On optimal diet in a patchy environment

Optimal foraging theory has dealt with the following questions independently: (1) On what prey types should an individual predator feed (optimal diet)? (2) How long should a predator stay in each patch if prey is patchily distributed (optimal allocation of time to patches) ? This paper explores optimal foraging in patches containing several different kinds of prey. Results obtained by simulation show that deviations from recent predictions are to be expected, particularly for long interpatch travel times and rapid depletion of profitable prey types. In these situations the tactics of feeding as either specialist or as a generalist can be inferior to a tactic which starts as a specialist and then expands the diet after some time in the patch. Furthermore, predators should not necessarily stay longer in a patch if interpatch travel time increases. Some experimental tests of these new predictions are proposed.     

Quantifying Predation Risk for Refuging Animals: A Case Study with Golden Marmots

Although a variety of behaviors expose animals to some risk of predation, there is no accepted way to compare their relative risk. For animals that retreat to refugia when alarmed by predators, the proportion of time devoted to each out-of-refuge behavior multiplied by the total time required to return to a refuge can be used to compare a behavior's relative predation risk. Total time to return to a refuge is a function of both response time - the time required to respond to an increased risk of predation — and travel time — the time required to flee to a refuge once alarmed. Quantifying these components can illustrate how animals minimize exposure to predators. Golden marmots (Marmota caudata aurea) were a refuging prey species used to examine the utility of this measure and to understand how marmots minimized their risk of exposure to predation. Golden marmots devoted different amounts of time to looking, foraging, self-grooming, and playing. To estimate the behavior-specific time required to return to refugia, the location of different activities was noted and a behavior-specific travel time was calculated. Alarm calls were played back to marmots engaged in different behaviors to determine, in a standardized manner, if there were behavior-specific response times. Marmots appeared to minimize their predation risk by performing most behaviors close to refugia. Results suggest that foraging was the riskiest behavior, largely because marmots foraged far from refugia and spent about 30% of their time foraging. While sample sizes were small, results also suggested that play, a rare adult behavior, exposed animals to predation because of a relatively long response time.     

Foraging tactics of an aquatic insect: Partial consumption of prey

Optimal patch foraging theory has recently been used to model partial consumption of prey by predators (Cook & Cockrell 1978; Sih 1980). I tested several predictions of this model with larvae of the predaceous diving beetle Dytiscus verticalis, which are predators of anuran tapdoles. The results of the study supported the qualitative predictions of the model. The prediction of decreasing search time with increasing prey density was confirmed. Handling time and the amount ingested per prey item also decreased with decreasing search time. A quantitative test of the optimal foraging model revealed that it accurately predicted handling times when prey densities were high but failed to do so at low prey densities. Two hypotheses based on mean extraction rates are proposed in an attempt to explain these results.     

Detecting predators and locating competitors while foraging: an experimental study of a medium-sized herbivore in an African savanna

Vigilance allows individuals to escape from predators, but it also reduces time for other activities which determine fitness, in particular resource acquisition. The principles determining how prey trade time between the detection of predators and food acquisition are not fully understood, particularly in herbivores because of many potential confounding factors (such as group size), and the ability of these animals to be vigilant while handling food. We designed a fertilization experiment to manipulate the quality of resources, and compared awareness (distinguishing apprehensive foraging and vigilance) of wild impalas (Aepyceros melampus) foraging on patches of different grass height and quality in a wilderness area with a full community of predators. While handling food, these animals can allocate time to other functions. The impalas were aware of their environment less often when on good food patches and when the grass was short. The animals spent more time in apprehensive foraging when grass was tall, and no other variable affected apprehensive behavior. The probability of exhibiting a vigilance posture decreased with group size. The interaction between grass height and patch enrichment also affected the time spent in vigilance, suggesting that resource quality was the main driver when visibility is good, and the risk of predation the main driver when the risk is high. We discuss various possible mechanisms underlying the perception of predation risk: foraging strategy, opportunities for scrounging, and inter-individual interference. Overall, this experiment shows that improving patch quality modifies the trade-off between vigilance and foraging in favor of feeding, but vigilance remains ultimately driven by the visibility of predators by foragers within their feeding patches.     

