Does acoustically simulated predation risk affect settlement and reproduction of a migratory passerine?

Nest predation is one of the most important drivers of avian life history evolution and population dynamics. Increasing evidence suggests that birds are able to assess nest predation risk and avoid settling in high‐risk areas to increase their reproductive performance. However, the cues used for settlement decisions are poorly known in most species. Population sizes of the migratory wood warbler Phylloscopus sibilatrix are characterized by strong annual fluctuations, which are negatively correlated with the number of forest rodents. Wood warblers might avoid rodent‐rich areas to reduce predation risk arising either from rodents, from rodent‐hunting predators attracted to such areas or from predators not linked to rodents. To evaluate these hypotheses, we conducted a large‐scale field experiment to test whether wood warblers avoided settling in plots with high predation risk simulated by broadcasting vocalizations of rodents or predators. Moreover, we tested whether reproductive performance varied in relation to simulated predation risk. Settlement patterns did not differ between plots with rodent, predator and noise control treatments. Likewise, measures of reproductive performance did not seem to differ across treatments. Thus, the broadcasted vocalizations of rodents and predators did not seem to be perceived as threat by wood warblers. Alternatively, the species might use other cues than those presented here, either other acoustic cues, visual and/or olfactory cues or a combination of cue types during settlement. Further experimental investigations to pin point cues and senses relevant for settlement decisions in wood warblers and birds in general are needed to better understand their life history and population dynamics.     

Rodent foraging is affected by indirect, but not by direct, cues of predation risk

We used foraging trays to determine whether oldfield mice, Peromyscuspolionotus, altered foraging in response to direct cues of predationrisk (urine of native and nonnative predators) and indirectcues of predation risk (foraging microhabitat, precipitation,and moon illumination). The proportion of seeds remaining ineach tray (a measure of the giving-up density [GUD]) was usedto measure risk perceived by mice. Mice did not alter theirGUD when presented with cues of native predators (bobcats, Lynxrufus, and red foxes, Vulpes vulpes), recently introduced predators(coyotes, Canis latrans), nonnative predators (ocelots, Leoparduspardalis), a native herbivore (white-tailed deer, Odocoileusvirginianus), or a water control. Rather, GUD was related tomicrohabitat: rodents removed more seeds from foraging trayssheltered beneath vegetative cover compared with exposed traysoutside of cover. Rodents also removed more seeds during nightswith precipitation and when moon illumination was low. Our resultssuggest that P. polionotus used indirect cues rather than directcues to assess risk of vertebrate predation. Indirect cues maybe more reliable than are direct scent cues for estimating riskfrom multiple vertebrate predators that present the most riskin open environments.     

Macro- and micro-habitat selection of small rodents and their predation risk perception under a novel invasive predator at the southern end of the Americas

Invasive predators are responsible for the extinction of numerous island species worldwide. The naïve prey hypothesis suggests that the lack of co-evolutionary history between native prey and introduced predators results in the absence of behavioral responses to avoid predation. The lack of terrestrial mammal predators is a core feature of islands at the southern end of the Americas. Recently, however, the American mink (Neovison vison) established as a novel terrestrial predator, where rodents became a main portion of its diet. Here, we investigated on Navarino Island, Chile, macro- and micro-habitat selection of small rodents using Sherman traps. Additionally, we experimentally tested behavioral responses of small rodents to indirect cues of native raptorial predation risk (vegetation cover) and direct cues of novel mink predation risk (gland odor) using Sherman traps and foraging trays (giving-up density (GUD)). At the macro-habitat level, we detected native rodents of the species Abrothrix xanthorhinus and Oligoryzomys longicaudatus and the exotic Mus musculus. In general, rodents preferred scrubland habitats. At the micro-habitat level, we only captured individuals of A. xanthorhinus. They preferred covered habitats with tall vegetation. GUD increased in opened areas (riskier for raptorial predation) regardless of the presence or not of mink odor. These results suggest that A. xanthorhinus can perceive predation risk by raptors, but not by mink, results that accord with the hypothesis that co-evolutionary history is important for rodents to develop antipredator behavior. Given that these rodents represent an important proportion of mink diet, the low abundances together with the apparent lack of antipredator response raise conservation concerns for the small rodent populations inhabiting the southernmost island ecosystems of the Americas.     

