Energy budgets and risk-sensitive foraging in starlings

The effect of energy budget on risk-sensitive foraging was assessedin a laboratory experiment using starlings (Sturnus vulgaris).Subjects chose between two options offering the same mean amountof food per trial, but differing in variance: a "fixed" optiongave 5 units food in every trial, and a "variable" option gave2 or 11 units food with probabilities 2/3 and 1/3, respectively.We manipulated energy budgets by controlling the cumulativeamount of food received by each bird at the end of a day. Inone treatment (positive budget) individuals were allowed toeat at the level of their own ad-libitum daily consumption,while for the other (negative budget), food was rationed toprovoke a steady drop in body weight during the experimentalperiod. No subject was allowed to drop below 80% of its ad libitumbody weight. Contrary to predictions from the "energy budgetrule" and contrary to reported results of some other studies, starlingssignificantly preferred the "fixed" option irrespective of energybudget conditions. Our results support the view that persistentrisk aversion for food amounts and risk proneness for food delaysare the norm, and shifts in risk attitude according to energybudget are exceptions. Several algorithms, which may have evolvedto maximize energetic pay off between variable food sources,can produce this trend as a side effect. We discuss two of thesealgorithms: (1) maximization of local (per trial) rate as opposedto global rate of gains, with longer handling time for largerrewards, and (2) choosing larger rewards and smaller delayssubject to Weber's law in the memory for the parameters of eachfood supply.     

Risk sensitivity in starlings: variability in food amount and food delay

Starlings' preferences for constant versus variable food sourceswere studied in the laboratory. The constant alternative gavea fixed amount of food after a fixed delay. The variable alternativeoffered either a varying amount of food after a fixed delay(treatment A) or a fixed amount of food after a variable delay(treatment B). In both treatments the ratio of amount of foodover trial length (the sum of intertrial interval plus delayand handling times) of the constant alternative equaled theaverage of the two ratios of the variable alternative. The variableratios were 30% higher and 30% smaller than the fixed ratio.In free-choice trials (both options available in each trial),the subjects were risk-averse or indifferent in treatment Aand indifferent or riskprone in treatment B. In no-choice trials(only one source available per trial), the latency to respondwas longer in the variable than in the constant source in treatmentA and the opposite in treatment B. The greater preference forvariability in time than for variability in reward amount isnot consistent with either maximizing the ratio of expectedenergy over expected time or the expected ratio of energy overtime for individual trials. There was a negative correlationbetween individual intake rate and degree of risk pronenessfor both kinds of variability. We present a model of choicebased on an information-processing theory for temporal memorythat accounts for the different effects of variability in delayand in amount but cannot explain the effects of intake rate.[Behav Ecol 1991;2:301–308]     

Use of water flow direction to provide spatial information in a small-scale orientation task

Experiments were designed to investigate whether three-spined sticklebacks Gasterosteus aculeatus can use direction of water flow as an orientation cue. The fish had to learn the location of a food patch in a channel where water flow direction was the only reliable indicator of the food patch position. Fish from two ponds and two rivers were trained and tested in the spatial task to determine whether river three-spined sticklebacks are more adept at using water flow as a spatial cue than fish from ponds. All fish were able to use water flow to locate the food patch but one of the two river groups was significantly faster at learning the patch location. When the task was reversed so that fish that had formerly been trained to swim downstream now had to learn to swim upstream and vice versa both river groups learned the reversed task faster than the two pond groups. In a second experiment, to investigate whether fish from ponds or rivers vary in the type of spatial cue that they prefer to use, fish from one pond and one river were given a choice between two different types of spatial cue: flow direction or visual landmarks. A test trial in which these two cues were put into conflict revealed that the river population showed a strong preference for flow direction whilst the pond population preferred to use visual landmarks.     

