Robust Fall Social Displays Predict Spring Reproductive Behavior in Brown‐Headed Cowbirds (Molothrus ater ater)

The ability to engage others in close proximity may be an essential component of social life and shapes the development of social skills. Variation in the willingness to initiate and sustain close interaction with conspecifics is known as sociability. The Brown‐headed Cowbird (Molothrus ater) uses an affiliative display called the head‐down to bring individuals into close proximity. During fall 2009, we manipulated a large flock of cowbirds in a fission–fusion perturbation and recorded the frequency of head‐downs and social approaches. During the fission–fusion perturbation, the rate of head‐downs remained both correlated and repeatable across perturbations. In spring 2010, we separated individuals into three aviaries, a high, intermediate, and low aviary, based on the frequency of head‐down displays they initiated during the previous fall 2009. When breeding, males in the high flock produced a higher number of songs within counter‐singing matches, and females laid more eggs in comparison with the other aviaries. These findings suggest that head‐down displays performed outside the breeding season may contribute to the development and maintenance of reproductive competence by providing intimate social interactions with others.     

Observational and Experimental Evidence for the Function of Tail Flicking in Eurasian Moorhen Gallinula chloropus

Tail movements such as wagging, flicking or pumping are reported from many bird species but their adaptive functions remain poorly understood. To investigate whether tail flicking functions as an alarm signal, either to predators or neighbouring birds, or as a signal of submission to conspecifics, I observed this behaviour in moorhen in a natural context, and conducted playback experiments using vocalizations of predators, conspecifics and heterospecifics. I found positive relationships between flicking and vigilance and nearest neighbour distance, and negative relationships between flicking and moorhen flock size and total flock size. Moorhen at the edge of a flock flicked at a higher rate. Single moorhen flicked more often compared with individuals in groups, both in single‐ and mixed‐species flocks, and there was a tendency that single moorhen flicked more often than single moorhen within a mixed‐species flock. Moorhen responded differently to conspecific and predator calls. While in both cases vigilance increased, tail flick rate was higher during predator playbacks and lower during conspecific playbacks. Furthermore, moorhen remained rather motionless when conspecific calls were played back, but not during predator calls, and, moorhen resumed to a baseline level of tail flicking more quickly after the playback of conspecific calls. Taken together, the results suggest that flicking may be considered as a honest signal of vigilance directed towards ambushing predators.     

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.     

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.     

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.     

Effects of position and flock size on vigilance and foraging behaviour of the scaled dove Columbina squammata

Amongst the benefits of foraging in flocks are the enhancement of food finding and predation avoidance. Characteristics such as size, individual position, as well as position and distance between members are factors that may influence vigilance and foraging. In a study using scaled doves, Columbina squammata, I observed a negative correlation between group size and vigilance and a positive correlation with time spent foraging, which suggests a reduction of costs and an increase of benefits as a consequence of larger group sizes. Individual position in the flock appeared to be an important factor in this trade-off. Peripheral individuals were more vigilant and foraged less than central ones, suggesting an edge effect in flocks of this species. The clustering of conspecifics may be related with fast transmission of information. Overall, aggressive interactions were rarely observed; when registered, they occurred mostly in larger groups, suggesting an effect of interference competition. These results imply that predation may be a strong pressure on the scaled dove's flock formation and behaviour.     

Does experimentally altered plumage ornamentation of female passerines influence nestling quality? A test in female tree swallows

It is widely recognized that female animals in a variety of taxa display ornamental traits, such as elaborate plumage, but elucidating whether such traits evolve by selection or genetic correlation remains a challenge because more ornamented females are often found to produce low‐quality offspring. While resource trade‐offs between the production of ornaments and offspring may underlie this negative relationship, it is not an adequate explanation for species where the timing of production of ornaments and offspring does not overlap. Instead, costs associated with engaging in agonistic interactions with conspecifics, which maintains the honesty of signals of quality, may also reduce resources available to invest in offspring. In this study, we enhanced and reduced the plumage brightness of female tree swallows (Tachycineta bicolor) relative to controls to test whether social costs associated with displaying bright plumage is a potential mechanism underlying the observation that more ornamented females produce low‐quality nestlings. Our results showed that nestlings reared by females in the enhanced plumage brightness treatment were structurally smaller, having shorter lengths of the combined head and bill than nestlings in the reduced plumage brightness treatment, and tended to grow their head and bills more slowly than nestlings in the reduced and control plumage brightness treatments. Nestlings in the enhanced plumage brightness treatment also tended to gain mass more slowly than nestlings in control treatment. Overall, that females with enhanced plumage brightness produced structurally smaller nestlings provides evidence that social costs paid by females may lead to the production of low‐quality offspring and should be considered in future studies.     

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.     

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).     

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.     

