Local prey vulnerability increases with multiple attacks by a predator

Theory and empirical evidence suggest that predator activity makes prey more wary and less vulnerable to predation. However if at least some prey in the population are energetically or spatially constrained, then predators may eventually increase local prey vulnerability because of the cumulative costs of anti‐predation behaviour. We tested whether repeated attacks by a predator might increase prey vulnerability in a system where redshanks on a saltmarsh are attacked regularly by sparrowhawks from adjacent woodland. Cumulative attack number led to a reduction in redshank numbers and flock size (but had no effect on how close redshanks fed to predator‐concealing cover) because some redshanks moved to safer but less profitable habitats, leaving smaller flocks on the saltmarsh. This effect held even though numbers of redshank on the saltmarsh increased with time of day. As a result of the change in flock size, predicted attack‐success increased up to 1.6‐fold for the sparrowhawk, while individual risk of capture for the redshank increased up to 4.5‐fold among those individuals remaining on the saltmarsh. The effect did not arise simply because hawks were more likely to attack smaller flocks because attack rate was not dependent on flock size or abundance. Our data demonstrate that when some individual prey are constrained in their ability to feed on alternative, safer foraging sites, their vulnerability to predation increases as predator attacks accumulate, although those, presumably better quality individuals that leave the immediate risky area will have lower vulnerability, so that the mean vulnerability across the entire population may not have changed substantially. This suggests that the selective benefits of multiple low‐cost attacks by predators on prey could potentially lead to 1) locally heightened trait‐mediated interactions, 2) locally reduced interference among competing predators, and 3) the evolution of active prey manipulation by predators.     

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

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

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.     

Hunting‐mediated predator facilitation and superadditive mortality in a European ungulate

Predator‐prey theory predicts that in the presence of multiple types of predators using a common prey, predator facilitation may result as a consequence of contrasting prey defense mechanisms, where reducing the risk from one predator increases the risk from the other. While predator facilitation is well established in natural predator‐prey systems, little attention has been paid to situations where human hunters compete with natural predators for the same prey. Here, we investigate hunting‐mediated predator facilitation in a hunter‐predator‐prey system. We found that hunter avoidance by roe deer (Capreolus capreolus) exposed them to increase predation risk by Eurasian lynx (Lynx lynx). Lynx responded by increasing their activity and predation on deer, providing evidence that superadditive hunting mortality may be occurring through predator facilitation. Our results reveal a new pathway through which human hunters, in their role as top predators, may affect species interactions at lower trophic levels and thus drive ecosystem processes.     

Interspecific differences in antipredator strategies determine the strength of non‐consumptive predator effects on stream detritivores

Animal species differ considerably in their response to predation risks. Interspecific variability in prey behaviour and morphology can alter cascading effects of predators on ecosystem structure and functioning. We tested whether species‐specific morphological defenses may affect responses of leaf litter consuming invertebrate prey to sit‐and‐wait predators, the odonate Cordulegaster boltonii larvae, in aquatic food webs. Partly or completely blocking the predator mouthparts (mandibles and/or extensible labium), thus eliminating consumptive (i.e. lethal) predator effects, we created a gradient of predator‐prey interaction intensities (no predator < predator – no attack < predator – non‐lethal attacks < lethal predator). A field experiment was first used to assess both consumptive and non‐consumptive predator effects on leaf litter decomposition and prey abundances. Laboratory microcosms were then used to examine behavioural responses of armored and non‐armored prey to predation risk and their consequences on litter decomposition. Results show that armored and non‐armored prey responded to both acute (predator – non‐lethal attacks) and chronic (predator – no attack) predation risks. Acute predation risk had stronger effects on litter decomposition, prey feeding rate and prey habitat use than predator presence alone (chronic predation risk). Predator presence induced a reduction in feeding activity (i.e. resource consumption) of both prey types but a shift to predator‐free habitat patches in non‐armored detritivores only. Non‐consumptive predator effects on prey subsequently decreased litter decomposition rate. Species‐specific prey morphological defenses and behaviour should thus be considered when studying non‐consumptive predator effects on prey community structure and ecosystem functioning.     

