Foraging Decision Rules and Prey Species Preferences of Northwestern Crows (Corvus caurinus)

Categorization of similar prey types and the application of decision rules by dietary generalists can enhance the efficiency of foraging decisions and facilitate the inclusion of novel prey types in the diet. While considerable research attention has been directed toward investigation of these concepts in invertebrates, few have assessed categorization and decision rules used by generalist vertebrate predators. In this study, we experimentally investigated decision rules and prey preferences of northwestern crows (Corvus caurinus) feeding on littleneck clams (Tapes philippinarum) and whelks (Nucella lamellosa). We presented crows with three species‐size combinations: small clams (2.0–2.9 cm length) paired with large whelks (4.0–4.9 cm), small clams paired with medium whelks (3.0–3.9 cm), and large clams (4.0–4.9 cm) with large whelks. Profitability estimates based on observations of crows feeding on these prey species indicated that clams were always the more energetically profitable option; however, in prey choice trials crows consistently selected the heavier prey species, regardless of differences in profitability. These results show that crows apply a general decision rule according to which they select heavier prey items when feeding on hard‐shelled prey requiring similar handling techniques, and that while such decision rules may approximate optimal choices they may not always follow predictions based solely on prey profitability. We discuss these results in the context of behavioural flexibility of generalist predators, and predicting impacts of intertidal avian predators on prey populations.     

A review of competitive resource sharing under constraints in sticklebacks

The best strategy for an animal competing intraspecifically for food depends on its relative competitive ability, its needs, and on the strategies its competitors are using. Three different investigations using threespined sticklebacks, Gasterosteus aculeatus , as predators and Daphnia or Tubifex worms as prey are reviewed: (1) six sticklebacks differing in competitive ability compete for two food patches, of which one is twice as profitable as the other; (2) two sticklebacks differing in competitive ability compete in one patch for two types of prey, of which one is three times as profitable as the other; (3) parasitised and healthy sticklebacks have to decide alone, and in competition with each other, how close they dare approach on their own to a live predator waiting close to profitable food. The best strategy from the point of view of the stickleback is different from that of its parasite. The two parasite species studied, Schistocephalus solidus and Glugea anomala, ought to influence their host's behaviour in opposite directions.     

Feeding rate and attack specialization: the roles of predator experience and energetic tradeoffs

Synopsis Individual mosquitofish, Gambusia affinis, can adopt a broad range of attack selectivities. In part, this variation can be explained by the past experiences of a fish. Individuals selected the more profitable Ceriodaphnia dubia (Cladocera) over less profitable cyclopoid copepods to a greater degree after being exposed to both prey types than did individuals experienced with only one of the prey types. Feeding rate (biomass ingested per unit time) declined with increased attack specialization on the profitable prey (Ceriodaphnia) when such prey were scarce, a result in agreement with assumptions of optimal diet theory. When profitable prey were abundant feeding rate was a bimodal function of the intensity of specialization on profitable prey; fish that specialized on cyclopoid copepods (the less profitable prey type) fed at higher rates than did generalists. This may be the result of antagonistic learning that precluded feeding efficiently on more than one type of prey at a time. The data are consistent with the hypothesis that rejection of unsuitable prey involves a time cost. The two preceeding aspects of foraging behavior, which are absent from most optimal diet models, could lead to failure in predicting the attack specialization of some predators, An additional aspect of the results was the generally weak relationship between feeding efficiency and specialization behavior. This suggests that feeding rate may not have been as tightly linked to the specialization behavior a predator adopts as is assumed by current foraging theory.     

Uniform persistence for sigmoidal diet selection with keystone prey species

In this paper we discuss uniform persistence (UP) criteria of two prey- one predator systems, where we consider that the predator's diet selection is a sigmoidal function of the most profitable prey type in place of a step function of conventional diet choice theory. We also derive UP results of the system with direct interspecific competition between the prey. The role of the most profitable prey item as a keystone species, the magnitude of its carrying capacity, the ability to withstand predation of both prey species, and the ratios of their profitability values (to predators) are important to whether or not adaptive foraging may promote UP. In general, foraging decision rules play no role in UP if the alternative prey item is the keystone species. The result is also not affected by the effect of direct competitive coexistence or dominance relationship of the prey. In some cases, dominance of one of the prey species provides the most advantageous situation for ensuring UP. Received: 1 February 1999 / Revised version: 20 September 1999 / Published online: 4 July 2000     

Prey selection by the larvae of three species of littoral fishes on natural zooplankton assemblages