Foraging strategy of a non-omniscient predator in a changing environment (I) model with a data window and absolute criterion

1. Almost all the models so far presented assume that predators are omniscient in the sense that they always have complete information about the spatial distribution of prey abundance and its change over time. But this type of model cannot cover the situation where the prey abundance in each patch changes over time due to factors other than predation. The model with a data window and absolute criterion (SAC) here enables us to treat such situations.
2. The strategy of non-omniscient predators can be generally devided into four procedures; collection of information, its memorization, decision of tactics and its execution. SAC involves only two tactics; to stay another time period in the patch the predator is staying presently or to move to another patch chosen at random. The choice of either one of the two tactics is made by comparing the profitability of the current patch estimated by the data window with a pre-determined absolute criterion.
3. Three changing patterns of prey abundance are considered. In the most general pattern good patches have a higher mean profitability than poor patches, but the profitability changes cyclically in each of patches.
4. There are only two possibilities for an optimal strategy; the “patch choice strategy” in which once the predator has taken a good patch, it tries to stay there even when the state becomes poor, and the ‘state choice strategy” in which the predator seeks for only good states in good patches. The condition for which either of the two foraging strategies is superior to the other is specified analytically.

     

Behavioral responses to prey density by three acarine predator species with different degrees of polyphagy

Behavioral responses by three acarine predators, Phytoseiulus persimilis, Typhlodromus occidentalis, and Amblyseius andersoni (Acari: Phytoseiidae), to different egg and webbing densities of the spider mite Tetranychus urticae (Acari: Tetranychidae) on rose leaflets were studied in the laboratory. Prey patches were delineated by T. urticae webbing and associated kairomones, which elicit turning back responses in predators near the patch edge. Only the presence of webbing affected predator behavior; increased webbing density did not increase patch time. Patch time increased with increased T. urticae egg density in the oligophagous P. persimilis, but was density independent in the polyphagous species T. occidentalis and A. andersoni. Patch time in all three species was more strongly correlated with the number of prey encounters and attacks than with the actual prey number present in the patch. Patch time was determined by (a) the turning back response near the patch edge; this response decayed through time and eventually led to the abandonment of the patch, and (b) encounters with, and attacks upon, prey eggs; these prolonged patch time by both an increment of time spent in handling or rejecting prey and an increment of time spent searching between two successive prey encounters or attacks. Although searching efficiency was independent of prey density in all three species, the predation rate by P. persimilis decreased with prey density because its searching activity (i.e. proportion of total patch time spent in searching) decreased with prey density. Predation rates by T. occidentalis and A. andersoni decreased with prey density because their searching activity and success ratio both decreased with prey density. The data were tested against models of predator foraging responses to prey density. The effects of the degree of polyphagy on predator foraging behavior were also discussed.     

Failure of simple optimal foraging models to predict residence time when patch quality is uncertain

Blue jays (Cyanocitta cristata) were presented with a foragingsituation in which half of the patches they encountered containedno prey and half contained a single prey item. Experimentallydetermined probability distributions controlled prey arrivaltimes in those patches that contained prey. Patch residencein empty patches was studied during four experiments. In thefirst, prey arrival was exponentially distributed. Residencetimes increased with travel time as predicted by a rate-maximizationmodel, but the bird stayed in empty patches much longer thanpredicted. During the second experiment, prey arrival was uniformlydistributed. The jays again stayed longer than optimal, andpatch residence times increased as travel time increased, althoughthe residence time that maximized rate of intake was independentof travel time under these conditions. In the third experiment,exponential and uniform patches were randomly intermixed. Thejays showed larger travel-time effects in the exponential thanin the uniform patch. However, the travel-time effect in theuniform patch was contrary to rate-maximization predictions,and the birds again overstayed in both patch types. In the fourthexperiment, prefeeding at the start of each foraging bout slightlyincreased overstaying rather than decreasing overstaying, aswould be expected if overstaying were due to underestimatingenvironmental quality. Consistent and dramatic overstaying anda travel-time effect under conditions where travel time hasno effect on optimal residence times suggest that the rate-maximizationapproach does not apply to foraging problems involving patchuncertainty.     