Human activity and habitat type affect perceived predation risk in urban white‐footed mice (Peromyscus leucopus)

Predation risk is one of the largest costs associated with foraging in small mammals. Small mammals select microhabitat features such as tree and shrub canopy cover, woody debris and vegetative ground cover that can lower the risk of detection from predators and provide greater protection if discovered. Small mammals also increase foraging activity and decrease selection for cover when cloud cover increases and moon illumination is less. Often researchers assume small mammals in urban areas respond to these cues in the same manner as in natural areas, but these cues themselves are altered in urban zones. In this study, we investigated how Amur honeysuckle (Lonicera maackii) and coarse woody debris (CWD) affected giving‐up density (GUD) in white‐footed mice (Peromyscus leucopus). Each of three habitat treatments (open flood channel, the edge and interior of the honeysuckle patch) contained cover treatments with coarse woody debris present or absent. The six treatment combinations were compared to environmental variables (temperature, humidity and illumination) and habitat variables to test their effect on GUD. Peromyscus leucopus foraged to lower densities in areas with CWD present and also under the honeysuckle canopy, using this invasive shrub to decrease predation risk, potentially increasing survivability within this urban park. Increased human presence negatively affected foraging behavior across treatments. Human presence and light pollution significantly influenced P. leucopus, modifying their foraging behavior and demonstrating that both fine‐ and coarse‐scale urban factors can affect small mammals. Foraging increased as humidity increased, particularly under the honeysuckle canopy. Changes in illumination due to moonlight and cloud cover did not affect foraging behavior, suggesting urban light pollution may have altered behavioral responses to changes in light levels. Lonicera maackii seemed to facilitate foraging in P. leucopus, even though it adversely affects the plant community, suggesting that its impact may not be entirely negative.     

Integrating the costs of plant toxins and predation risk in foraging decisions of a mammalian herbivore

Foraging herbivores must satisfy their nutrient requirements in a world of toxic plants while also avoiding predators. Plant toxins and perceived predation risk at food patches should both reduce patch residency time, but the relative strengths of these factors on feeding decisions has rarely been quantified. Using an arboreal generalist herbivore, the common brushtail possum Trichosurus vulpecula, we tested the effects on food intake of the plant toxin, cineole, and regurgitated pellets from one of its predators, the powerful owl Ninox strenua at the small spatial scale of the food patch. We used the giving-up density (GUD) framework, with animals harvesting food items (sultanas) in an inedible matrix (small pebbles). We ran two consecutive field experiments in a eucalypt woodland in eastern Australia, 1 month apart in the same location. In experiment 1, there was a significant interaction between cineole [at 17% of dry matter (DM)] and owl pellets. The GUD was lowest in the absence of both cineole and owl pellet, intermediate in the presence of owl pellet; and highest with cineole ± owl pellet. The effect of owl pellet diminished over time. In experiment 2, only cineole (at 10% DM) increased the GUD significantly. The difference in effect of owl pellet was probably due to both habituation and freshness of the cue. Our study demonstrates the importance of synthesising predator–prey and plant–herbivore ecology to better understand the complex set of constraints influencing foraging herbivores. The greater effect of toxin than fear on possums is likely to be due to its high, but ecologically relevant concentration. This highlights the need to explore the relative and net impacts of a range of concentrations of plant toxins and predation risks.     

The effects of water on patch use by two Simpson Desert granivores (Corvus coronoides and Pseudomys hermannsburgensis)

Abstract Water loss while foraging may affect the overall value of food to desert animals. When water is scarce, foragers may alter activity and shun certain types of food due to elevated water loss. When water is abundant, foragers can exploit food patches more thoroughly and remain active over a broader range of ambient conditions. In short, food and water may be complementary resources. The presence of water raises the marginal value of food, particularly those foods low in water content. We tested for the complementarity of food and water to foragers at a sand dune site in the Simpson Desert of arid Australia. To do so, we quantified patch exploitation of foragers in the presence or absence of bowls filled with water. In order to quantify patch use, we provisioned feeding trays with granulated peanuts mixed into a sand substrate. In these trays we measured giving-up densities (GUD; the amount of food left in a tray after a foraging bout) of diurnal (mostly Australian ravens, Corvus coronoides) and nocturnal foragers (mostly sandy inland mouse, Pseudomys hermannsburgensis). The presence of water affected the GUD of ravens but not of rodents. For the ravens, GUD dropped about 50% in response to added water. For ravens, water and food are strongly complementary. In addition, ravens had lower GUD in the open than the bush microhabitat, and lower GUD at the bottom than the tops of sand dunes.     