Differential vulnerability determines the diet of a slow-moving predatory stream insect

1. We investigated the diet and prey electivity of Rhyacophila obliterata, a slow‐moving invertebrate predator capable of hunting in high‐flow microhabitats, and quantified the components of the predation sequence of fifth‐instar larvae foraging on mobile (Baetis mayflies, Amphinemura stoneflies) versus semi‐sessile (larval blackflies) prey. 2. In the field, fifth‐instar Rhyacophila consistently took more larval blackflies than more mobile prey. In behavioural trials, the number of attacks by Rhyacophila differed significantly between prey types, mobile prey being attacked more often than blackflies. Capture success, by contrast, was highest for blackflies, whereas Amphinemura and Baetis were rarely captured. In mixed‐prey feeding trials, Rhyacophila showed strong preference for blackflies and equally strong avoidance of Amphinemura and Baetis. 3. For mobile prey, the risk of being captured by this sluggish predator is very low, so they can afford to be in close contact with it. Rhyacophila was almost unable to capture any other prey but blackflies, resulting in strong passive selection for blackflies. 4. Therefore, the diet of fifth‐instar Rhyacophila can be predicted from laboratory observations and prey behaviour is the major determinant of the diet of this invertebrate predator.     

Roosting Behavior of Colonial and Solitary Flying Foxes in American Samoa (Chiroptera: Pteropodidae)1

We examined characteristics of roosting sites utilized by two flying fox species (Pteropus tonganus and P. samoensis) in American Samoa. The colonial roosting sites of P. tonganus were observed over a ten‐year period, including two years when severe hurricanes devastated bat populations and destroyed roost trees. Prior to the hurricanes, roosts were located on cliff faces above the ocean or steep mountainsides, locations that were either inaccessible to people or in protected areas where hunting was not allowed. In the years immediately following the hurricanes, P. tonganus colonies split into smaller groups that moved frequently to different locations. Four years after the second hurricane, colonies had coalesced and returned to many of the traditional roosting sites used before the hurricanes. Common tree species in upland and coastal forest were selected as roosts. The isolated locations selected for P. tonganus roosts were apparently the result of hunting pressure on the colonies. The solitary roosts of P. samoensis were observed during 29 months. Roosting bats were well concealed and hard to detect within the forest; even bats on exposed branches were cryptic. Mature primary forest was favored as roosting habitat. Individual bats used specific branches or trees as roosts and returned to them for up to 29 months. Unlike P. tonganus, people did not alarm roosting P. samoensis easily and some roosts were located near houses and along roads.     

Coevolution of a marine gastropod predator and its dangerous bivalve prey

The fossil record of the interaction between the predatory whelk Sinistrofulgur and its dangerous hard‐shelled bivalve prey Mercenaria in the Plio‐Pleistocene of Florida was examined to evaluate the hypothesis that coevolution was a major driving force shaping the species interaction. Whelks use their shell lip to chip open the shell of their prey, often resulting in breakage to their own shells, as well as to their prey. Mercenaria evolved a larger shell in response to an intensifying level of whelk predation. Reciprocally, an increase in attack success (ratio of successful to unsuccessful attacks) and degree of stereotypy of attack position by the predator suggest reciprocal adaptation by Sinistrofulgur to increase efficiency in exploiting hard‐shelled prey. A decrease in prey effectiveness (ratio of unsuccessful to total whelk predation attempts) and an increase in the minimum boundary of a size refuge from whelk predation for Mercenaria may indicate that predator adaptation has outpaced prey antipredatory adaptation. Evolutionary size increase in Sinistrofulgur most likely occurred in response to prey adaptation to decrease the likelihood of feeding‐induced shell breakage and unsuccessful predation when encounters with damage‐inducing prey occur, coupled with (or reinforced by) an evolutionary response to the whelk's own predators. Predator adaptation to Mercenaria best explains temporal changes in whelk behaviour to decrease performance loss (shell breakage) associated with feeding on hard‐shelled prey; this behavioural change limits attacks on prey to when the whelk's shell lip is thickest and most resistant to breakage. Despite evidence of reciprocal adaptation between predator and prey, the contribution of Mercenaria to Sinistrofulgur evolution is likely only a component of the predator's response to dangerous bivalve prey. This study highlights the importance of understanding the interactions among several species in order to provide the appropriate context to test evolutionary hypotheses about any specific pair of species. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society, 2003, 80 , 409–436.     