Foraging behavior of an estuarine predator, the blue crab Callinectes sapidus in a patchy environment

To define general principles of predator‐prey dynamics in an estuarine subtidal environment, we manipulated predator density (the blue crab, Callinectes sapidus) and prey (the clam, Macoma balthica) patch distribution in large field enclosures in the Rhode River subestuary of the central Chesapeake Bay. The primary objectives were to determine whether predators forage in a way that maximizes prey consumption and to assess how their foraging success is affected by density of conspecifics. We developed a novel ultrasonic telemetry system to observe behavior of individual predators with unprecedented detail. Behavior of predators was more indicative of optimal than of opportunistic foraging. Predators appeared responsive to the overall quality of prey in their habitat. Rather than remaining on a prey patch until depletion, predators appeared to vary their patch use with quality of the surrounding environment. When multiple (two) prey patches were available, residence time of predators on a prey patch was shorter than when only a single prey patch was available. Predators seemed to move among the prey patches fairly regularly, dividing their foraging time between the patches and consuming prey from each of them at a similar rate. That predators more than doubled their consumption of prey when we doubled the number of prey (by adding the second patch) is consistent with optimizing behaviors ‐ rather than with an opportunistic increase in prey consumption brought about simply by the addition of more prey. Predators at high density, however, appeared to interfere with each other's foraging success, reflected by their lower rates of prey consumption. Blue crabs appear to forage more successfully (and their prey to experience higher mortality) in prey patches located within 15–20 meters of neighboring patch, than in isolated patches. Our results are likely to apply, at least qualitatively, to other crustacean‐bivalve interactions, including those of commercial interest; their quantitative applicability will depend on the mobility of other predators and the scale of patchiness they perceive.     

Ecological consequences of body size: a model for patch choice in desert rodents

Summary Recent models exploring the ecological consequences of body size have assumed that its primary effect is to determine how easily individual prey of different sizes can be pursued or handled. However, for predators that eat small, particulate food, size-related costs associated with finding and harvesting prey should be at least as important as those associated with consuming individual prey once thay have been harvested. Such predators should have generalized diets, and body size differences would not be expected to influence substantially the sizes of prey eaten. The effect of body size on spatial patterns of foraging could, however, be substantial for these predators if prey have a patchy distribution.I develop a simple model for a particle feeder foraging in patchy environments and use it to examine the special case of patch choice by seed-eating desert rodents. The model implies that for most parameter values large and small animals should specialize to different extents on the most profitable patches. Size differences among coexisting desert rodents therefore can be expected to promote partitioning of food by differential patch choice. Preliminary observations of desert rodent seed dispersion and microhabitat preferences indicate that interspecific differences in patch choice do exist.The model predicts that the nature of the relationship between size and patch choice depends on the values taken by certain model parameters. Thus, although the model predicts that patch choice generally should vary with body size, the spatial scale of patchiness and the way in which within-patch harvest rates and between-patch travel velocities scale with size determine whether, and in what way, body size should affect patch choice. As yet estimates of these parameters for heteromyid rodents are not precise enough for us to have much confidence in specific model predictions about this system. However, it will only be a matter of time before we can derive better estimates; in principle the model is testable, and when suitably modified should be applicable to many systems.     

