Daily Patterns of Optimal Producer and Scrounger Use under Predation Hazard: A State-Dependent Dynamic Game Analysis

Feeding in groups often gives rise to joining: feeding from other's discoveries. The joining decision has been modeled as a producer-scrounger game where the producer strategy consists of searching for one's food and the scrounger strategy consists of searching for food discovered by others. Previous models revealed that the evolutionarily stable proportion of scrounging mostly depends on the fraction of each food patch available only to its producer. These early models are static and state independent and are therefore unable to explore whether the time of day, the animal's state, and the degree of predation hazard influence an individual's decision of whether to use the producer or scrounger strategy. To investigate these issues, we developed a state-dependent dynamic producer-scrounger game model. The model predicts that, early in the day, low reserves promote a preference for the scrounger strategy, while the same condition late in the day favors the use of the producer strategy. Under rich and clumped food, the availability of scrounging can improve the daily survival of any average group member. The model suggests only weak effects of predation hazard on the use of scrounging. Future developments should consider the effects of dominance asymmetries and allowing foragers a choice between foraging alone or in a group harboring an evolutionarily stable frequency of scrounger.     

Experimental evidence that group foragers can converge on predicted producer-scrounger equilibria

When foraging together, animals are often observed to feed from food discoveries of others. The producer-scrounger (PS) game predicts how frequently this phenomenon of food parasitism should occur. The game assumes: (1) at any moment all individuals can unambiguously be categorized as either playing producer (searching for undiscovered food resources) or scrounger (searching for exploitation opportunities), and (2) the payoffs received from the scrounger tactic are negatively frequency dependent; a scrounger does better than a producer when the scrounger tactic is rare, but worse when it is common. No study to date has shown that the payoffs of producer and scrounger conform to the game's assumptions or that groups of foragers reach the predicted stable equilibrium frequency (SEF) of scrounger, whereby both tactics obtain the same payoff. The current study of three captive flocks of spice finches, Lonchura punctulata, provides the first test of the PS game using an apparatus in which both assumptions of the PS game are met. The payoffs to the scrounger, measured as feeding rate (seeds/s), were highly negatively frequency dependent on the frequency of scrounger. The feeding rate for scrounger declined linearly while the rate for producer either declined only slightly or not at all with increasing scrounger frequency. When given the opportunity to alternate between tactics, the birds changed their use of each, such that the group converged on the predicted SEF of scrounger after 5-8 days of testing. Individuals in this study, therefore, demonstrated sufficient plasticity in tactic use such that the flock foraged at the SEF of scrounger. Copyright 2000 The Association for the Study of Animal Behaviour.     

The Effect of Within-Flock Spatial Position on the Use of Social Foraging Tactics in Free-Living Tree Sparrows

The benefit of producer (searches for own food) or scrounger (exploits the others’ food discoveries) foraging tactic in a group of socially feeding animals may depend on where the individual searches for food within the group. Scrounging may be more advantageous in the centre of the group, having more individuals around to join, while producing may be more beneficial at the edges, where more unexplored food patches may be found. This study shows within‐flock position correlates with foraging tactic use of feeding birds in socially foraging tree sparrows, Passer montanus. Sparrows staying closer to the centre of the flock found their food patches more frequently by joining (i.e. use more frequently the scrounging tactic) than those staying toward the edges. To our knowledge this is the first field study demonstrating the relationship between spatial position and foraging tactic use. We investigated this relationship under different perceived predation hazard, and found that under elevated risk of predation, central individuals may increase their use of joining more than individuals on the periphery of the flock. Moreover, we show that extremely specialized use of searching tactics may be very infrequent in tree sparrows. As both within‐flock position and search tactic use can be altered very quickly and without leaving the flock, individuals may easily alter them in order to adjust their behaviour.     

