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.     

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.     

The influence of flock size and geometry on the scanning behaviour of spotted turtle doves,Streptopelia chinensis

Abstract The negative correlation between the time individuals spend scanning the environment for predators and group size is usually explained by the benefit of corporate vigilance. However, this negative correlation may be explained additionally in terms of the ‘dilution effect’ and ‘selfish herd geometry’. Our experimental investigation of the scanning behaviour of free-living spotted turtle doves foraging at different shaped feeders revealed that flock geometry influenced individual scanning rates. The time spent scanning declined with group size less rapidly among birds foraging in linear flocks than among those foraging in more two-dimensional flocks. These results were not confounded by aggressive behaviour, and indicate that the benefits of foraging in groups include the so-called selfish herd geometry.     

Vigilance in Greater Flamingos Wintering in Southern Tunisia: Age-Dependent Flock Size Effect

Decrease in individual vigilance with flock size is a widely recognized pattern in group‐living species. However such a relationship may be affected by other factors, such as age and flock composition. For instance, because young animals generally lack experience and have higher nutritional needs than adults, they can be expected not only to be less vigilant than adults but also to decrease their vigilance level by a greater extent when flock size increases than adults do. We investigated this issue using data on greater flamingos wintering in the gulf of Gabès, in southern Tunisia. Flamingos tended to congregate in small single‐age flocks for feeding, but as flock size increased, flocks became mixed. We found that when flock size increased, young flamingos significantly decreased their vigilance time, while adult did not, suggesting an age‐dependent flock size effect on vigilance. However, when flock composition (single‐age vs. mixed) was taken into account, a more complex pattern was found. Within single‐age and small flocks, no difference was found between young flamingos and adult ones regarding their vigilance level and their response to increasing flock size. However, within mixed and large flocks, adult flamingos were more vigilant than young ones, while variation in flock size did not result in a significant change in vigilance. These results suggest that young birds relied on the presence of adults, and hence more experienced individuals in detecting dangers, to reduce their vigilance and to increase their foraging time in order to satisfy their higher nutritional requirements. They could also be interpreted as a possible consequence of increasing competition with flock size which constrained more nutritionally stressed young flamingos to increase their foraging time to the detriment of vigilance.     

How do vigilance and feeding by common cranes Grus grus depend on age,habitat, and flock size?

Animals often spend less time vigilant and more time feeding when foraging in larger groups. This group-size effect does not, however, consider if larger groups differ systematically from smaller ones: Large groups could form in different habitats than small groups or be composed of a different mix of ages or classes than small groups. We examined how habitat differences and flock size and composition explain feeding and vigilance rates in common cranes Grus grus , wintering in holm oak Quercus ilex dehesas of Spain. Flock size and composition were related to habitat type in cranes: flocks formed in areas sown with cereal crops were larger than flocks formed in set aside areas. Vigilance rate depended on habitat but decreased with increasing flock size in a similar way across all habitats. Juveniles were less vigilant than adults and showed little change in vigilance with flock size. Vigilance increased and feeding time decreased over months from November through February. Our results show that vigilance is affected by habitat but that the group size effect on vigilance is not the product of differences between habitats in group size or composition.     

暂时与群体分离的个体藏马鸡的反捕食警戒

通过与同伴分担捕食风险 ,生活在稳定群体中的个体动物可以获得长期的适合度利益 ,但同时它们不得不承担食物竞争所带来的潜在代价。这种代价常常取决于食物资源的类型。当好的食物资源出现时 ,一些个体可以离开群体而独享这种资源。了解这些临时游离者如何组织其反捕食行为 ,在进化生态学上是有意义的。藏马鸡 (Crossoptilonharmani)是西藏雅鲁藏布江中游高山灌丛植被的一种典型的非繁殖季节集群鸟类。野外观察表明 ,为了独享好的食物资源 ,一些个体常常远离当前群体的活动范围。分离事件更可能发生于大的群体 ,但其发生率与参与者的数量呈负相关 ;而参与分离的个体愈多 ,分离持续的时间就愈长。分离者的个体警惕水平随着临时群体大小的增加而下降 ,遵从在其它自然大小鸟类群体所发现的一般性规律。分离行为的发生和持续时间被认为是个体对当前食物回报和捕食风险进行权衡的结果。这种利益 -代价权衡也可以解释藏马鸡所具有的强烈集群行为     