Foraging and Calling Behavior of Carolina chickadees (Poecile carolinensis) in Response to the Head Orientation of Potential Predators

When animals detect predators they modify their behavior to avoid predation. However, less is known about whether prey species modify their behavior in response to predator body and behavioral cues. Recent studies indicated that tufted titmice, a small songbird, decreased their foraging behavior and increased their calling rates when they detected a potential predator facing toward a feeder they were using, compared to a potential predator facing away from that feeder. Here, we tested whether related Carolina chickadees, Poecile carolinensis, were also sensitive not just to the presence of a predator model, but to its facial/head orientation. Although chickadees are closely related to titmice, recent studies in different populations suggest chickadees respond to risky contexts involving predators differently than titmice. We conducted two field studies near feeders the birds were exploiting. In Study One, a mask‐wearing human observer stood near the feeder. In Study Two, a model of a domestic cat was positioned near the feeder. In both studies, the potential threatening stimulus either faced toward or faced away from the feeder. Chickadees avoided the feeder more in both studies when the potential predator was present, and showed strongest feeder avoidance when the potential predator faced toward the feeder. Chickadee calling behavior was also affected by the facial orientation of the potential predator in Study 1. These results suggest that, like titmice, chickadees exhibit predation‐risk‐sensitive foraging and calling behavior, in relation to facial and head orientation of potential threats. These small birds seem to attend to the likely visual space of potential predators. Sensitivity to predator cues like behavior and body posture must become more central to our theories and models of anti‐predator behavioral systems.     

Prey selection and foraging period of the predaceous rocky intertidal snail,Acanthina punctulata

Summary The diet and foraging period of the neogastropod Acanthina punctulata were investigated in order to test various aspects of recent optimal foraging strategy models. This intertidal snail is an actively searching predator which preys on snails and barnacles by boring a hole in the shell and rasping out the flesh. Unlike many gastropod predators, Acanthina drill its gastropod prey at a very specific location on the columella, the thickest portion of the shell. Acanthina's foraging period can be interpreted as a compromise between maximizing the energy obtained by feeding and minimizing risk of mortality from exposure to wave action. That foraging period minimizing risk of being dislodged by waves appears to be during low tide when the predators can be in shallow pools. However, prey cannot be captured and consumed during one low tide. Thus Acanthina must be exposed during some high tides, and its strategy appears to be to restrict movement while exposed. Thus search is not initiated during high tide, but drilling and prey consumption are continued during that time. A snail not drilling or consuming prey seeks the protection of crevices or large anemones during high tide. A model is presented to indicate the relative amounts of risk and net energy for Acanthina at successive low and high tides. Predictions from the model, e.g., minimizing search time to avoid being exposed for an additional high tide and no movement during high tide are supported by field data. Acanthina commences foraging at the beginning of low tide, searches initially for preferred prey, but if unsuccessful, settles for a less preferred prey and begins drilling this prey before the end of low tide. Drilling and ingestion of prey occur during the following high and sometimes low tides. These handling times take 95% of the total foraging time in the field, while search time takes only 5% (pursuit time is negligible). Drilling alone accounts for 48–70% of the total drilling and eating time. In the laboratory, drilling and eating time for littorine food ranged from 15–60 hrs per item. The time to drill and eat a littorine increases exponentially with prey length. Since handling and processing prey items represents such a large investment of time, Acanthina would be expected to be very selective with respect to choice of prey items. Electivity coefficients from field data suggest that littorines are preferred over barnacles. Acanthina in the laboratory optimizes the amount of biomass ingested per time by choosing larger littorines over smaller ones and by preferring the more readily drilled species.It is suggested that Acanthina obtains information about the range of prey available initially by encountering and evaluating quite a few prey before making a selection, but usually by comparing an item of prey encountered to the prey it recently ingested. This latter method should provide a basis for evaluating prey encountered and has the advantage of reducing search time, the total amount of time spent feeding and thus the high-tide time exposed to wave action.In a similar manner, the decrease in the level of acceptability of prey as search time increases represents a compromise between maximizing energy obtained and minimizing risk from mortality.     