Living and dying in a multi‐predator landscape of fear: roe deer are squeezed by contrasting pattern of predation risk imposed by lynx and humans

The theory of predation risk effects predicts behavioral responses in prey when risk of predation is not homogenous in space and time. Prey species are often faced with a tradeoff between food and safety in situations where food availability and predation risk peak in the same habitat type. Determining the optimal strategy becomes more complex if predators with different hunting mode create contrasting landscapes of risk, but this has rarely been documented in vertebrates. Roe deer in southeastern Norway face predation risk from lynx, as well as hunting by humans. These two predators differ greatly in their hunting methods. The predation risk from lynx, an efficient stalk‐and‐ambush predator is expected to be higher in areas with dense understory vegetation, while predation risk from human hunters is expected to be higher where visual sight lines are longer. Based on field observations and airborne LiDAR data from 71 lynx predation sites, 53 human hunting sites, 132 locations from 15 GPS‐marked roe deer, and 36 roe deer pellet locations from a regional survey, we investigated how predation risk was related to terrain attributes and vegetation classes/structure. As predicted, we found that increasing cover resulted in a contrasting lower predation risk from humans and higher predation risk from lynx. Greater terrain ruggedness increased the predation risk from both predators. Hence, multiple predators may create areas of contrasting risk as well as double risk in the same landscape. Our study highlights the complexity of predator–prey relationship in a multiple predator setting. Synthesis In this study of risk effects in a multi‐predator context, LiDAR data were used to quantify cover in the habitat and relate it to vulnerability to predation in a boreal forest. We found that lynx and human hunters superimpose generally contrasting landscapes of fear on a common prey species, but also identified double‐risk zones. Since the benefit of anti‐predator responses depends on the combined risk from all predators, it is necessary to consider complete predator assemblages to understand the potential for and occurrence of risk effects across study systems.     

Do prey select for vacant hunting domains to minimize a multi‐predator threat?

Many ecosystems contain sympatric predator species that hunt in different places and times. We tested whether this provides vacant hunting domains, places and times where and when predators are least active, that prey use to minimize threats from multiple predators simultaneously. We measured how northern Yellowstone elk (Cervus elaphus) responded to wolves (Canis lupus) and cougars (Puma concolor), and found that elk selected for areas outside the high‐risk domains of both predators consistent with the vacant domain hypothesis. This enabled elk to avoid one predator without necessarily increasing its exposure to the other. Our results demonstrate how the diel cycle can serve as a key axis of the predator hunting domain that prey exploit to manage predation risk from multiple sources. We argue that a multi‐predator, spatiotemporal framework is vital to understand the causes and consequences of prey spatial response to predation risk in environments with more than one predator.     

Prey: predator ratio dependence in the functional response of a freshwater amphipod

1. First known for their shredding activity, freshwater amphipods also behave as active predators with consequences for prey population regulation and amphipod coexistence in the context of biological invasions. 2. A way to quantify predation is to determine the average consumption rate per predator, also known as its functional response (FR). 3. Although amphipods are gregarious and can display social interactions that can alter per capita consumption rates, previous studies using the FR approach to investigate amphipod predation ignored such potential mutual interference because they did not consider variations in predator density. 4. We investigated the FR of Echinogammarus berilloni feeding on dipteran larvae with joint variations in prey and predator densities. This bivariate experimental design allowed us to estimate interference and to compare the fits of the three main classes of theoretical FR models, in which the predation rate is a function of prey density alone (prey‐dependent models), of both prey and predator densities (predator‐dependent models) or of the prey‐to‐predator ratio (ratio‐dependent models). 5. The Arditi–Ginzburg ratio‐dependent FR model provided the best representation of the FR of E. berilloni, whose predation rate showed a decelerating rise to a horizontal asymptote as prey abundance increased. 6. Ratio dependence means that mutual interference between amphipods leads to prey sharing. Mutual interference is likely to vary between amphipod species, depending on their level of aggressiveness.     

Semi‐intensive shrimp farms as experimental arenas for the study of predation risk from falcons to shorebirds

