be constrained or offset by increased interaction (or competition) within larger groups even in a population that faces negligible predation risk.     

Number of neighbors instead of group size significantly affects individual vigilance levels in large animal aggregations

The group size effect states that animals living in groups gain anti‐predator benefits through reducing vigilance levels as group size increases. A basic assumption of group size effect is that all individuals are equally important for a focal individual, who may adjust its vigilance levels according to social information acquired from them. However, some studies have indicated that neighbors pose greater influences on an individual's vigilance decisions than other group members, especially in large aggregations. Vigilance has also been found to be directed to both predators (anti‐predation vigilance) and conspecifics (social vigilance). Central individuals might rely more on social vigilance than peripheral individuals. To test these hypotheses, we examined the effects of flock size, number of neighbors and position within a flock on vigilance and competition of greater white‐fronted goose Anser albifrons that form large foraging flocks in winter, controlling the effects of other variables (group identity, winter period and site). We found that individual vigilance levels were significantly affected by number of neighbors and position within a flock, whereas flock size showed no effect. Individuals devoted a large component of vigilance to nearby flock mates. Central individuals directed a relatively larger proportion of vigilance to monitor neighbors than peripheral ones, indicating that central individuals more relied on social information acquired from neighbors, possibly caused by the more blocked visual field of central individuals. Moreover, some social vigilance may function as conducting or preventing agonistic interactions since competition intensity was positively correlated with number of neighbors. Our study therefore demonstrate that the number of neighbors is more important than group size in determining individual vigilance in large animal groups. Further studies are still needed to unravel which neighbors pose greater influence on individual vigilance, and the factors that influence individuals to acquire information from their neighbors to adjust vigilance behaviors.     

PREDATOR VIGILANCE AND GROUP SIZE IN MAMMALS AND BIRDS: A CRITICAL REVIEW OF THE EMPIRICAL EVIDENCE

One commonly cited benefit to animals that forage in groups is an increase in the probability of detecting a predator, and a decrease in the time spent in predator detection. A mathematical model (Pulliam 1973) predicts a negative relationship between group size and vigilance rates. Over fifty studies of birds and mammals report that the relationship at least partly explains why individuals forage in groups. This review evaluates the strength of these conclusions based on their evidence. Those variables that may confound the relationship between vigilance and group size are outlined, and their control is assessed for each study. The variables I consider to be important include the density and type of food; competition between individuals; the proximity to both a safe place and the observer; the presence of predators; the visibility within the habitat; the composition of the group; the ambient temperature and the time of day. Based on these assessments, most of the studies fail to adequately demonstrate an unambiguous relationship between vigilance behavior and group size. Nevertheless, many studies reveal interesting features of the relationship between vigilance and group size that should provide fruitful avenues for future research.     

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.     

Vigilance and Collective Detection of Predators in Degus (Octodon degus)

Individuals of social and partially social species typically reduce their vigilance activity when foraging in groups. As a result, per capita risk of predation decreases and individuals allocate more time to foraging and other fitness rewarding activities. Reduction of per capita risk is hypothesized to occur because there are more individuals to detect potential predators. If so, collective (i.e. total) vigilance is expected to increase with foraging group size. Increased surveillance during group foraging may occur if group members scan independently of one another, or sequentially to avoid the overlapping of their vigilance bouts. Intriguingly, such coordinated vigilance assumes that individuals monitor not only the presence, but the vigilance behaviour of group mates. We used seasonal records on time budget and grouping patterns of individually marked degus (Octodon degus), a social rodent, to examine if (a) individual vigilance decreases and/or foraging increases with group size, (b) collective vigilance increases with group size and (c) foraging degus coordinate their vigilance. When foraging, degus decreased their individual vigilance and increased their foraging time when in larger groups. Despite this, degus in larger groups increased their collective vigilance, supporting the hypothesis that socially foraging degus decrease predation risk through an improved ability to detect and escape potential predators. Additionally, patterns of collective vigilance suggested that degus scan independently of each other and so, they do not coordinate their vigilance to prevent its temporal overlapping. This finding does not support that foraging degus monitor the vigilance activity of group mates.     

Multi-region response to conservation buffers targeted for northern bobwhite

We coordinated a large-scale evaluation of northern bobwhite (Colinus virginianus) population response to establishment of 9-m to 37-m linear patches (buffers) of native herbaceous vegetation along row-crop field margins as part of the Conservation Reserve Program practice Habitat Buffers for Upland Birds (CP33). We compared northern bobwhite covey densities on 1,088 paired row-crop fields with and without native herbaceous buffers in 13 states during autumn, 2006–2008. We used a 2-stage random effects modeling approach that incorporates the effective area as an offset in generalized linear mixed models to assess regional relationships among autumn bobwhite covey densities and covariates of field type (i.e., fields with vs. without native herbaceous buffers), ecological region, year, survey week, and contracted vegetative cover (i.e., planting native grasses and forbs vs. establishing through natural regeneration). Covey density was correlated with year and interaction effects of field type and ecological region. The year effect suggested annual variation in covey densities, whereas the field type by ecological region interaction suggested covey response to buffers was dependent on spatial location, likely reflecting differences in buffer establishment, succession, and characteristics of the surrounding landscape among regions. Mean fitted covey density on fields across all survey sites was 0.047 (±0.008 bootstrap standard error [BSE]) and 0.031 coveys/ha (±0.003 BSE) on row-crop fields with and without herbaceous buffers, respectively. Covey density was greater on fields with buffers relative to matched, comparison fields without buffers in the Mississippi Alluvial Valley (241%; P < 0.001) and both the eastern (123%; P < 0.001) and western (60%; P = 0.01) portions of the Southeastern Coastal Plain region. Covey density was an order of magnitude greater in the central Texas region compared to other regions, but exhibited a small response to native herbaceous buffers, as did density of coveys in the Eastern Tallgrass Prairie and Central Hardwoods regions. Disproportionate response to buffers in the Mississippi Alluvial Valley and Southeastern Coastal Plain suggests native herbaceous habitats might be limiting during autumn in these regions, whereas lack of response in the Eastern Tallgrass Prairie, Central Hardwoods, and central Texas regions suggests that herbaceous habitat either was not limiting or buffers failed to provide adequate requirements for bobwhites during autumn. Selection of other habitats to meet security and thermoregulatory needs might have resulted in lack of response in these regions. Native herbaceous cover provided by buffers can provide critical habitat in row-crop agricultural systems in some regions, and can contribute to regional population recovery objectives of the Northern Bobwhite Conservation Initiative (NBCI). However, range-wide NBCI recovery objectives will best be met through multiple conservation practices in row-crop agricultural systems. © 2013 The Wildlife Society.     

Effect of Group Size on Individual and Collective Vigilance in Greater Rheas

We studied the effect of group size on the proportion of time that greater rheas, Rhea americana, allocated to vigilance and feeding during the non‐breeding season. We tested whether: (1) the proportion of time that one bird allocates to vigilance (individual vigilance) decreases with group size, and (2) the proportion of time that at least one bird of the group is vigilant (collective vigilance) increases with group size. We analyzed video‐recordings of birds that were foraging alone or in groups from two to 12 birds. The proportion of time allocated to individual vigilance decreased and the proportion of time spent feeding increased with group size. In both cases the main significant difference was between birds foraging alone and in groups. Collective vigilance did not vary with group size and it was lower than expected if vigilance bouts were random or sequential. Our results indicate that rheas foraging in large groups would not receive the benefit of an increase in collective vigilance, although they could still benefit from a reduction of predation risk by the dilution effect.