The fear diet: Risk,refuge, and biological control by omnivorous weed seed predators

Weed seed biocontrol by omnivorous mice and insects can limit weed seedbanks, but this ecosystem service can be difficult to predict given the broad diet breadth of seed predators and their potential for intraguild predation. Seed foraging behavior is further modified by fluctuating cues of predation risk from higher trophic levels and the availability of refuge habitat. Uncertainty about whether co-occurring insects and mice additively contribute to weed biocontrol or interfere with each other via intraguild predation limits our ability to recommend habitat management strategies that reliably promote seed destruction. Using seed removal assays, fluorescent powder tracking, and stable isotope analyses, we assessed effects of a predation risk cue (moonlight) on mouse foraging patterns in a patchwork of vegetated and exposed plots in a cultivated field. Mouse foraging activity decreased on exposed ground during the full moon, compared to dark nights, yet foraging movements were unaffected by moon cycle within refuge patches. Weed seed consumption was more than three times higher in cover than exposed soil, and 78% of that difference was attributable to invertebrate granivores. Mice and invertebrate granivores both exhibited higher foraging activity in cover, indicating co-occurrence of intraguild predators and prey. However, stable isotope analyses of fecal samples revealed that mice captured in refuge habitats fed at slightly lower trophic levels than those in exposed habitats (suggesting minimal intraguild predation in refuge habitat), and mouse diet was unaffected by moonlight. Despite increased availability of invertebrate prey in cover patches, mice do not appear to preferentially exploit prey when avoiding their own predators or interfere with weed seed predation. Therefore, functional redundancy of mice and invertebrate seed predators in cover crops and other refuge habitats may strengthen and stabilize weed seed biocontrol.     

Behavior of fish predators and their prey: habitat choice between open water and dense vegetation

Synopsis Behavior of largemouth bass, Micropterus salmoides, and northern pike, Esox lucius, foraging on fathead minnows, Pimephales promelas, or bluegills, Lepomis macrochirus, was quantified in pools with 50% cover (half the pool had artificial stems at a density of 1000 stems m⁻²). Both predators spent most of their time in the vegetation. Largemouth bass searched for bluegills and ambushed minnows, whereas the relatively immobile northern pike ambushed all prey. Minnows were closer to predators and were captured more frequently than bluegills. Even when minnows dispersed, they moved continually and eventually wandered within striking distance of a predator. Bluegills dispersed in the cover with predators. Bass captured the few bluegills that strayed into the open and pike captured those that approached too closely in the cover. The ability of predators to capture prey while residing in habitats containing patches of dense cover may explain their residence in areas often considered to be poor ones for foraging. The unit is sponsored jointly by the United States Fish and Wildlife Service, Ohio Department of NaturalResources, The Ohio State University, and the Wildlife Management Institute     

Foraging in yellow dung flies: testing for a small-male time budget advantage

1. Mating and foraging are generally mutually exclusive activities. Individuals are thus faced with a continuous trade-off between time and energy expended in foraging and mating, but different phenotypes should respond to this trade-off in different ways. 2. Sexual selection theory predicts that females should maximize their time and energy spent gathering resources, whereas males should maximize their time and energy spent obtaining mates, thus minimizing their time spent foraging, subject to the constraint that they need to forage minimally to sustain their activity. 3. Smaller individuals require less food to maintain their activity. Small males in particular could therefore increase mating effort at the expense of foraging effort and, all else being equal, may thus enjoy a time budget advantage relative to large males. On the other hand, larger individuals may compensate by being more efficient at finding prey and/or extracting nutrients. 4. The effects of sex, body size, and prey density on foraging time budgets of male and female yellow dung flies, Scathophaga stercoraria, were investigated in the laboratory. 5. Higher prey density (Drosophila melanogaster) resulted in reduced feeding (= handling) and hunting (= waiting or search) times for both sexes, as predicted by the marginal value theorem applied to foraging theory. Females fed longer on a prey item than did males, and also caught the next prey item more quickly. Large individuals extracted nutrients more quickly, but were not faster at catching prey. Small individuals satiated more quickly than larger individuals and also ate fewer prey items. 6. These results are largely consistent with the predictions and suggest a small-male time budget advantage in the yellow dung fly. Integrating the various predictions to test directly for a small-male time budget advantage is difficult in the laboratory, however, because hunting times are unlikely to reflect the natural situation. To what extent these results lead to increased probabilities for small males of obtaining matings in the field remains to be demonstrated.     