The prey selection of larvae of three common littoral fish species (pike, Esox lucius; roach,Rutilus rutilus; and three-spined stickleback, Gasterosteus aculeatus) was studied experimentally in the laboratory by using natural zooplankton assemblages. Zooplankton prey was offered at four different concentrations to study the functional responses of the planktivores. The diets of pike and sticklebacks were formed mainly of copepod juveniles and adults, which dominated the prey communities, although sticklebacks ate also cladocerans. The diet of roach larvae consisted of rotifers, cladocerans and copepods, without prey selection, in equal proportions indicating a more omnivorous diet. All fish larvae were able to feed selectively although in sticklebacks prey selection was less pronounced. Pike and roach larvae preferred large prey to smaller prey types. Patterns of prey selection are discussed in the context of size-selection theory and apparent vs. true selection.     

Influence of predation risk on diet selection: a simple example in the grey squirrel

Current models of the optimal diet are special cases of a more general (and complex) model which incorporates the effects of predation risk on diet selection; this follows from an assumption implicit in current models that all prey items are eaten where they are encountered. Relaxing this assumption so that a forager might carry a prey item to protective cover for consumption leads to the conclusion that the value of a prey item is a function of its distance to cover as well as its energy content and handling time. Such considerations can significantly alter the outcome of diet selection relative to that expected from simple diet theory. We found that grey squirrels (Sciurus carolinensis) may reject more energetically profitable, but small food items in favour of locating larger, less energetically profitable items that can be carried to protective cover for consumption without greatly sacrificing foraging efficiency. The squirrel's tendency to reject a more profitable item is a function of its distance from cover and the size of the less profitable items. Such behaviour is inconsistent with predictions of current diet models, but is consistent with our qualitative predictions based on a previously determined predation-risk-foraging-efficiency trade-off in the grey squirrel.     

Tubes and foraging behavior in larval Chironomidae: implications for predator avoidance

Summary In laboratory experiments, I studied differential susceptibility of four co-occurring species of chironomids to a predatory damselfly. The chironomids differed in foraging behavior and could be ranked according to the amount of time they spent outside of their tubes. In choice experiments, the predator consistently selected the prey which spent more time out of the tube, and time out of tube was a significant predictor of the predation rate coefficient. Electivity indices, calculated from field samples and diet analyses of the predator, supported the laboratory results. The data suggest that exposure to predators in a heterogeneous prey community is largely determined by tubedwelling behavior.     

The costs of mixed prey for a planktivorous cyprinid,Chalcalburnus chalcoides mento

Synopsis Experiments with the Danube bleak, Chalcalburnus chalcoides mento, an obligatory planktivore, were carried out to test some basic assumptions of foraging theory regarding prey selection. The results of experiments in which two prey types were offered simultaneously were compared with results of corresponding experiments with single prey types. Although the fish always selected for the more profitable prey they always did far worse than theoretical predictions. Selectivity cannot compensate for the costs accruing from the presence of alternate prey. Under conditions which are closer to natural prey assemblages, characterized by low densities of highly profitable prey, the inability to cope adequately with a mixed prey supply may be of little disadvantage. Because of relatively low locomotion costs, patch selection may be more important than prey selection for optimizing feeding.     

Prediction of Switching and Counter-Switching Based on Optimal Foraging

It is shown that optimally foraging predators can switch or counter-switch depending on prey types and on environmental conditions, due to changes in the profitability of the prey types involved. Subsequently rules are developed to predict switching or counter-switching by the predator when prey densities change, using examples from the literature as well as new data on prey selection in sticklebacks.     

Horseshoe bats make adaptive prey-selection decisions, informed by echo cues

Foragers base their prey-selection decisions on the information acquired by the sensory systems. In bats that use echolocation to find prey in darkness, it is not clear whether the specialized diet, as sometimes found by faecal analysis, is a result of active decision-making or rather of biased sensory information. Here, we tested whether greater horseshoe bats decide economically when to attack a particular prey item and when not. This species is known to recognize different insects based on their wing-beat pattern imprinted in the echoes. We built a simulation of the natural foraging process in the laboratory, where the bats scanned for prey from a perch and, upon reaching the decision to attack, intercepted the prey in flight. To fully control echo information available to the bats and assure its unambiguity, we implemented computer-controlled propellers that produced echoes resembling those from natural insects of differing profitability. The bats monitored prey arrivals to sample the supply of prey categories in the environment and to inform foraging decisions. The bats adjusted selectivity for the more profitable prey to its inter-arrival intervals as predicted by foraging theory (an economic strategy known to benefit fitness). Moreover, unlike in previously studied vertebrates, foraging performance of horseshoe bats was not limited by costly rejections of the profitable prey. This calls for further research into the evolutionary selection pressures that sharpened the species's decision-making capacity.     