Varying environmental conditions and energetic demands can affect habitat use by predators and their prey. Anthropogenic habitats provide an opportunity to document both predation events and foraging activity by prey and therefore enable an empirical evaluation of how prey cope with trade‐offs between starvation and predation risk in environments of variable foraging opportunities and predation danger. Here, we use seven years of observational data of peregrine falcons Falco peregrinus and shorebirds at a semi‐intensive shrimp farm to determine how starvation and predation risk vary for shorebirds under a predictable variation in foraging opportunities. Attack rate (mean 0.1 attacks/hr, equating 1 attack every ten hours) was positively associated with the total foraging area available for shorebirds at the shrimp farm throughout the harvesting period, with tidal amplitude at the adjacent mudflat having a strong nonlinear (quadratic) effect. Hunt success (mean 14%) was higher during low tides and declined as the target flocks became larger. Finally, individual shorebird vigilance behaviors were more frequent when birds foraged in smaller flocks at ponds with poorer conditions. Our results provide empirical evidence of a risk threshold modulated by tidal conditions at the adjacent wetlands, where shorebirds trade‐off risk and rewards to decide to avoid or forage at the shrimp farm (a potentially dangerous habitat) depending on their need to meet daily energy requirements. We propose that semi‐intensive shrimp farms serve as ideal “arenas” for studying predator–prey dynamics of shorebirds and falcons, because harvest operations and regular tidal cycles create a mosaic of foraging patches with predictable food supply. In addition, the relatively low hunt success suggests that indirect effects associated with enhanced starvation risk are important in shorebird life‐history decisions.     

Turbidity amplifies the non‐lethal effects of predation and affects the foraging success of characid fish shoals

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Behavioural responses to changing landscapes: flock structure and anti-predator strategies of tits wintering in fragmented forests

We analysed the effects of forest fragmentation on the flock structure of insectivorous forest passerines (Parus, Aegithalos, Certhia, Regulus, etc.), and on the anti‐predator behaviour and energy management of blue tits in these flocks. We surveyed flocks in Central Spain during two winters. Flocks in fragments comprised fewer individuals and species than flocks in unfragmented forests. The most abundant species in forest flocks (blue tit, Parus caeruleus, and firecrest, Regulus ignicapillus) were also the most abundant in fragments, while the rarest species in the area never occurred in small woodlots. We investigated how fragmentation and related changes in flock structure affect anti‐predator behaviour of blue tits, a widely distributed species in the area. In fragments but not in forests, blue tits increased scanning rates with decreasing flock size. Vigilance was relaxed when great tits, Parus major, were abundant as flock mates, suggesting that the absence of this dominant species in fragments could intensify anti‐predator behaviour of blue tits. Blue tits enhanced anti‐predator behaviour in the second winter parallel to an increase in the abundance of raptors. This behavioural change was stronger in fragments, where blue tits foraged deeper in the canopy and increased scanning and hopping rates. Under increased predation risk, birds are expected to reduce body mass to improve predator avoidance. On average, blue tits weighed similar in fragments and forests the second winter. However, they accumulated fat along the day in fragments only, and adjusted body mass to body size more closely in that habitat type. This suggests that blue tits perceived fragments as unpredictable habitats where fattening would help avoid starvation, but also as dangerous sites where overweight would further increase the risk of predation. In summary, our results support that fragmentation affects individual behaviour of blue tits, and show the potential of behavioural approaches to unravel how different species face the advancing fragmentation of their habitats.     

Differential mortality of wintering shorebirds on the Banc d'Arguin, Mauritania, due to predation by large falcons

Predators may influence many aspects of the daily life and seasonal movements of their prey. Here we quantify direct, and evaluate indirect effects of predation by three falcon species (Lanner Falcon Falco biarmicus , Barbary Falcon Falco pelegrinoides and Peregrine Falcon Falco peregrinus ) on coastal shorebirds wintering on the Banc d'Arguin, Mauritania, an area hosting approximately 30% of the East Atlantic Flyway population of shorebirds. On the basis of 754 h of observation over five winters, 97 witnessed attacks and 585 collected prey remains, we show that shorebirds were safer in larger flocks, which tended to be attacked less often. Furthermore, species that forage relatively close to shore and in small flocks were depredated more often than expected from their relative abundance. In three species, Red Knot Calidris canutus canutus , Bar-tailed Godwit Limosa lapponica taymyrensis and Dunlin Calidris alpina , the juveniles were more vulnerable than adults. We estimated that on average 1% of the juvenile and 0.1% of the adult Red Knots present were killed by large falcons each winter. For Red Knots we simultaneously quantified annual survival on the basis of an individual colour-marking programme: mortality due to predation by falcons accounted for an estimated 6.2% (juveniles) and 0.8% (adults) of annual mortality. We suggest that juvenile Red Knots are 10 times as likely to be killed by falcons because they use riskier habitats, i.e. early and late tide foraging areas closer to shores where surprise attacks are both more common and more successful. These results indicate that the strength of indirect effects of predation operating in a shorebird population largely outweigh the effects of mortality per se .     