Risky decisions: a test of risk sensitivity in socially foraging flocks of Lonchura punctulata

Group foraging allows for individuals to exploit the food discoveriesof other group members. If searching for food and searchingfor exploitation opportunities within a group are mutually exclusivealternatives, the decision to use one or the other is modeledas a producer-scrounger game because the value of each alternativeis frequency dependent. Stochastic producer-scrounger modelsgenerally assume that producer provides a more variable anduncertain reward than does the scrounger and hence is a riskierforaging alternative. Socially foraging animals that are attemptingto reduce their risk of starvation should therefore alter theiruse of producer and scrounger alternatives in response to changesin energy budget. We observed flocks of nutmeg mannikins (L.punctulata) foraging in an indoor aviary to determine whethertheir use of producer and scrounger alternatives were risk sensitive.Analyses of the foraging rewards of three flocks of seven birdsconfirm that producer is a riskier foraging strategy than isscrounger, although the difference in risk is rather small.We then submitted two other flocks to two different energy budgetsand observed the foraging decision of four focal birds in eachflock. All but one bird increased their relative use of theriskier producer strategy in the low food reserve treatment,but the overall use of producer did not differ significantlybetween treatments, providing evidence for a small but consistenteffect.     

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.      

Producer–Scrounger Games in a Spatially Explicit World: Tactic Use Influences Flock Geometry of Spice Finches

Group-foraging animals can either search for their food (producer) or search for opportunities to join the food discoveries of others (scrounger). To maximize food returns, producers should distance themselves from potential competitors whereas scroungers should increase proximity to potential producers. We investigated the extent to which playing one or the other tactic affected an individual's location in captive flocks of ground-feeding spice finches ( Lonchura punctulata ) as they foraged for hidden clumps of food on an aviary floor. We constrained some individuals to use the producer tactic by pre-training them to find food hidden under lids. Constrained producers foraged significantly further from the center of flocks than constrained scroungers. Flocks with many scroungers were significantly more compact than flocks with fewer scroungers. The results are consistent with published simulations of spatially explicit producer–scrounger models and suggest that the use of producer and scrounger foraging tactics be included as a factor that affects an individual's position within foraging groups.     

Phenotypic Correlates of Scrounging Behavior in Zebra Finches: Role of Foraging Efficiency and Dominance

In flocks, individuals can search for their own food using the producer tactic or exploit the discoveries of companions using the scrounger tactic. Models of the producer–scrounger game usually assume that tactic payoffs are independent of individual phenotypic traits. However, factors such as dominance status or foraging efficiency may constrain the use of tactics and lead to asymmetric tactic use among individuals. For instance, in flocks composed of foragers with unequal foraging efficiency, foragers that are less efficient at obtaining food are expected to rely on the scrounger tactic to a greater extent. I examined the role of foraging efficiency and dominance status as potential correlates of scrounging behavior in small aviary flocks of zebra finches (Taenopygia guttata). Individual foraging efficiency was documented in each flock in a treatment that prevented scrounging. In a subsequent treatment that allowed scrounging, higher levels of scrounging occurred as predicted in foragers with lower foraging efficiency. Dominance status was a poor predictor of tactic choice. Birds that arrived later on the foraging grid foraged less efficiently when scrounging was prevented and used scrounging to a greater extent when allowed, suggesting a link between boldness, foraging efficiency and the choice of foraging tactics in small flocks of zebra finches.     

The effect of prior residence and pair bond on scrounging choices in flocks of zebra finches, Taenopygia guttata

In groups, animals can use the producer tactic to locate food patches and the scrounger tactic to join the food discoveries of other companions. At equilibrium, models predict a mixture of the two tactics with equal payoffs. Several factors may constrain the use of tactics and lead to biases in scrounging choices. I explored the effect of prior residence and pair bond as potential constraints on scrounging choices in flocks of zebra finches (Taenopygia guttata). Experimental flocks contained two birds already established in an aviary (prior residents) and two birds recently released in the aviary for the first time (new residents). All birds were previously trained to find food on a foraging grid. In the aviary, new residents followed prior residents from perches to the grid and relied heavily on prior residents to locate food patches. Low initial success by new residents probably favoured heavy reliance on the scrounger tactic. New residents that formed pair bonds with prior residents foraged closer to their mates and scrounged selectively from their mates in some cases. Prior residence, and pair bond to a lesser extent, influenced scrounging choices in zebra finches and could lead to deviation from the expected use of foraging tactics.     