Differences in feeding selectivity and efficiency between young-of-the-year European perch (Perca fluviatilis) and roach (Rutilus rutilus) — field observations and laboratory experiments on the importance of prey movement apparency vs. evasiveness

Feeding selectivity and efficiency of young-of-the-year European perch and roach were compared under field and laboratory conditions. In laboratory experiments, the importance of prey evasiveness versus prey movement conspicuousness for fish selectivity was evaluated with respect to changing Cladocera/Copepoda prey ratio. Feeding efficiency was additionally investigated in relation to feeding time (5, 10, 20 min) and prey density (approx. 50, 200, 700 ind. L⁻¹). In Říov Reservoir, the diet of both fish species was nearly exclusively composed of crustacean zooplankton. In roach, diet shifted from rotifers and bosminids in May, towards Daphnia sp. and Leptodora kindtii in June and July. Daphnia contributed almost exclusively to the roach diet since June, composing on average more than 94% of total prey. Cyclopoid copepods, occurred in the roach’s diet only on the first sampling date; later on both cyclopoid and calanoid copepods were completely absent. On the other hand, copepods played an important role in the diet of perch. In early and mid-June when their share in the zooplankton was particularly high, copepods contributed by more than 50% to the diet of perch. Although their contribution dropped with their decline in zooplankton in June/July, by the end of July they again comprised about one third of perch’s diet. In both fish species, the increase in numbers of cladocerans in their diet was related to increase in SL. In roach, the numbers of consumed prey were doubled every twenty days during the investigated period. In perch the increase was not so consistent, but significantly higher efficiency of perch was reported on three out of six sampling dates. In laboratory experiments, roach showed a distinct avoidance for copepods and a preference for cladocerans. Both prey categories were only fed non-selectively when they dominated the prey mixture. Perch selectivity was more diversified. Contrary to roach, perch were fed copepods non-selectively on a balanced prey ratio. Further, with an increasing share of Cladocera, a situation resembling that of roach and Copepoda was avoided. However, when the share of copepods in the prey mixture dropped below ten percent, they were consumed non-selectively and with their ongoing decline in the prey mixture their preference even increased. Feeding efficiency differed significantly between perch and roach when foraging on copepods exclusively or on a prey mixture where copepods predominated. In the short time feeding experiment (5 min) with copepods, perch consumed on average 5.9 times more prey than roach. Although roach increased their success with increasing time it was still 1.7 times greater than for perch in the long time feeding experiment (20 min). Total numbers of prey consumed were positively affected by prey density and feeding time. With increasing feeding time, the consumption rate generally declined. With a fourfold increase in feeding time, the numbers of consumed prey increased on average only two times. Only in roach feeding on copepods did the numbers of prey consumed per minute of feeding increase with increasing feeding time. However, the overall numbers were low. Differences in feeding selectivity and efficiency between perch and roach juveniles were found to be significant both in the field and laboratory experiments. In roach, selectivity was determined solely by prey evasiveness. By contrast, perch’s selectivity was influenced by prey movement conspicuousness; prey escape abilities did not play an important role. Perch were more efficient foragers on evasive prey, but its feeding efficiency for non-evasive prey was not lower than that of roach. According to our observations, we suggest feeding behaviour to be responsible for the roach’s inefficiency in capturing evasive copepods.     

Optimal diet choice for large herbivores: an extended contingency model

1. A more general contingency model of optimal diet choice is developed, allowing for simultaneous searching and handling, which extends the theory to include grazing and browsing by large herbivores.
2. Foraging resolves into three modes: purely encounter-limited, purely handling-limited and mixed-process, in which either a handling-limited prey type is added to an encounter-limited diet, or the diet becomes handling-limited as it expands.
3. The purely encounter-limited diet is, in general, broader than that predicted by the conventional contingency model.
4. As the degree of simultaneity of searching and handling increases, the optimal diet expands to the point where it is handling-limited, at which point all inferior prey types are rejected.
5. Inclusion of a less profitable prey species is not necessarily independent of its encounter rate and the zero-one rule does not necessarily hold: some of the less profitable prey may be included in the optimal diet. This gives an optimal foraging explanation for herbivores' mixed diets.
6. Rules are shown for calculating the boundary between encounter-limited and handling-limited diets and for predicting the proportion of inferior prey to be included in a two-species diet.
7. The digestive rate model is modified to include simultaneous searching and handling, showing that the more they overlap, the more the predicted diet-breadth is likely to be reduced.     