Head raising during foraging and pausing during intermittent locomotion as components of antipredator vigilance in chipmunks

Although many studies of vigilance examine head raising in foraging, grooming or resting animals, pauses during intermittent locomotion are rarely considered from the perspective of vigilance, and no studies have compared head raising and pausing in the same system. We videotaped central place foraging chipmunks, Tamias striatus, as they approached a patch, collected sunflower seeds, and left to return to their burrows. There was a strong similarity between head raising during foraging and pausing during intermittent locomotion. Chipmunks paused more frequently when moving towards the patch than when leaving the patch. Chipmunks in the patch raised their heads at an intermediate rate, which tended to decrease with time in the patch. Pauses and the duration of motionless periods during head raises were very short (∼0.4 s), and their frequency distributions were similar. Animals remained motionless during 22% of the time spent approaching the patch, 14% of the time spent in the patch and 7% of the time spent leaving the patch. Rates of pausing and head raising generally decreased with short-term familiarity (number of trips to the patch) and with long-term familiarity (proximity of the patch to the burrow). Trials with higher pause rates when approaching the patch had higher head-raising rates in the patch. Whether the focal individual was solitary, dominant or subordinate in a dyad, or competing with multiple chipmunks in the patch had no effect on pausing or head raising. In a separate experiment, exposure to a model hawk increased pause and head-raising rates. We conclude that head raising during foraging and pausing during locomotion serve a similar vigilance function, that this vigilance is directed towards detection of predators rather than conspecifics, and that time allocated to vigilance is sufficient to significantly reduce foraging rates and affect many space use and foraging decisions.     

Sexual Selection and the Psychophysics of Female Choice

Woodland birds of 20 species were captured in mist-nets in winter (September to February) and summer (April—August) and scored for whether or not they made distress calls while being handled. Birds of only two species showed significant tendencies to be consistent in calling or failing to call when recaptured, but most species had weak trends in. the same direction. Some species changed seasonally in the tendency to call, but there was no general pattern. In winter, but not summer, the incidence of calling was correlated across species with an index of the number of conspecifics likely to hear the call, but there was no comparable correlation with variations within species. In the long-tailed tit, calling was associated with struggling, and with eyelid colour, which reflects a bird's mood. This implies that calling may be directed at a captor as well as at flock companions.     

To scan or not to scan? Occurrence of the group‐size effect in a seasonally nongregarious forager

Group‐living requires a compromise between safety and direct/indirect costs for individuals. The larger is the group, the greater is the collective vigilance, leading to a greater net food intake per forager because of the time saved individually from scanning behaviour. In turn, individual alertness usually decreases with increasing group size (“group‐size effect”). Information on the occurrence of group‐size effect is still unclear. Previous studies have shown that it may fail to occur or even reverse, for example when costs of interference between conspecifics are high. In turn, assessing whether the group‐size effect would occur in weakly or seasonally gregarious species may help to understand its drivers. We evaluated the occurrence and the extent of group‐size effect in a seasonally nongregarious herbivore, the roe deer Capreolus capreolus. We examined the roles of sex/age class and season as drivers of vigilance behaviour. In roe deer, the group‐size effect did not depend on sex/age class: time spent foraging increased with increasing group size; in turn, vigilance increased with decreasing group size, in all sex/age classes. Females with fawns were the most vigilant sex/age class, thus revealing the cost of offspring protection. Accordingly, the higher spring vigilance levels of does could be related to reproductive costs (e.g., defence of newborn fawns). Conversely, the greater summer vigilance of bucks could result from patrolling/defence of territories. Both adult males and females also showed the higher vigilance in winter, likely because of an increase in the perception of predation risk and/or, possibly, hormones linked to an increase in intolerance of conspecifics, in males. However, the group‐size effect occurred in all the seasons, for adult males and females. Our findings suggest that foraging benefits provided by the group‐size effect may have overcome costs of group‐living, even in a weakly gregarious forager.     

Incompatibility between antipredatory vigilance and scrounger tactic in nutmeg mannikins, Lonchura punctulata

Most social foragers must search for food while avoiding predators. Group-foraging nutmeg mannikins engaged in a producer-scrounger game search for their own food (play producer) by hopping with the head down and search for others' food discoveries (play scrounger) by hopping with the head up. If the scrounger tactic is compatible with antipredatory vigilance, then an increase in antipredatory vigilance should lead to the detection of more joining opportunities, and hence to more joining by foragers. We tested this prediction as well as the extent to which stationary birds use head up exclusively for antipredatory purposes and hopping birds use head up for foraging purposes only. We observed three flocks of nutmeg mannikins searching for hidden clumps of food in an indoor aviary. We used a 2×2 factorial design in which both the distance to a safe refuge and the food distribution were manipulated. The use of head up by stationary and eating birds increased significantly with increased distance to cover. Distance to cover, however, had no effect on the use of the scrounger tactic or on the level of joining. We found no evidence of compatibility between the scrounger tactic and antipredatory vigilance. Our results provide the first unambiguous evidence for the existence of two distinct and incompatible patterns of vigilance for predators and for conspecifics. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.      