Effects of predation and dispersal on Mastomys natalensis population dynamics in Tanzanian maize fields

1. We investigate the effects of different levels of predation pressure and rodent dispersal on the population dynamics of the African pest rodent Mastomys natalensis in maize fields in Tanzania. 2. Three levels of predation risk were used in an experimental set-up: natural level (control), excluding predators by nets and attracting avian predators by nest boxes and perch poles. Because dispersal of the rodents could mask the predation pressure treatment effects, control and predator exclusion treatments were repeated with enclosed rodent populations. 3. Population growth during the annual population rise period was faster in the absence of predators and peak population size was higher, but otherwise dynamics patterns were similar for populations where predators had access or were attracted, indicating that compensatory mechanisms operate when rodents are exposed to high levels of predation risk. Reducing dispersal of rodents removed the effect of predation on population growth and peak size, suggesting that local predators may play a role in driving rodent dispersal, but have otherwise little direct effect on population dynamics.     

Microhabitat use, giving-up densities and temporal activity as short- and long-term anti-predator behaviors in common voles

We used depletable food patches to determine the effect of microhabitat (mowed versus unmowed adjacent grasslands) and time (day versus night) on the foraging behavior of common voles (Microtus arvalis). The food remaining after 12‐h periods (giving‐up density, GUD) measured the vole's habitat selection under predation risk. In accord with several other rodent species and the effects of avian predators, voles had significantly lower GUDs in the unmowed than mowed portion of the grassland. GUDs in patches along the border between adjacent habitats were more similar to the risky mowed grassland than the safe unmowed grass. Time interacted strongly with microhabitat. In the mowed grass, voles had significantly higher GUDs at night than day. Whereas in the unmowed grass, GUDs were significantly higher during the day than night. Vole GUDs did not vary with time along the boundary. This suggests that predators are more abundant or effective in the mowed grass at night (owls?), and in the unmowed grass during the day (weasels?). In terms of predation risk, the voles perceived the mowed grass at night as the riskiest and the unmowed grass at night as the safest. Voles may have difficulties assessing resources under high predation risk: GUDs among patches were well equalized in the unmowed microhabitat whereas in the mowed grass only day GUDs did not vary significantly among patches. We linked these results to the vole's day‐night‐activity and life span. For the 533 voles live‐trapped at the study area, the ratio of day versus night captures for each individual served as an activity index and the span between first and last capture measured minimum life span. In accord with higher GUDs at night, very few individuals behaved selectively towards the night, but individual life expectancy increased with temporal opportunism. Microhabitat differences in GUDs reflect short‐term strategies of predator avoidance and the trapping data reflect long‐term patterns of anti‐predator behavior.     

Spicing up restoration: can chili peppers improve restoration seeding by reducing seed predation?

Seed predation by rodents presents a significant barrier to native plant recruitment and can impede restoration seeding efforts. In nature, some plants contain secondary defense compounds that deter seed predators. If these natural defense compounds can be applied to unprotected seeds to inhibit rodent granivores, this approach could improve restoration seeding. Capsaicin is the active ingredient in chili pepper (Capsicum spp.) seeds that creates the burning sensation associated with human consumption of hot peppers. This compound has a similar effect on other mammals and is believed to have evolved as a deterrent to rodent seed predators. We used seed‐coating techniques to attach powder ground from Bhut Jolokia (Capsicum chinense) peppers to native plant seeds and evaluated the efficacy of these seed coatings for deterring rodent seed predation and enhancing native plant recruitment using laboratory and field experiments. Laboratory feeding trials demonstrated that native deer mice (Peromyscus maniculatus) consumed far fewer pepper‐coated seeds compared to untreated control seeds. Field seed‐addition experiments consistently demonstrated that rodent seed predation reduced native plant recruitment over the 4‐year study. Coating techniques used in the first 3 years were not persistent enough to reduce rodent seed predation effects on plant recruitment. However, a more persistent coating applied in conjunction with late‐winter sowing negated rodent seed predation effects on recruitment in year 4. Our results demonstrate that coating seeds with natural plant defense compounds may provide an effective, economical way to improve the efficacy of plant restoration by deterring seed predation by ubiquitous rodent granivores.     