Co-evolution of learning complexity and social foraging strategies

Variation in learning abilities within populations suggests that complex learning may not necessarily be more adaptive than simple learning. Yet, the high cost of complex learning cannot fully explain this variation without some understanding of why complex learning is too costly for some individuals but not for others. Here we propose that different social foraging strategies can favor different learning strategies (that learn the environment with high or low resolution), thereby maintaining variable learning abilities within populations. Using a genetic algorithm in an agent-based evolutionary simulation of a social foraging game (the producer-scrounger game) we demonstrate how an association evolves between a strategy based on independent search for food (playing a producer) and a complex (high resolution) learning rule, while a strategy that combines independent search and following others (playing a scrounger) evolves an association with a simple (low resolution) learning rule. The reason for these associations is that for complex learning to have an advantage, a large number of learning steps, normally not achieved by scroungers, are necessary. These results offer a general explanation for persistent variation in cognitive abilities that is based on co-evolution of learning rules and social foraging strategies.     

Testing dynamic variance-sensitive foraging using individual differences in basal metabolic rates of zebra finches

Social foragers can alternate between searching for food (producer tactic), and searching for other individuals that have located food in order to join them (scrounger tactic). Both tactics yield equal rewards on average, but the rewards generated by producer are more variable. A dynamic variance-sensitive foraging model predicts that social foragers should increase their use of scrounger with increasing energy requirements and/or decreased food availability early in the foraging period. We tested whether natural variation in minimum energy requirements (basal metabolic rate or BMR) is associated with differences in the use of producer–scrounger foraging tactics in female zebra finches Taeniopygia guttata . As predicted by the dynamic variance-sensitive model, high BMR individuals had significantly greater use of the scrounger tactic compared with low BMR individuals. However, we observed no effect of food availability on tactic use, indicating that female zebra finches were not variance-sensitive foragers under our experimental conditions. This study is the first to report that variation in BMR within a species is associated with differences in foraging behaviour. BMR-related differences in scrounger tactic use are consistent with phenotype-dependent tactic use decisions. We suggest that BMR is correlated with another phenotypic trait which itself influences tactic use decisions.     

Producers and scroungers: A general model and its application to captive flocks of house sparrows

Many forms of interaction within and between species appear to be based on ‘scrounger’ individuals or species exploiting a limited resource provided ‘producers’. A mathematical model is presented which shows whether or not scroungers are maintained in a group, depending on their frequency and the group size. Some of the predictions of the model were tested in captive flocks of house sparrows Passer domesticus L. Here the scroungers obtained most of their food (mealworms) by interaction and the producers found most of their food by actively foraging: the pay-off to each type was measured as mealworm capture rate. Neither type changed strategy opportunistically in response to instantaneous flock composition but, not surprisingly, scroungers fared better when one of more producers were present. However, scrougers did much worse than expected when greatly outnumbered by producers, perhaps because producers then found the available food very quickly.     

Learning rules for social foragers: implications for the producer-scrounger game and ideal free distribution theory

In population games, the optimal behaviour of a forager depends partly on courses of action selected by other individuals in the population. How individuals learn to allocate effort in foraging games involving frequency-dependent payoffs has been little examined. The performance of three different learning rules was investigated in several types of habitats in each of two population games. Learning rules allow individuals to weigh information about the past and the present and to choose among alternative patterns of behaviour. In the producer-scrounger game, foragers use producer to locate food patches and scrounger to exploit the food discoveries of others. In the ideal free distribution game, foragers that experience feeding interference from companions distribute themselves among heterogeneous food patches. In simulations of each population game, the use of different learning rules induced large variation in foraging behaviour, thus providing a tool to assess the relevance of each learning rule in experimental systems. Rare mutants using alternative learning rules often successfully invaded populations of foragers using other rules indicating that some learning rules are not stable when pitted against each other. Learning rules often closely approximated optimal behaviour in each population game suggesting that stimulus-response learning of contingencies created by foraging companions could be sufficient to perform at near-optimal level in two population games.     