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.     

A communication-based spatial model of antipredator vigilance

Many animals spend their lives performing two often mutually exclusive tasks: feeding and watching out for predators (anti-predator vigilance). There have been many theoretical and empirical studies investigating this trade-off, especially for birds. An important characteristic of a flock of real birds is the area occupied by the flock. Individuals feeding close together experience increased competition so that the feeding rate decreases. Widely spaced individuals may suffer a loss in vigilance efficiency, since communication between individuals is more difficult, such that the predation risk increases. A vigilance model is developed which allows birds to control their spacing (and so the area of the flock) as well as their vigilance rate. The best strategy for the birds is found under a variety of environmental conditions, under the assumption that each individual acts selfishly to maximize its own fitness.     

Seeing without being seen: a removal experiment with mixed flocks of Willow and Crested Tits Parus montanus and cristatus

This paper tests the hypothesis that foraging site selection reflects a trade-off between the various needs for concealment from predators, to find food, and for the individual to maintain some view of its surroundings. After removal of Crested Tits Parus cristatus (the dominant species in mixed flocks), Willow Tits P. montanus did not decrease their foraging heights as expected but remained in the most exposed parts of young pines. In contrast, after removal of Willow Tits, Crested Tits increased their foraging height from the sheltered lower canopy to sites previously occupied by Willow Tits. When flock size was reduced, the birds maintained the same high levels of vigilance without concealing themselves in dense vegetation. I suggest that flock members may benefit from foraging in sites that afford good anti-predator vigilance.     

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.     

Intraflock variation in the speed of escape-flight response on attack by an avian predator

The benefits of flocking to prey species, whether through collective vigilance,dilution of risk, or predator confusion, depend on flock members respondingin a coordinated way to attack. We videotaped sparrowhawks attackingredshank flocks to determine if there were differences in thetiming of escape flights between flock members and the factorsthat might affect any differences. Sparrowhawks are surpriseshort-chase predators, so variation in the time taken to takeflight on attack is likely to be a good index of predation risk.Most birds in a flock flew within 0.25 s of the first bird flying,and all birds were flying within 0.7 s. Redshanks that werevigilant, that were closest to the approaching raptor, and thatwere close to their neighbors took flight earliest within aflock. Birds in larger flocks took longer, on average, to takeflight, measured from the time that the first bird in the flockflew. Most birds took flight immediately after near neighbors tookoff, but later flying birds were more likely to fly immediatelyafter more distant neighbors took flight. This result, alongwith the result that increased nearest neighbor distance increasedflight delay, suggests that most redshanks flew in responseto conspecifics flying. The results strongly suggest that thereis significant individual variation in predation risk withinflocks so that individuals within a flock will vary in benefitsthat they gain from flocking.     

年龄、集群、生境及天气对鄱阳湖白鹤越冬期日间行为模式的影响

于2012年1—3月和2012年12—2013年3月,采用瞬时扫描法研究了鄱阳湖国家级自然保护区越冬白鹤(Grusleucogeranus)的昼间时间分配模式和日活动节律,分析了年龄、集群大小、生境类型和天气对白鹤越冬行为的影响。结果表明,白鹤越冬期的主要行为有觅食、警戒、理羽、游走、休息和争斗。其中,觅食是鄱阳湖越冬白鹤主要行为,占其昼间活动时间的82.94%;警戒占11.94%,理羽占3.52%,游走占1.88%,休息占0.15%,争斗占0.02%。白鹤在越冬期间的觅食、游走、休息和争斗行为存在明显的昼间节律性,其中游走、觅食和争斗行为在每天的上午和下午各存在1个高峰,休息行为在中午出现一个高峰;警戒和理羽行为则没有显著的昼间节律变化。年龄、集群大小、天气和生境对白鹤的越冬行为具有显著的影响:幼鹤取食的时间显著多于成鹤,而警戒时间则显著的小于成鹤;与家庭群中的个体相比,集体群中的个体取食和争斗时间显著增加,警戒时间显著减少;在天气晴朗时,白鹤的警戒时间显著大于阴天的警戒时间,取食时间则显著少于阴天时白鹤的取食时间;白鹤在大风天气显著增加理羽行为;在有雾的天气会显著增加取食时间,减少警戒时间。在草洲和浅水两种生境中,白鹤的游走和理羽行为存在显著差异,而觅食和警戒行为没有显著的差异,这可能源于草洲和浅水生境相互毗邻,并且具有相同程度的人为干扰。GLM分析结果表明,年龄和集群大小、年龄和生境、集群大小和天气、集群大小和生境、天气和生境对白鹤昼间时间分配模式的影响存在显著交互效应。