Cleaner Wrasses Keep Track of the ‘When’ and ‘What’ in a Foraging Task1

In recent years evidence has accumulated that at least some animals can remember the ‘what’, ‘where’ and ‘when’ of personal experiences. Currently, evidence for such ability is taxonomically restricted to birds and mammals. Here, we demonstrate for the first time that cleaner wrasses Labroides dimidiatus are able to remember when they interacted with what after a single event. In nature, cleaners remove ectoparasites from other reef fishes, so‐called clients. Clients are depleted, non‐stationary food patches at the end of an interaction and replenished only after a delay. In our experiments, we presented twelve cleaners every 2.5 min, a choice between two of a total of four plates with different colours and patterns. One plate was always accessible but contained a non‐preferred food item while the other three contained a preferred food item, but allowed a next feeding event only after 5, 10 or 15 min. Thus, to maximise food intake, cleaners had to remember for each choice when they had last interacted with which plate. When confronted with two plates offering preferred food, cleaners showed an overall significant preference for the plate that allowed access during the trial. For six cleaners, the preference was significant. Also, on trials involving the always accessible plate, cleaners discriminated between trials in which they had to eat the non‐preferred food and trials on which they could eat the preferred food. In conclusion, cleaners are able to track the ‘when’ and ‘what’ (or possibly ‘who’) within a biologically meaningful time period.     

Exogenous melatonin reduces the resynchronization time after phase shifts of a nonphotic zeitgeber in the house sparrow (Passer domesticus)

Continuous melatonin administration via silastic implants accelerates the resynchronization of the circadian locomotor activity rhythm in house sparrows (Passer domesticus) after exposure to phase shifts of a weak light-dark cycle. Constant melatonin might induce this effect either by increasing the sensitivity of the visual system to a light zeitgeber or by reducing the degree of self-sustainment of the circadian pacemaker. To distinguish between these two possible mechanisms, two groups of house sparrows, one carrying melatonin implants and the other empty implants, were kept in constant dim light and subjected to advance and delay shifts of a 12-h feeding phase. The resynchronization times of their circadian feeding rhythm following the phase shifts were significantly shorter when the birds carried melatonin implants than when they carried empty implants. In a second experiment, melatonin-implanted and control birds were released into food ad libitum conditions 2 days after either a delay or an advance phase shift. The number of hours by which the activity rhythms had been shifted on the second day in food ad libitum conditions was assessed. Melatonin-implanted house sparrows had significantly larger phase shifts in their circadian feeding rhythm than control birds. This is in accordance with the first experiment since a larger phase shift at a given time reflects accelerated resynchronization. Additionally, the second experiment also excludes any possible masking effects of the nonphotic zeitgeber. In conclusion, constant melatonin accelerates resynchronization even after phase shifts of a nonphotic zeitgeber, indicating that constant high levels of melatonin can reduce the degree of self-sustainment of the circadian pacemaker independent of any effects on the photoreceptive system.     

Effects of habitat complexity,dominance and personality on habitat selection: Ideal despotic cichlids

Habitat structure can impede visibility and movement, resulting in lower resource monopolization and aggression. Consequently, dominant individuals may prefer open habitats to maximize resource gain, or complex habitats to minimize predation risk. We explored the role of dominance on foraging, aggression and habitat choice using convict cichlids (Amatitlania nigrofasciata) in a two‐patch ideal free distribution experiment. Groups of six fish of four distinct sizes first competed for shrimp in one‐patch trials in both an open and complex habitat; half the groups experienced each habitat type first. Following these one‐patch trials, each group then chose between habitat types in a two‐patch trial while competing for food. Finally, each fish underwent an individual behavioural assessment using a battery of “personality” tests to determine if behaviour when alone accurately reflected behaviour within a social context. In the one‐patch trials, dominant fish showed similar food consumption between habitats, but chased more in the complex habitat. In the two‐patch choice trials, dominants preferred and defended the complex habitat, forming an ideal despotic distribution with more than half the fish and competitive weight in the open habitat. Within the groups, individual fish differed in foraging and chasing, with repeatabilities of 0.45 and 0.23 across all treatments. Although a higher foraging rate during the individual assessment predicted foraging rate and use of the complex habitat during the group trials, aggression and boldness tests were not reflective of group behaviour. Across groups, heavier dominants and those with higher foraging rate in the open habitat used the open habitat more, suggesting that both risk and energetic state affect habitat preference in dominant convict cichlids.     

A field study of prey selection in planktivorous fish larvae

Summary The food selection of larval fish was studied from field samples collected in two areas that differ in productivity. In the area where planktonic primary and secondary production was high the fish larvae showed a tendency to specialise and they selected the largest prey species available as food; in the area of lower production fish were generalists and they fed equally on all size classes.Abundance of prey was found to be one of the decisive factors in the prey selection of planktivorous fish larvae. Large prey species (calanoids) were selected when their absolute abundance was high but when their abundance was low, small-sized cladocerans were preferred. Visibility and stage of life history were also assumed to affect the mode of selection.     