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.     

Within‐patch oviposition site shifts by spider mites in response to prior predation risks decrease predator patch exploitation

In egg‐laying animals with no post‐oviposition parental care, between‐ or within‐patch oviposition site selection can determine offspring survival. However, despite the accumulation of evidence supporting the substantial impact predators have on oviposition site selection, few studies have examined whether oviposition site shift within patches (“micro‐oviposition shift”) reduces predation risk to offspring. The benefits of prey micro‐oviposition shift are underestimated in environments where predators cannot disperse from prey patches. In this study, we examined micro‐oviposition shift by the herbivorous mite Tetranychus kanzawai in response to the predatory mite, Neoseiulus womersleyi, by testing its effects on predator patch exploitation in situations where predatory mites were free to disperse from prey patches. Adult T. kanzawai females construct three‐dimensional webs on leaf surfaces and usually lay eggs under the webs; however, females that have experienced predation risks, shift oviposition sites onto the webs even in the absence of current predation risks. We compared the predation of eggs on webs deposited by predator‐experienced females with those on leaf surfaces. Predatory mites left prey patches with more eggs unpredated when higher proportions of prey eggs were located on webs, and egg survival on webs was much higher than that on leaf surfaces. These results indicate that a micro‐oviposition shift by predator‐experienced T. kanzawai protects offspring from predation, suggesting adaptive learning and subsociality in this species. Conversely, fecundity and longevity of predator‐experienced T. kanzawai females were not reduced compared to those of predator‐naïve females; we could not detect any costs associated with the learned micro‐oviposition shift. Moreover, the previously experienced predation risks did not promote between‐patch dispersal of T. kanzawai females against subsequently encountered predators. Based on these results, the relationships of between‐patch oviposition site selection and micro‐oviposition shift are discussed.     

Indirect food web interactions affect predation of Tengmalm's Owls Aegolius funereus nests by Pine Martens Martes martes according to the alternative prey hypothesis

Although population cycles of rodents are geographically widespread and occur in a number of rodent species, higher‐order food web interactions mediated by predator–rodent dynamics have primarily been described from boreal and arctic biomes. During periods of low rodent abundance, predators may switch to alternative prey, which may affect other predators directly or indirectly. Using a long‐term dataset, we assessed the frequency of Pine Marten Martes martes predation on the nests of Tengmalm's Owl Aegolius funereus during periods of fluctuating rodent abundance in Central Europe. The number of nests predated by Pine Martens was positively correlated with the annual number of nests available. The probability of predation by Pine Martens on Tengmalm's Owl nests decreased with increasing spring abundance index of Apodemus mice, but was not related to the abundance index of Myodes and Microtus voles, pooled rodent abundance or age of the nestbox. Additionally, we found no relationship between the breeding frequency (i.e. the number of nesting attempts per nestboxes available) and an abundance index of Microtus and Myodes voles, Apodemus mice or overall rodent abundance. Our results demonstrate, for the first time in a temperate area, that during periods of low Apodemus mouse abundance, the switching response of an opportunistic mammalian predator can lead to indirect food web interactions through an increase in nest predation on a sympatric avian predator.     

The influence of moss on grazers in high‐altitude streams: food,refuge or both?

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Foraging and habitat use of common duikers,Sylvicapra grimmia,in a heterogeneous environment within the Soutpansberg,South Africa

We investigated aspects of the foraging behaviour and activity patterns of free‐ranging common duikers (Sylvicapra grimmia) within the Soutpansberg, South Africa. We used giving‐up densities (GUD) and camera traps to test for habitat selection and patch‐use behaviour by common duikers inhabiting a grassland containing distinct ‘islands' of woody vegetation. Foraging in or around a wooded island was affected by its surrounding vegetation. GUDs were significantly lower in portions dominated by tall grass and scattered ferns and highest in areas with open short grass and thick fern. Using grids of 5 × 5 stations, we mapped the duikers' foraging on a larger scale that incorporated neighbouring rocky hillsides. The duikers preferred feeding in areas with tall grass and scattered fern (sufficient cover and escape routes), followed by the wooded islands and thick fern (lack of sightlines/escape routes and presence of predator‐ambush sites), whereas little foraging occurred at the edges and rocky areas (hard substrate that impede escape potential). Photos (total 873) revealed solitary activity, highest in the late afternoon. Photos of vigilant individuals were mostly from rocky and fern habitats. Our results suggest that the duikers allocated their feeding efforts, activity and vigilance patterns to attune to their perceived risk of predation within their heterogeneous environment.     