The Effects of Predation Risk, Food Abundance, and Population Size on Group Size of Brown-Headed Cowbirds (Molothrus ater)

Social and ecological conditions can influence flock formation (e.g. number of flocks, flock size, etc.) depending on the degree of social attraction of a species. We studied group formation in brown‐headed cowbirds (Molothrus ater) over short time periods (30 min) in two semi‐natural experiments conducted under controlled conditions. First, we determined the shape of the relationship between intake rate and flock size by manipulating group size in a single enclosure. Second, we assessed the role of population size, food abundance, and predation risk, and their interactions, in flock size formation in a system of four enclosures (two with and two without food) connected to a central refuge patch. In the first experiment, we found that pecking rates peaked at intermediate flock sizes (three to six individuals), which was influenced by greater availability of foraging time and more aggressive interactions in large groups. In the second experiment, flock sizes in the patches with food increased with population size likely due to the benefits of patch exploitation in groups. Flock size decreased after predator attack probably because refuge availability reduced perceived predation risk more than flocking in larger groups. Food abundance had minor effects, varying flock sizes between the two patches with food, under high food availability conditions when population size was high, probably due to social cohesion effects. Our results suggest that: (1) this species has an inverted‐U food intake–group size relationship with a range of intake‐maximizing flock sizes rather than a single peak, (2) the presence of a near refuge modifies the expected benefits of group patch exploitation under high predation risk, and (3) an increase in population size would more likely be translated into rapid increases in the size of the flocks rather than in more new flocks.     

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.     

Common Myna Roosts Are Not Recruitment Centres

We studied communal roosting in the Common Myna (Acridotheres tristis) in the light of the recruitment centre hypothesis and predation at the roost. The number and sizes of flocks departing from and arriving at focal roosts were recorded over a two year period. We also recorded the sizes and behaviour of foraging flocks. We found that flock sizes of birds departing from roosts at sunrise were larger than those at the feeding site, suggesting that there was no recruitment from the roosts. Flocks entering the roosts during sunset were larger on average than those leaving the following sunrise, suggesting no consolidation of flocks in the morning. Flocks entering the roosts at sunset were also larger on average than those that had left that sunrise, although there was no recruitment at the feeding site. There was no effect of group size on the proportion of time spent feeding. Contrary to expectation, single birds showed lower apparent vigilance than birds that foraged in pairs or groups, possibly due to scrounging tactics being used in the presence of feeding companions. Thus, the recruitment centre hypothesis did not hold in our study population of mynas. Predation at dawn and dusk were also not important to communal roosting: predators near the roosts did not result in larger flocks, and resulted in larger durations of arrival/departure contrary to expectation. Since flock sizes were smallest at the feeding site and larger in the evening than in the morning, but did not coincide with predator activity, information transfer unrelated to food (such as breeding opportunities) may possibly give rise to the evening aggregations.     

Advantages of social foraging of Willow Tits Parus montanus

The mean number of Willow Tits Parus montanus in single-species flocks was significantly larger than in mixed-species flocks of Willow and Coal Tits P. ater. Both flock size and the tendency of Willow Tits to join mixed-species flocks were negatively correlated with ambient temperature, probably because each bird, when the metabolic rate of the birds increased, could allocate more time to foraging due to improved predator detection by many eyes. The vigilance time of Willow Tits decreased with flock size and was determined by the total number of individuals in a flock rather than by the number of Willow Tits in mixed-species flocks of Willow and Coal Tits.     

Relation of group size and daily activity patterns to southern lapwing (Vanellus chilensis) behaviour

Behavioural patterns of birds commonly vary according to flock size and daily activity pattern. Southern lapwing behaviours and their relation with flock size were studied, as well as the relationship between the frequency of behaviours and the period of the day. Results showed that the proportion of time spent in foraging and vigilance was higher during the morning, when small groups were more common, and decreased from midday on, when group size increased. Maintenance and inactivity (sleeping) behaviours presented the opposite pattern. Correlation between flock size and period of the day, and their similar effects on bird behaviour may be evidence that groups of different sizes could have different functions throughout the day. Our results show that southern lapwings seem to form smaller flocks for feeding in the beginning of the day and larger flocks later for different activities (e.g., maintenance and sleeping). In this sense, it is possible that group size variation throughout the day is related to different demands for specific behaviours (functions) of groups, according to the daily activity patterns of the birds.     