The influence of size, shape, and phenolic content on the selection of winter foods by the Azores bullfinch (Pyrrhula murina)

Although the quality of foods may change through the seasons, few studies have assessed shifts in food preferences. I studied winter food selection of the Azores bullfinch to evaluate whether physical characteristics (size and accessibility), or phenolic content of food items better explained changes of winter diet. The birds preferred larger items of all winter foods: seeds, sori and flower buds and more accessible seeds. In autumn, seeds seemed preferred to sori, but in spring birds switched from seeds to sori and flower buds, once the latter became larger. Phenols may influence food preference but they did not explain diet shifts or preferences for individual trees. Food selection is a complex mechanism, depending on the alternatives available and seasonal shifts in food preference may continually change the availability of 'good quality food'.     

Effects of food availability on temporal activity patterns and growth of Atlantic salmon

1. Patterns of sheltering and activity are of fundamental importance in the ecology of animals and in determining interactions among predators and prey. Balancing decreased mortality risk when sheltering with increased feeding rate when exposed is believed to be a key determinant of diel patterns of sheltering in many animals. 2. Despite lower foraging efficiency at night than during the day, Atlantic salmon Salmo salar parr are nocturnal during winter and at low summer temperatures. Nocturnal activity also occurs at warm water temperatures during summer, but little is known about the functional significance of this behaviour. 3. This study aimed to determine: (1) the preferred activity and shelter pattern of Atlantic salmon parr during warm summer months, and (2) their response to variations in food availability when balancing growth rate (G) and mortality risk (M), as expressed through time out of shelter. We differentiated among four potential responses to reduced food availability: (1) no response; (2) G decreases but M remains constant; (3) G remains constant but M increases; and (4) G decreases and M increases. 4. Time exposed from shelter was inversely related to food availability. Fish subject to high food availability were significantly less active during the day than those with restricted rations. However, food availability had no significant effect on the extent to which fish were active at night. There was no evidence of variation in growth rate with food availability. 5. Salmon were predominantly nocturnal at high ration levels, consistent with their previously reported behaviour during winter. Rather than switching to diurnal behaviour at high temperatures per se, as previously was supposed, it appears that the fish are diurnal only to the extent needed to sustain a growth rate, and this extent depends on food availability. 6. Atlantic salmon parr modulate the amount of time they are active rather than growth when responding to variations in food availability over an order of magnitude.     

Time–Place Learning Based on Food but not on Predation Risk in a Fish, the Inanga (Galaxias maculatus)

Time–place learning, or the ability to learn to be in different places at different times of day, is already known to occur in response to daily spatio-temporal patterns of food availability. However, the ability to learn daily patterns of predation risk and move between areas at the right time of day in order to avoid predation has never been tested. This study asked whether inangas, Galaxias maculatus , are capable of time–place learning based on food availability only, predation risk only, or the antagonistic combination of food availability and predation risk. Shoals of five inangas were kept in aquaria partially divided into a right and left section. Every day they were exposed to a stimulus on one side in the morning and on the other side in the afternoon. Depending on the experiment, the stimulus could be two deliveries of food, two simulated heron strikes, or both of the above within the same hour. After 14 d the stimuli were not given and the position of the fish was noted in both the morning and the afternoon. The majority of the fish learned to switch sides at the correct daily time in order to get food, but they remained on the same side at both daily times in response to either predation risk alone or the combination of predation risk and food. It seems that the potential for time–place learning based on predation risk is less than that based on food, and that predation risk can even curtail the expression of time–place learning based on food. Fish may resort to other tactics, such as shoaling and reduced movement, in response to predation risk. Daily habitat shifts could still be present in nature and rooted in the avoidance of predation, but instead of being the direct result of learning they would be mostly innate.     