Evolution of handling time can destroy the coexistence of cycling predators

Several consumers (predators) with Holling type II functional response may robustly coexist even if they utilize the same resource (prey), provided that the population exhibits nonequilibrium dynamics and the handling time of predators is sufficiently different. We investigate the evolution of handling time and, in particular, its effect on coexistence. Longer handling time is costly in terms of lost foraging time, but allows more nutrients to be extracted from a captured prey individual. Assuming a hyperbolically saturating relationship between handling time and the number of new predators produced per prey consumed, we obtain three results: (i) There is a globally evolutionarily stable handling time; (ii) At most two predator strategies can coexist in this model; (iii) When two predators coexist, a mutant with intermediate handling time can always invade. This implies that there is no evolutionarily stable coexistence, and the evolution of handling time eventually leads to a single evolutionarily stable predator. These results are proven analytically and are valid for arbitrary (not only small) mutations; they however depend on the relationship between handling time and offspring production and on the assumption that predators differ only in their prey handling strategy.     

Effects of Foraging Experiences on Residence Time of the Predatory Mite <Emphasis Type="BoldItalic">Neoseiulus womersleyi</Emphasis> in a Prey Patch

We investigated the effects of foraging experiences on the residence time of Neoseiulus womersleyi in a currently inhabited prey (Tetranychus urticae) patch. Satiated predators that had experienced starvation stayed longer in a current patch than those that had not experienced starvation. Satiated predators that had experienced a prey-rich patch showed approximately the same residence time in the current patch irrespective of the number of prey therein. By contrast, satiated predators that had experienced a prey-poor patch stayed longer in a current patch of high prey density than in one of low prey density. N. womersleyi appears to determine residence time in the current patch based on foraging experiences together with the quantity of prey in the current patch.     

Carrying food items to cover for consumption: the behavior of ten bird species feeding under the risk of predation

Summary In earlier work (Lima et al. 1985; Lima 1985), we found that gray squirrels (Sciurus carolinensis) and black-capped chickadees (Parus atricapillus) when exploiting a patch of food in the open often carried individual food items to protective cover for consumption. Their tendency to carry (i) decreased as distance of the patch from cover increased, and (ii) increased as size of the available food items increased. A simple model indicated that this behavior was consistent with a trade-off between efficient foraging and predation risk. Maximal feeding efficiency was achieved by always eating at the patch, whereas minimal time exposure to predators was achieved by carrying all items to cover for consumption. Because predation-riskrelated trade-offs are likely to be of importance in the determination of feeding behavior, we surveyed the behavior of 10 bird species feeding under similar conditions to assess both the generality of the above results and the adequacy of some simple assumptions concerning the assessment and perception of predation risk.We observed considerable interspecific variability in behavior. Of the 10 species studied, 4 behaved in a manner similar to the squirrels and chickadees. Five other species showed an increased tendency to carry with larger items but no clear tendency to decrease carrying from longer distances. The one remaining species exhibited neither behavioral trend.The model that predicted squirrel and chickadee behavior failed to account for all observed behavior. The behavior of all species, however, was influenced by predation risk, and the discrepancy between theory and observation most likely reflects shortcomings of the model. These discrepancies indicate that other factors, in addition to exposure time, may be of significance in the perception of predation risk by several (or all) of the species studied. Of particular importance may be a distance-dependent probability of escaping attack. Other results indicate that predation risk may influence handling times via aspects of the digestive process.     