Constraints on prey size selection by the three-spined stickleback: energy requirements and the capacity and fullness of the gut

An experiment was designed to study how gut fullness and encounter with 5-mm Asellus aquaticus influenced acceptance or rejection of less profitable 8-mm Asellus . 45-mm sticklebacks were found to always accept 5-mm prey whereas 8-mm prey were accepted with an initial probability of about 0.9. This probability decreased as the gut filled. Fish of differing sizes and sex had similar daily energy intakes per unit body size, however the acceptance of 8-mm prey was related to fish size. Whenever a fish orientated to a prey it was followed by pursuit and manipulation independently of prey size. The decision to accept or reject prey occurred after one manipulation, a criterion that was more variable for the larger prey. For one feeding session per day the total energy intake was almost constant despite the changing combination of prey sizes eaten. The fish ate prey with long handling times if the energetic contents of the stomach had not reached 450 J. Calculations were made of how many of each millimetre prey size group would satisfy the 450 J demand and how long the estimated number would take to handle. This showed that the best option is to consume 5-mm prey if given the choice.     

Food selection in juvenile three-spined sticklebacks studied in relation to size,abundance and biomass of prey

The food selection and diet of juvenile three-spined sticklebacks (Gasterosteus aculeatus L.) was studied in the littoral of a brackish-water sea area. The sampling stations were located in a eutrophicated area, where the plankton was characterized by rotifers, in a noneutrophicated zone, where copepods predominated, and in the intermediate zone, characterized by cladocerans. In all areas the fish preferred microcrustaceans to rotifers, but they did not feed strictly according to the size-selection hypothesis. Nor did they take the most abundant prey species. With some exceptions the species chiefly eaten by and also selected by fish, were those which had the largest biomass in the plankton.     

Optimal foraging by the swallow (Hirundo rustica, L): Prey size selection

The foraging ecology of the swallow (Hirundo rustica) was investigated in the field and compared with predictions of optimal foraging theory. Prey items were selected primarily by size. The inclusion of an item in the diet depended on the absolute abundance of the high-ranking prey and not of the low-ranking ones. Small items of low profitability were, however, included when food was abundant and more small items were taken when they were relatively abundant. Energetics data are used to show that it is profitable in terms of net energy gain for the swallow to take a mixture of large and small items, whereas optimal foraging theory would predict exclusive specialization on large, high-ranking items. Other possible explanations for the inclusion of small items in the diet are discussed.     

The influence of predation risk on diet selectivity: A theoretical analysis

Studies that have examined the effect of experimental increases in predation risk on diet selectivity have shown both decreased and increased diet selectivity. A possible explanation for these disparate results emerges from an examination of the prey sets used in these studies, which differed in the relationship between the values of risk components associated with the capture of different prey types (‘danger’) and their profitabilities. When less profitable prey were more dangerous, selectivity increased with predation risk. When prey were equally dangerous, selectivity did not change. Finally, when the more profitable prey were also more dangerous, selectivity decreased with risk. Here, we examine theoretically the influence of a forager's estimate of the probability that a predator is present (φ) on the selection of diets from prey sets with varying danger–profitability relationships. A dynamic programming model is used to determine the maximum attack time (or distance) for each of two types of prey, differing in their energetic content, for a range of forager energy state and φ levels. The diets which would result if foragers attacked prey according to the rules provided by the dynamic model are then determined. The model results indicate that the prey danger–profitability relationship determines the diet selectivity response to φ, confirming that variation in this relationship could be responsible for the range of experimental results. This revised version was published online in July 2006 with corrections to the Cover Date.     

The influence of prey size and abundance, and individual phenotype on prey choice by the three-spined stickleback, Gasterosteus aculeatm L.

Prey selection behaviour of three-spined sticklebacks, Gasterosteus aculeatus L., was studied in two experiments. Where possible, the experimental apparatus satisfied the assumptions of the simplest optimal diet model (the basic prey model); prey were presented sequentially, the fish could not search for and handle prey at the same time, and net energy gain, handling time and encounter rate were fixed. Experiment 1 presented fish with a range of Asellus sizes so that pursuit ( p ) and handling ( h ) time could be related to prey size. Published energy values of Asellus together with pursuit and handling times were used to calculate E /( p+h ) for Asellus measuring 3,4,5,6,7 and 9 mm. Pursuit times did not differ with prey size but handling times did. E /( p+h ) was very variable particularly at the larger prey sizes. Experiment 2 presented fish with two sequences of prey differing in the encounter rate with the most profitable prey sizes. Fish did not select the diet predicted by the basic prey model tending to always ignore the largest prey even when net energy gain would have been maximized by including them in the diet. Further analysis showed that the probability of a prey size being taken was a function of prey size, fish stomach fullness and encounter rate. It is concluded that the basic prey model is too simple to capture the behaviour of the fish. One of its main faults is that the changing state of the fish through the feeding bout is ignored.     