The relative importance of food and shelter for seagrass-associated invertebrates: a latitudinal comparison of habitat choice by isopod grazers

The generality of mechanisms affecting habitat choice and grazing in seagrass meadows was evaluated in a latitudinal comparison of seagrass grazers from the temperate (60°N) Baltic Sea and the subtropical (30°N) Gulf of Mexico. Using similar habitat choice experiment set-ups in Finland and the USA, the role of food type, habitat complexity and predation hazard on habitat choice of the isopods Idotea baltica (Pallas) and Erichsonella attenuata Harger were tested. When shelter was provided by both living and artificial seagrass, epiphytic food resources on artificial vegetation were clearly preferred by both species, although Idotea was attracted to epiphyte-free seagrass when no alternative food was present. When choosing between food and shelter, both species preferred epiphytic food over shelter. However, under predation hazard of fish, Erichsonella clearly switched to the habitat offering shelter, while the presence of a predatory fish produced no preference for shelter by Idotea. Food type may be considered as an universal mechanism that partly determines the presence of grazers in seagrass habitats and is, in the absence of a predator, more important than shelter. Predation risk affected the behaviour of the grazers, but the response varied between species possibly due to varying importance of fish predation in the areas studied. Received: 16 November 1998 / Accepted: 13 February 1999     

Effects of food value, predation risk, and pilferage on the caching decisions of Dipodomys merriami

Animals that scatter cache their food face a trade-off betweenthe benefits of protecting caches from pilferers and the costsassociated with caching. Placing food into a large number ofwidely spaced caches helps to protect it from pilferage butalso involves costs such as greater exposure to predators. Ipredicted that animals would disperse food into a larger numberof more widely spaced caches when caching (1) a preferred foodversus a less preferred food and (2) under conditions of lowpredation risk versus high predation risk. To test these predictions,I examined the scatter-caching decisions of Merriam's kangaroorats (Dipodomys merriami). D. merriami distributed caches inclumped patterns, regardless of food preference, but they showeda tendency to invest more in a preferred food by distributingcaches more widely. Under the relative safety of the new moon,they did not disperse caches more widely, rather they partitionedthe same amount of food into a larger number of caches thanthey did under the full moon, when predation risk is higher.To examine whether their cache spacing decisions had a significantimpact on the success of cache pilferers, I measured discoveryby pilferers of artificial caches of two food types at differentcaching distances. Results indicate that the cache spacing behaviorof D. merriami functions to protect caches from pilferers, becauseincreased spacing of artificial caches decreased the probabilityof pilferage for both types of food.     

The Influence of Alarm Substance on Feeding in Zebra Danio Fish (Brachydanio rerio)

The effects of alarm substance on feeding behaviour of zebra danio fish (Brachydanio rerio) were tested by offering them high and low densities of enclosed waterfleas (Daphnia magna). Normally the fish attacked high densities of prey, but when exposed to alarm substance they preferred lower and presumably less confusing prey densities — also lowering their feeding rate.     

Food provisioning lowers inter-clutch interval in Moorhens Gallinula chloropus

Food supplements were provided to Moorhens Gallinula chloropus during production of their first brood, to test whether food availability limited the number of broods they could rear in a season. Supplemented territories had shorter inter-clutch intervals when individual differences were held constant, although they did not produce more broods. Because unmanipulated birds with short inter-clutch intervals produced more broods, food availability may be one of several factors limiting the production of multiple broods and therefore reproductive success.     

Does Feeding Competition Influence Tammar Wallaby Time Allocation?

Animals may aggregate to reduce predation risk, but this potentially incurs the cost of increased competition. We studied the degree to which competition for food influenced the time tammar wallabies (Macropus eugenii) allocate to foraging and vigilance by experimentally manipulating access to food, while holding other factors constant. Groups of six wallabies were observed when they had access to either one or six non‐depleting bins of supplemental food. Food availability had no effect on the time allocated to foraging, looking or affiliative interactions, and this was true whether individuals or groups were treated as the unit of analysis. However, wallabies engaged in substantially more aggressive acts in the high‐competition treatment. These results, when combined with other findings, suggest that the moderately social tammar wallaby receives an antipredator benefit by aggregating with conspecifics which is not reduced significantly by foraging competition.     

Competitive resource sharing: An experimental test of a learning rule for ESSs

Six sticklebacks (Gasterosteus aculeatus) distributed themselves between two simulated drift food patches in the ratio of patch profitabilities (Milinski 1979). The present study investigated the rule with which each fish decides to stay in one or the other patch, and whether the fish have more difficulties in reaching a decision when the prey is supplied irregularly as is the case under natural conditions. With frame by frame analysis of filmed experiments the hunting success and sampling effort of each individual fish could be determined continuously. The results support the hypothesis that the fish used the relative payoff sum (RPS) learning rule (Harley 1981; Regelmann 1984). There was a close correlation between all decisions of individual fish during a trial, whether to stay or leave a patch, and decisions predicted by the RPS learning rule using the actual experience of the fish. Differences in competitive ability between the fish influenced the dynamics of the distribution in the predicted way, the more successful competitors deciding earlier where to stay than the less successful ones. This had the effect that the good competitors as well as the poor ones were distributed between the patches in the ratio of patch profitabilities. It is also compatible with the RPS learning rule that the fish reached their decision as quickly whether they were supplied with prey regularly or irregularly.     