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

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

Visual obstruction and vigilance: a natural experiment

Visual obstructions can cause an increase in antipredator vigilance in prey animals by making predator detection more difficult. However, visual obstructions can also skew the perception of group size and inter‐individual distances and impair the detection of alarm signals by conspecifics. These changes within the group alone can cause an increase in vigilance. To disentangle the contribution of these various factors to changes in vigilance, I documented vigilance in a gregarious species, the semipalmated sandpiper Calidris pusilla, foraging in a habitat where a naturally‐occurring visual barrier partially prevented predator detection without altering the transfer of information about predation risk within the group. I used a matched sampling design to collect vigilance data for birds using adjacent areas with and without the visual barrier. In the visually‐obstructed area, sandpipers maintained a higher level of vigilance, occurred farther away from cover and in smaller flocks, and preferentially scanned the area of danger with one eye in particular. All these changes suggest that visual obstruction increased perceived predation risk. I conclude that it is the inability to get a good view of any approaching predator, rather than changes in intra‐group communication that caused the increase in vigilance in the visually‐obstructed area.     

Hunting Activity Effects on Roost Selection by Male Wild Turkeys

Roosting is an important component of wild turkey (Meleagris gallopavo; turkey) ecology as roosts provide security from predators and inclement weather. Males call (gobble) from roosts during the reproductive season, and roost locations are important for maximizing access to females and transmission of calls across the landscape, while also minimizing predation risk. Spring hunting of male turkeys occurs during the reproductive season, and hunting activity influences male behaviors and calling. Because roost sites are important for wild turkey ecology, we evaluated roost site selection and fidelity of male turkeys relative to land cover types, vegetative characteristics, and the presence of hunting activity during 2017–2018 in Georgia, USA. Prior to onset of hunting, males selected roosts nearest to hardwood and pine (Pinus spp.) forests. Roost site fidelity was low and distances between roosts were large. After onset of hunting, males selected pine forests less and exhibited greater plasticity in roost selection while fidelity remained minimal, suggesting that males may have altered selection to mitigate risk from hunting while maintaining the strategy of moving about their ranges and roosting at different sites on consecutive nights. Future research should examine potential effects of hunting-induced shifts in resource selection on other aspects of male turkey behavior and ecology. © 2019 The Wildlife Society.     

Gregariousness and repellent defences in the survival of phytophagous insects

Group living has both costs and benefits for plant‐feeding insects, but defence against predators is the most widely acknowledged benefit. Gregarious folivores typically have warning coloration and elaborate anti‐predator defences. Do these defences protect these species from predation? To see if protection from predators generally results from gregariousness, I compared the shapes of published survivorship curves of externally feeding, gregarious and solitary Lepidoptera and Symphyta. Gregarious species are less likely than solitary species to die in the larval stages. However, solitary species that have anti‐predator defences do not have higher larval survival compared to gregarious species. This result, along with evidence from experimental manipulations of group size, suggests that repellent defences per se do not increase survival of gregarious larvae. Group behaviour is undoubtedly important in affecting the higher larval survival of gregarious species, but we currently cannot determine whether predator learning, dilution of risk, or rapid development contribute most to increasing survival.     

Low Foraging Success of Semipalmated Sandpipers at the Edges of Groups

Variation in foraging success in relation to spatial position in a group is little known in species that feed on mobile prey that can hide or flee upon disturbance by foragers. I examined the foraging success of individuals located either at the edge or at the centre of flocks of semipalmated sandpipers (Calidris pusilla) feeding on a burrowing amphipod (Corophium volutator) during migration stopover in the Bay of Fundy, Canada. The rates of pecking, prey capture and success were lower for individuals foraging at the edge than at the centre of flocks. Edge birds spent more time running and more time flying than centre birds. Edge birds moved away from the centre of the flock and made frequent short flights towards the centre. In contrast, centre birds rarely moved in a specific direction and flew mostly to relocate elsewhere with the whole flock. Sandpiper flocks foraged over a large area in a relatively short amount of time. In addition, amphipod density is high in this habitat. It thus appears unlikely that prey depletion or low food availability at the edges of groups could explain the spatial variation in foraging success. Low foraging success at the edges of flocks thus arose mainly because of time costs related to flock expansion and retraction. The effect of mutual interference among foragers and of predation risk by falcons is discussed with respect to flock expansion and retraction.