Sex differences in giraffe foraging behavior at two spatial scales

We test predictions about differences in the foraging behaviors of male and female giraffes (Giraffa camelopardalis tippelskirchi Matchie) that derive from a hypothesis linking sexual size dimorphism to foraging behavior. This body-size hypothesis predicts that males will exhibit specific behaviors that increase their dry-matter intake rate relative to females. Foraging behavior was examined at two hierarchical levels corresponding to two spatial and temporal scales, within patches and within habitats. Patches are defined as individual trees or shrubs and habitats are defined as collections of patches within plant communities. Males were predicted to increase dry-matter intake rate within patches by taking larger bites, cropping bites more quickly, chewing less, and chewing faster. Within habitats, males were expected to increase intake rate by increasing the proportion of foraging time devoted to food ingestion as opposed to inter-patch travel time and vigilance. The predictions were tested in a free-ranging population of giraffes in Mikumi National Park, Tanzania. Males spent less total time foraging than females but allocated a greater proportion of their foraging time to forage ingestion as opposed to travel between patches. There was no sex difference in rumination time but males spent more time in activities other than foraging and rumination, such as walking. Within patches, males took larger bites than females, but females cropped bites more quickly and chewed faster. Males had longer per-bite handling times than females but had shorter handling times per gram of intake. Within habitats, males had longer average patch residence times but there was no significant sex difference in inter-patch travel times. There was no overall difference between sexes in vigilance while foraging, although there were significant sex by habitat and sex by season interactions. Although not all the predictions were confirmed, overall the results agree qualitatively with the body-size hypothesis. Sex-related differences in foraging behavior led to greater estimated intake rates for males at the within-patch and within-habitat scales. Received: 20 November 1995 / Accepted: 5 November 1996     

Different responses of congeneric consumers to an exotic food resource: who gets the novel resource prize?

Exotic species can provide abundant food resources for native consumers, but predicting which native species will respond positively remains a challenge. We studied the foraging behavior of black-capped (Poecile atricapillus) and mountain (P. gambeli) chickadees in western Montana to compare the degree to which these congeric and syntopic consumers exploited larvae of Urophora, an exotic biological control insect living within the seedheads of the invasive forb, spotted knapweed (Centaurea stoebe). Chickadees typically forage within tree or shrub cover, whereas knapweed and hence Urophora larvae thrive in open grassland away from cover. We found that black-capped chickadees were much more likely than mountain chickadees to forage for Urophora. Black-capped chickadees strategically minimized time spent in open habitats by flying out from cover to retrieve knapweed seedheads and immediately returning to cover to extract the larvae. Black-capped chickadees also employed an atypical hovering technique nearly twice as often as their congeners did, particularly when foraging away from cover. Via this hovering technique, birds were able to gather knapweed seedheads from erect plants rather than searching for seedheads on the ground. These shifts in foraging behavior allowed black-capped chickadees to exploit Urophora larvae to a much greater degree than their congeners while minimizing exposure to a high-risk habitat, an outcome with potentially important community-wide consequences. Behavioral flexibility has been used to predict the success of invading species. We suggest that behavioral flexibility may also be used to predict how native species will respond to invasions, particularly the availability of exotic food resources.     

Sex-Related Differences in the Trade-Off between Foraging and Vigilance in a Granivorous Forager

The relationship between intake rate and food density can provide the foundation for models that predict the spatiotemporal distribution of organisms across a range of resource densities. The functional response, describing the relationship between resource density and intake rate is often interpreted mechanistically as the relationships between times spend searching and handling. While several functional response models incorporate anti-predator vigilance (defined here as an interruption of feeding or some other activity to visually scan the environment, directed mainly towards detecting potential predators), the impacts of environmental factors influencing directly anti-predator vigilance remains unclear. We examined the combined effects of different scenarios of predation risk and food density on time allocation between foraging and anti-predator vigilance in a granivorous species. We experimentally exposed Skylarks to various cover heights and seed densities, and measured individual time budget and pecking and intake rates. Our results indicated that time devoted to different activities varied as a function of both seed density and cover height. Foraging time increased with seed density for all cover heights. Conversely, an increased cover height resulted in a decreased foraging time. Contrary to males, the decreased proportion of time spent foraging did not translate into a foraging disadvantage for females. When vegetation height was higher, females maintained similar pecking and intake rates compared to intermediate levels, while males consistently decreased their energy gain. This difference in anti-predator responses suggests a sexually mediated strategy in the food-safety trade-off: when resource density is high a females would adopt a camouflage strategy while an escape strategy would be adopted by males. In other words, males would leave risky-areas, whereas females would stay when resource density is high. Our results suggest that increased predation risk might generate sexually mediated behavioural responses that functional response models should perhaps better consider in the future.