Multiple predator effects in an intertidal food web

1. We examined the effects of multiple predators from an intertidal boulder food web to test whether and how three different predator species affected the survival of a small amphipod species. 2. Predators were chosen because they differ in their foraging mode, two feeding at the bottom and in benthic refuges (nemertean and shrimp) and one in the water-column (juveniles of a fish). 3. Mortality of amphipods was not affected by nemerteans, but was high in the presence of shrimp or fish. Highest mortalities were observed in predator-combinations that contained both shrimp and fish. Amphipods responded to shrimp by escaping into the water column, while they avoided fish by remaining in the refuge. We conclude that predator-specific defence causes conflicts for prey when both shrimp and fish are present. 4. Comparing observed effects of multiple predators with expected effects revealed risk enhancement for the shrimp + fish combination. A comparison of different predictive models revealed that the multiplicative model was most appropriate, although additive models may work well under certain conditions. 5. Based on known consumption-ranges of the predators used, we conclude that nemerteans were saturated with prey while fish were far from their saturation point. A predator's functional response curve (prey consumption in relation to prey abundance) determines its impact on prey populations. This knowledge appears essential in order to predict whether prey organisms face risk enhancement, risk reduction or additive effects of multiple predators.     

Behavioural Response of Juvenile Bluegill Sunfish to Variation in Predation Risk and Food Level

The behavioural response of juvenile bluegill sunfish (Lepomis macrochirus) to predation risk when selecting between patches of artificial vegetation differing in food and stem density was investigated. Bluegill foraging activity was significantly affected by all three factors. Regardless of patch stem density or risk of predation bluegills preferred patches with the highest prey number. During each trial bluegill foraging activity was clearly divided into a between- and within-patch component. In the presence of a predator bluegills reduced their between-patch foraging activity by an equivalent amount regardless of patch stem density or food level, apparently showing a risk-adjusting behavioural response to predation risk. Within patches, however, foraging activity was affected by both food level and patch stem density. When foraging in a patch offering a refuge from predation, the presence of a predator had no effect on bluegill foraging activity within this patch. However, if foraging in a patch with only limited refuge potential, bluegill foraging activity was reduced significantly in the presence of a predator. Further, this reduction was significantly greater if the patch contained a low versus a high food level, indicating a risk-balancing response to predation with respect to within-patch foraging activity. Both these responses differ from the risk-avoidance response to predation demonstrated by juvenile bluegills when selecting among habitats. Therefore, our results demonstrate the flexibility of juvenile bluegill foraging behaviour.     

The influence of food distribution upon the development of aggressive and competitive behaviour in juvenile chum salmon, Oncorhynchus keta

Groups of juvenile chum salmon were reared on food that was either dispersed throughout, or localized in one area of, the rearing tank. Groups receiving localized food displayed more aggression than those receiving dispersed food. This led to differences in growth, with fish reared on localized food having greater individual growth variability, i.e. growth depensation. However, after several months of rearing in these different feeding/social environments, fish reared on dispersed food were just as aggressive when first exposed to localized food as were fish reared on localized food. Furthermore, in competitive contests between fish of the two rearing histories, those reared on dispersed food were just as likely to become dominant as those reared on localized food. These results suggest that the behavioural development of aggressiveness is not amenable to alteration by manipulation of food distribution.     

Feeding behaviour of the central mudminnow,Umbra limi,in the field and laboratory

Synopsis The field component of this study compared feeding preferences of Umbra limi with available resources in the environment. The mudminnows preferred benthic coleopterans, anisopterans, and amphipods while avoiding planktonic and other benthic food types. The laboratory component tested for possible switching behaviour by offering different ratios of two food types (Enchytraeus worms and Culex larvae). Both components indicated a benthic feeding preference in mudminnows. Although preferences were variable across individuals, switching did not occur, a result consistent with recent predictions on predatory behaviour. Feeding patterns reflected individual differences among fish, runs of feeding on one food type, and a constancy in the total number of the two food types eaten.