Costs and benefits of flocking in foraging white-fronted geese (Anser albifrons): effects of resource depletion

This study investigated the costs and benefits of flocking in white-fronted geese Anser albifrons foraging on rice grains in Japan. The time budgets of focal geese were recorded, and the effects of flock size on the proportions of time spent in vigilant and agonistic behaviour were tested. The results showed that the decline in vigilance level and consequent increase in foraging time were beneficial results of flocking whereas agonistic interactions, a potential cost of flocking, did not increase with increasing flock size. However, seasonal variation in flock size suggested that exploitative competition could be a cost of flocking; the sizes of flocks in spring, when resource depletion had progressed, were significantly reduced compared with those in autumn. An experimental increase in rice density resulted in a significant increase in flock size. We conclude that the flock size of foraging white-fronted geese is a result of compromise between a constant benefit of flocking (i.e. decline in vigilance level) and a cost of flocking varying with food abundance (i.e. exploitative competition).     

An Experimental Analysis of Spatial Position Effects on Foraging and Vigilance in Brown-Headed Cowbird flocks

Several observational studies have found that the costs and benefits of social foraging vary as a function of spatial position in the group. However, it is difficult to make mechanistic inferences because several confounding factors, such as food deprivation levels, food availability, neighbor distance, and group size can mask or amplify spatial position effects. We attempted to address experimentally the effect of spatial position on foraging and vigilance in a group, controlling for many confounding factors. We used enclosures that restricted physical but not visual interactions between brown-headed cowbirds and manipulated spatial position, flock size, and neighbor distance. Pecking rate (number of pecks per trial duration) was not related with position, but instantaneous pecking rate (number of pecks per foraging bout duration) was higher at the edge. The proportion of time spent head-up (scanning and food-handling) was also higher at the edge. For pecking rate and proportion of time spent scanning, changes in neighbor distance influenced the behavior of edge birds to a lesser extent than central birds. These results suggest that cowbirds at the edge perceived greater predation risk and that during the limited foraging time available, edge birds tried to compensate by foraging at a faster rate.     

The influence of flocking on the foraging behaviour of the chough (Pyrrhocorax pyrrhocorax) and the Alpine chough (P. graculus) coexisting in the Alps

We studied the flocking and foraging behaviour of the chough Pyrrhocorax pyrrhocorax and the Alpine chough P. gruculus coexisting in the south-western Italian Alps in order to evaluate the costs and benefits of foraging in single- and mixed-species flocks.
In the single-species context, flock size significantly affected the foraging behaviour of the Alpine chough; in larger flocks, the birds stayed for a shorter time in a patch and fed more quickly than in smaller flocks. Flock size did not significantly affect the foraging behaviour of the chough, probably because of the small number of individuals per flock.
The propensity for mixed-species flocking was rather low. The observed frequencies of single-species flocks of choughs and Alpine choughs were significantly higher than those expected on the basis of random flocking, whereas the observed frequencies of mixed-species flocks of the two species were lower than those expected. The stay times became significantly shorter for the chough in the presence of the Alpine chough. Moreover, feeding rates of the Alpine chough were significantly lower in the presence of the dominant chough.
The present study does not confirm the hypothetical foraging advantages of flocking. In single-species flocks, the benefits for the Alpine chough (higher feeding rates in larger flocks) were roughly compensated by the costs (shorter stay times in larger flocks), whereas the chough apparently neither gained benefits nor endured costs.
In mixed-species flocks, the Alpine chough sustained costs due to a reduction of feeding rates and the chough suffered costs due to a reduction of stay times. Hence, on average, single-species flocking gives no evident foraging advantages to either the chough or the Alpine chough, whereas mixed-species flocking provides some disadvantages for both species.     