Prey profitability for adult Grey Herons Ardea cinerea and the constraints on prey size when feeding young nestlings

Patterns of handling time and profitability are examined for adult Grey Herons feeding on carp, eels and catfish. Handling times generally increased with prey size but were influenced markedly by the morphology and behaviour of the prey. Profitability was highest for carp (max. 0.9 g/s for 15–20 cm fish), lowest for catfish (max. 0.05 g/s) and intermediate for eels (max. 0.1 g/s). Nestlings were unable to ingest the sizes of fish most profitable for the adults to consume until aged 20 days; by the age of 30 days, they could consume the full size-range of prey taken by the adults. In order to feed their young chicks, adults must therefore either select smaller prey, or break their large prey into smaller pieces. The diet of nestling Grey Herons in the Camargue is examined for evidence to support or refute the former hypothesis.
Young nestlings (≤20 days) regurgitated smaller carp than old nestlings (> 20 days). Comparison of prey types in the diet of the two groups showed that small prey species occurred significantly more often in the diet of young chicks, while the converse was true for larger prey species. The occurrence of particular prey types in the diet only of young chicks suggests that adults may forage in more marginal, shallower water (where small prey are probably more abundant) to meet the requirements of their brood during the early part of the nestling phase. The second hypothesis, that the adults break down large prey into smaller pieces, was not examined, although evidence from other studies suggests that this does occur; both mechanisms may therefore be important.     

Adaptive omnivory and species coexistence in tri-trophic food webs

The commonness of omnivory in natural communities is puzzling, because simple dynamic models of tri-trophic systems with omnivory are prone to species extinction. In particular, the intermediate consumer is frequently excluded by the omnivore at high levels of enrichment. It has been suggested that adaptive foraging by the omnivore may facilitate coexistence, because the intermediate consumer should persist more easily if it is occasionally dropped from the omnivore's diet. We explore theoretically how species permanence in tri-trophic systems is affected if the omnivore forages adaptively according to the "diet rule", i.e., feeds on the less profitable of its two prey species only if the more profitable one is sufficiently rare. We show that, compared to systems where omnivory is fixed, adaptive omnivory may indeed facilitate 3-species persistence. Counter to intuition, however, facilitation of 3-species coexistence requires that the intermediate consumer is a more profitable prey than the basal resource. Consequently, adaptive omnivory does not facilitate persistence of the intermediate consumer but enlarges the persistence region of the omnivore towards parameter space where a fixed omnivore would be excluded by the intermediate consumer. Overall, the positive effect of adaptive omnivory on 3-species persistence is, however, small. Generally, whether omnivory is fixed or adaptive, 3-species permanence is most likely when profitability (=conversion efficiency into omnivores) is low for basal resources and high for intermediate consumers.     

The evolution of locomotory behavior in profitable and unprofitable simulated prey

Prey that are unprofitable to attack (for example, those containing noxious chemicals) frequently exhibit slower and more predicable movement than species that lack these defenses. Possible explanations for the phenomenon include a lack of selection pressure on unprofitable prey to avoid predators and active selection on unprofitable prey to advertise their noxiousness. We explicitly tested these and other hypotheses using a novel artificial world in which the locomotory characteristics (step size, waiting time, and angular direction) of artificial profitable and unprofitable computer-generated prey were subject to continued selection by humans over a number of generations. Unprofitable prey evolved significantly slower movement behavior than profitable prey when they were readily recognized as unprofitable, and also when they frequently survived predatory attacks. This difference arose primarily as a consequence of more intense selection on profitable prey to avoid capture. When unprofitable prey were very similar (but not identical) in morphological appearance to profitable prey, unprofitable prey evolved particularly slow movement behavior, presumably because when they were slow-moving they could be more readily recognized as being unprofitable. When unprofitable prey were constrained to move slowly, a morphologically identical profitable prey species evolved locomotor mimicry only when it had no more effective means of avoiding predation. Overall, our results provide some of the first empirical support for a number of earlier hypotheses for differences in movement between unprofitable and profitable prey and demonstrate that locomotor mimicry is not an inevitable outcome of selection even in morphologically similar prey.