Behavioural responses during feather replacement in house sparrows

Birds lose feathers, whether during molt or by accident, and replace them by processes that are energetically demanding. We hypothesized that house sparrows Passer domesticus biblicus use behavioral means to save energy when feathers are lost, and tested the general prediction that house sparrows growing new feathers adjust their behavior to minimize the energy costs of foraging and to increase net energy gain from their food. To test these predictions we divided 18 house sparrows into three groups: 1) plucked – house sparrows from which we plucked 15 flight feathers; 2) cut – house sparrows in which the same 15 feathers were cut off at the calamus below the barbs; and 3) control – unmanipulated house sparrows with plumage intact. We recorded both the quantity of seeds the house sparrows ate and the time they spent foraging from assay food patches. We found that ‘plucked’ sparrows growing new feathers adjust their foraging behavior by reducing their feeding time and the number of visits to a food patch. This allowed them to increase their patch harvest rate while maintaining a steady body mass.     

Effect of Group Size on Feeding Rate when Patches are Exhaustible

One benefit of group foraging is that individual foragers can join the food discoveries of companions and thus increase encounter rate with food patches. When food patches are exhaustible, however, individual shares of each patch will decrease with group size negating the effect of increased encounter rate. Mean feeding rate may actually decrease with group size as a result of aggression or time wasted joining already depleted patches, or when searching to join the food discoveries of others, which is referred to as scrounging, precludes finding food. I examined the relationship between mean feeding rate and group size in captive flocks of zebra finches (Taenopygia guttata) foraging for small clumps of seeds. Finches in groups of two or four fared better than solitary birds in terms of mean feeding rate despite the fact that birds in groups scrounged a large proportion of their food. Solitary birds initiated feeding activity after a longer delay, which led to their lower success. Early departures by food finders from food patches joined by others may have lessened the impact of scrounging on mean feeding rate. As a result of benefits from the presence of companions, group foraging in zebra finches appears a viable alternative to foraging alone despite the cost of sharing resources.     

You watch my back,I'll watch yours: Emergence of collective risk monitoring through tacit coordination in human social foraging

Predation risk is a significant concern when social animals including humans engage in foraging. When people search for resources together, individuals often find themselves in a producer–scrounger game, in which some individuals bear the cost of risk monitoring while others can free ride on those efforts. A theoretically rational strategy is to mix foraging and risk monitoring randomly with the same probability across all members, but such uncoordinated action often yields inefficiencies of under- or over-supply of risk monitoring in a group. Here, we examined whether people could spontaneously develop a coordinated risk-monitoring system, alternating vigilance and foraging in a pair. Given that human cooperation is vulnerable to fear of exploitation and emotional arousal under risk, we hypothesized that such sources of anxiety would be potential disruptors to coordination. In a laboratory experiment, two participants worked on a “treasure hunt” task simultaneously, in which they chose between low or high vigilance against predators during foraging without verbal communication. If one chose high vigilance with personal cost, it yielded a spillover benefit to the other. Besides behavioral choices, each participant's physiological arousal (skin conductance response) and cognitive effort (tonic pupil dilation) were measured during the task. Results showed that some pairs were actually able to develop a role-alternating system over time through tacit coordination, but coordinated action was also vulnerable to anxiety and mistrust among participants. Overall, these results imply that, besides the mutual behavioral control that often characterizes repeated interaction, cognitive control of emotional arousal may be a critical psychological factor for the emergence of coordinated cooperation.     

Wild Carib grackles play a producer scrounger game

Producer–scrounger (PS) game-theoretical foraging modelsmake predictions about the decision of group-feeding animalseither to look for food (produce) or for opportunities to exploitthe discoveries of other foragers (scrounge). We report themost complete demonstration to date of the applicability ofthe PS foraging game in a free-living animal, the Carib grackle(Quiscalus lugubris) of Barbados. As assumed by PS games, thepayoffs obtained by scroungers were negatively frequency dependent.Experimentally, increasing the cost of scrounging led to a decreasein the observed proportion of scroungers, whereas raising thecost of producing increased the proportion of scroungers. Observationsof marked birds revealed that group-level changes could be broughtabout by individual flexibility in tactic use. Despite consistentindividual differences in tactic use, most birds used both tacticsand could alter their use of producing and scrounging when conditionschanged. We found no difference in the payoffs obtained by producersand scroungers, suggesting a symmetrical game equilibrium. Ourresults call for testing the PS foraging game in a broader rangeof biological systems that include different types of scroungingbehavior (e.g., scramble, stealthful, or aggressive scrounging)as well as the exploitation of different phases of food production(e.g., searching, handling).     