Collective Vigilance in the Wintering Hooded Crane: The Role of Flock Size and Anthropogenic Disturbances in a Human‐Dominated Landscape

Vigilance achieved at the group level, known as collective vigilance, can enhance the ability to assess threats and confer benefits to gregarious prey species. Examining the factors that influence collective vigilance and exploring how individual vigilance is organized at the group level can help to understand how prey groups respond to potential threats. We quantified collective vigilance and determined its temporal pattern in a natural wintering population of the hooded crane Grus monacha in the Shengjin Lake reserve in China. We examined the role of flock size and anthropogenic disturbances in the human‐dominated landscape on collective vigilance and level of synchronization. The proportion of time during which at least one individual scanned the surroundings (collective vigilance) increased with flock size and was higher in the more disturbed buffer zone of the lake. Synchronization of vigilance occurred more frequently in the smaller flocks but was not related to the risk of disturbance. Synchronization implies that individuals tend to monitor and copy the vigilance of neighbors. In the degraded wetlands, the wintering hooded crane benefits from foraging in groups and synchronizing their vigilance in response to human disturbances.     

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.     

Age-Related Microhabitat Segregation in Willow Tit Parus montanus Winter Flocks

It is expected that through flexibility in behaviour, flock living birds respond to the asymmetries in resource access derived from dominance relationships. We analysed the microhabitat use of willow tits in winter flocks and assessed possible factors which shape habitat segregation between adults and juveniles in different temperature regimes. When foraging in mild conditions (ambient temperature > 0°C), flocks split up into subgroups with adults foraging in inner parts of trees more often than juveniles. However, no differences were recorded in the vertical position occupied in trees. In harsh conditions (< ? 4°C), flocks re‐united and juveniles further moved to outer parts of trees, increasing horizontal segregation between age classes. In mild conditions, vigilance behaviour was not related to the position of birds in trees, but in harsh conditions, scanning frequency was higher in outer parts of trees only for adults. In mild weather, juvenile position in trees was associated with body size and mass. The foraging microhabitat segregation detected in harsh conditions fits the age‐related hoarding distribution previously described in the same population. This supports the hypothesis that hoarded food is important in determining future foraging habitat use. Adult preference and intraspecific competition for safer or richer inner parts of trees as foraging sites during harsh conditions seems to determine the habitat segregation between adults and juveniles. Furthermore, we suggest that in mild weather, when foraging in the absence of adults, juveniles balance the costs of using a potentially dangerous microhabitat with the benefits of building energetically cheap and large food reserves through hoarding. The expected patterns of microhabitat segregation may differ in parids, depending on whether predation risk or other factors such as food availability are the main factors controlling habitat quality.     

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’.     

Vigilance during Preening in Coots Fulica atra

In birds, vigilance during feeding is usually linked with (1) the many‐eyes‐hypothesis, (2) the dilution effect, and (3) the intraspecific scramble competition hypothesis. To exclude competition for resources as a driving force in vigilance research into activities other than feeding is necessary. Additionally, variables such as nearest neighbour distance and position within a flock are supposed to influence vigilance and vigilance of singletons should be compared with vigilance of flock members. I studied vigilance during preening in Eurasian Coots (Fulica atra) and counted the number of scans per minute. A total number of 117 coots were sampled with 16 of them preening alone. Coots preening alone showed a significantly higher scan rate. I found a significant negative correlation between vigilance and flock size while nearest neighbour distance correlated positively with vigilance. Further, vigilance was higher in individuals at the edge. A general linear model using nearest neighbour distance and flock size within 10 m as covariates also revealed a significant influence of flock size but not of nearest neighbour distance. The centre‐edge effect still remained significant. These results indicate that flock size is the most influential predictor of vigilance in preening coots, followed by spatial organization. However, using nearest neighbour distance instead of flock size also produced a significant model as did the flock size measurement within a radius of 20 m. As scramble competition could be ruled out, the flock size effect may be indeed related to predation.     

The advantages of social foraging in downy woodpeckers

I investigated the advantages gained by downy woodpeckers (Picoides pubescens) which join mixed-species winter flocks. Woodpeckers foraging alone showed high levels of vigilance as measured by head-cocking rates, and low feeding rates. Woodpeckers foraging with one or two flock members showed intermediate rates of head-cocking and feeding, while woodpeckers foraging with flocks of three or more birds showed low head-cocking rates and high feeding rates. Although local enhancement and copying may contribute to the woodpeckers' increased foraging efficiency in a flock, these do not appear to be the main factors. As downy woodpeckers spend less time on vigilance, they devote more time to foraging, thereby increasing their foraging efficiency