Effects of arboriculture stands on European hare <Emphasis Type="Italic">Lepus europaeus</Emphasis> spring habitat use in an agricultural area of northern Italy

Habitat changes due to agricultural intensification is widely considered a main cause of European hare (Lepus europaeus Pallas, 1778) decline. We investigated the potential of arboriculture stands to enhance landscape diversity and increase farmland suitability for European hares in an intensive agricultural area of northern Italy. In spring 2005, we recorded hare habitat use during feeding activity and resting time by nocturnal censuses and searching for daytime resting sites, respectively. Hare presence/absence, based on faecal pellets, was assessed in 150 1-m radius plots. Hares positively selected edges and avoided woods and stubble during both feeding activity and resting time, which highlights the key role of permanent linear cover in providing the variety of vegetation types required by hares for shelter and feed. Forms were typically located in covered places, surrounded by denser vegetation than sample plots, in order to gain protection from avian and terrestrial predators. Arboriculture stands seem to be able to satisfy a part of hare habitat requirement, thanks to ground flora that provide tasty food, and tree cover that offers refuge against predators and bad weather conditions. However, more focused studies are necessary to improve knowledge of the long-term effects of these crops on farmland biodiversity.     

Abundance and diversity of rodents at the human–wildlife interface in Western Serengeti, Tanzania

Rodent species abundance and diversity in Western Serengeti are evaluated and discussed in relation to different levels of conservation status [Unprotected Area (UA), Game Reserve (GR) and National Park (NP)] and broad site differences in human livelihood activities. A total of 2170 individuals, spread over 16 rodent species, were caught in a capture‐mark‐recapture study which covered both the dry and wet seasons. The more humid site (Tabora B) in the northern part of Serengeti had the highest diversity of rodents followed by the Mihale site at the western extension. The driest site at Robanda had the lowest overall species diversity. Diversity also varied between the three levels of conservation status whereby the UA had the least diversity while the NP, which enjoyed the highest level of conservation status, had the highest diversity of rodents. Unprotected Area and NP plots at Tabora B showed a rodent species similarity index of 40%; all the other paired plots scored over 50% similarity indices, suggesting that, within a site, species composition did not vary significantly between the three levels of conservation status. The Robanda site had the highest (56%) overall abundance of rodents; Mihale and Tabora B sites had about the same level of rodent abundance (20 and 24% respectively). For the Mihale site, Mastomys natalensis ranked first followed by Arvicanthis niloticus and Tatera robusta, each of which contained 40, 38 and 16%, respectively, of all individuals caught at the site. For the Robanda site, the figures were 66%A. niloticus, 22%M. natalensis and 9%T. robusta; while for the Tabora B site the scores were 37%M. natalensis, 18%T. robusta and 11%Lemniscomys barbarus. The differences in diversity, species composition and population abundance appear to result largely from physiognomic vegetation types, and habitat perturbations caused by livelihood activities in Western Serengeti.     

On the value of information: studying changes in patch assessment abilities through learning

Little is known about how animals acquire and use prior information, particularly for Bayesian patch assessment strategies. Because different patch assessment strategies rely upon distinct capabilities to obtain information, we analyzed whether foragers can alter their foraging strategy when they exploit predictable patches with periodic renewal. For this, we evaluated if learning contribute to increase foraging efficiency by improving patch assessment abilities in degus (Octodon degus), a diurnal caviomorph rodent from central Chile. Single degus exploited pairs of depleting patches that were renewed daily. During the initial two days of the experiment, degus exploited patches in agreement with a fixed‐time strategy, i.e. at the population level, giving‐up densities (GUD) were not distinguishable from density‐independence (i.e. consumption proportional to initial patch densities), and richer patches were under‐exploited. After day five, degus improved significantly their assessment strategy, showing agreement with Bayesian information updating. However, on day 15 and afterwards, degus foraged patches in agreement with a prescient strategy, because GUDs across patches indicated positive density‐dependence and equalization of GUDs. Although highly variable, the GUD ratio between rich and poor patches decreased significantly throughout time. Within‐subject data showed that as subjects learned patch qualities they showed a tendency toward GUD equalization and differentiation from density‐independence. By the end of the experiment, degus allocated more time to richer patches during the initial period of each trial, and allocated similar amounts of time by the end of trials. Further, the first visit of a session was significantly biased toward the rich patch by the final days of the experiment. The results suggest that assessment abilities can change when exploiting novel but predictable patches. When degus can incorporate adequate environmental information, prior and current information may become accurate enough to make animals exploit patches efficiently.     