Geometry for a selfish foraging group: a genetic algorithm approach

The advantages of group living are not shared equally among all group members and these advantages may depend on the spatial position occupied by a forager within the group. For instance, it is thought that socially dominant individuals prefer the predator-safe central position of groups forcing subordinates to the periphery. Uneven spread of benefits among group members can occur when some animals (the scroungers) parasitically exploit the food findings of other foragers (the producers). Here we focus on how playing producer or scrounger affects an individual''s spatial position within a group. We model the movement of foraging animals playing scrounger or producer using a spatially explicit simulation and use a genetic algorithm to establish movement rules. We find that groups containing producers and scroungers are more compact compared to an equivalent group of producers only. Furthermore, the position occupied by strategies varies: scroungers are mainly found in central positions, while producers in the periphery, suggesting that the best position for strategies differs. Dominants, therefore, should prefer movement rules which lead to central positions because of the positional benefits provided to the scrounger strategy they use. Moreover, position within a group will introduce an asymmetry among otherwise phenotypically symmetric individuals.     

A dynamic habitat selection game

A patch selection game is formulated and analyzed. Organisms can forage in one of H patches. Each patch is characterized by the cost of foraging, the density and value of food, the predation risk, and the density of conspecifics. The presence of conspecifics affects the finding and sharing of food, and the predation risk. Optimal foraging theory can be viewed as a "1-person" game against nature in which the optimal patch choice of a specific organism is analyzed assuming that the number of conspecifics in other patches is fixed. In the general game theoretic approach, the behavior of conspecifics is included in the determination of the distinguished organism's strategy. An iterative algorithm is used to compute the solution of the "n-person" game or dynamic ESS, which differs from the optimal foraging theory solution. Experiments to test the proposed theory using rodents and seed trays are briefly discussed.     

栗喉蜂虎捕食差异

2010年4～5月和2011年4～6月,用焦点动物取样法对云南省保山市潞江镇怒江河谷栗喉蜂虎捕食差异进行了观察。对栗喉蜂虎雌性成年鸟、雄性成年鸟、青年鸟捕食种类、数量、成功率、食物组成、食物资源量、处理食物成功率及处理时间进行了统计。结果表明,栗喉蜂虎捕食种类包括7类昆虫,以膜翅目Hymenoptera、蜻蜓目Odonate、鳞翅目Lepidoptera昆虫为主;成年雄鸟捕食频次高于成年雌鸟和青年鸟;成年鸟与青年鸟处理食物成功率无显著差异,但二者捕食成功率和处理食物时间差异显著。     

The effects of predation risk on the use of social foraging tactics

The effects of predation on the use of social foraging tactics, such as producing and scrounging, are poorly known in animals. On the one hand, recent theoretical models predict increased use of scrounging with increasing predation risk, when scroungers seeking feeding opportunities also have a higher chance of detecting predators. On the other hand, there may be no relation between tactic use and predation when antipredator vigilance is not compatible with scanning flockmates. We investigated experimentally the effects of predation risk on social foraging tactic use in tree sparrows, Passer montanus. We manipulated predation risk in the field by changing the distance between shelter and a feeder. Birds visited the feeder in smaller flocks, spent less time on it and were somewhat more vigilant far from shelter than close to it. Increased predation risk strongly affected the social foraging tactic used: birds used the scrounger tactic 30% more often far from cover than close to it. Between-flock variability in scrounging frequency was not related to the average vigilance level of the flock members, and within-flock variability in the use of scrounging was negatively related to the vigilance of birds. Our results suggest that in tree sparrows, the increased frequency of scrounging during high predation risk cannot simply be explained by an additional advantage of increasing antipredator vigilance. We propose alternative mechanisms (e.g. increased stochasticity in food supply, and that riskier places are used by individuals with lower reserves) that may explain increased scrounging when animals forage under high predation risk.