The landscape of fear as an emergent property of heterogeneity: Contrasting patterns of predation risk in grassland ecosystems

The likelihood of encountering a predator influences prey behavior and spatial distribution such that non‐consumptive effects can outweigh the influence of direct predation. Prey species are thought to filter information on perceived predator encounter rates in physical landscapes into a landscape of fear defined by spatially explicit heterogeneity in predation risk. The presence of multiple predators using different hunting strategies further complicates navigation through a landscape of fear and potentially exposes prey to greater risk of predation. The juxtaposition of land cover types likely influences overlap in occurrence of different predators, suggesting that attributes of a landscape of fear result from complexity in the physical landscape. Woody encroachment in grasslands furnishes an example of increasing complexity with the potential to influence predator distributions. We examined the role of vegetation structure on the distribution of two avian predators, Red‐tailed Hawk (Buteo jamaicensis) and Northern Harrier (Circus cyaneus), and the vulnerability of a frequent prey species of those predators, Northern Bobwhite (Colinus virginianus). We mapped occurrences of the raptors and kill locations of Northern Bobwhite to examine spatial vulnerability patterns in relation to landscape complexity. We use an offset model to examine spatially explicit habitat use patterns of these predators in the Southern Great Plains of the United States, and monitored vulnerability patterns of their prey species based on kill locations collected during radio telemetry monitoring. Both predator density and predation‐specific mortality of Northern Bobwhite increased with vegetation complexity generated by fine‐scale interspersion of grassland and woodland. Predation pressure was lower in more homogeneous landscapes where overlap of the two predators was less frequent. Predator overlap created areas of high risk for Northern Bobwhite amounting to 32% of the land area where landscape complexity was high and 7% where complexity was lower. Our study emphasizes the need to evaluate the role of landscape structure on predation dynamics and reveals another threat from woody encroachment in grasslands.     

Is Foraging Time Limited During Winter? – A Feeding Experiment with Tree Sparrows Under Different Predation Risk

Small passerines are faced with a trade‐off when foraging during winter. Increasing energy reserves makes them more vulnerable to predators, while a low level of reserves exposes them to a high risk of starvation. Whether small birds under these circumstances are allowed to reduce their foraging activity under increased predation risk, for example in feeding sites more exposed to predators, remains controversial in former behavioural and ecological researches. In this study, we investigated the foraging activity of free‐living Tree Sparrow Passer montanus flocks feeding on an artificial feeding platform. The predation risk perceived by the sparrows was manipulated by placing the platform either close to or far from a bushy shelter. Foraging activity, assessed as cumulative activity of sparrows per unit time on the platform, did not differ between the low‐risk and the high‐risk conditions and did not significantly change during the day. Feeding efficiency, assessed as pecking rate, was not either reduced under the high‐risk condition. Our results suggest that sparrows were forced to feed almost continuously during the day in order to maintain their preferred level of energy reserves. However, several behavioural changes helped sparrows to adopt a safer foraging policy when feeding far from cover, as we found in another study. Altogether, sparrow flocks feeding far from cover decreased the overall foraging time (the time when any sparrow stayed on the platform) by approximately 20% as compared to the near cover condition. A possible way to maintain the same level of foraging activity despite of the reduction in overall foraging time is discussed.     

Opportunity costs influence food selection and giving‐up density of dabbling ducks

The interaction of animals with their food can yield insights into habitat characteristics, such as perceived predation risk and relative quality. We deployed experimental foraging patches in wetlands used by migrating dabbling ducks Anas spp. in the central Illinois River Valley to estimate variation in seed removal and giving‐up density (GUD; i.e. density of food remaining in patches following abandonment) with respect to seed density, seed size, seed depth in the substrate, substrate firmness, perceived predation risk, and an energetic profitability threshold (i.e. critical food density). Seed depth and the density of naturally‐occurring seeds outside of experimental plots affected seed removal and GUD in experimental patches more than perceived predation risk, seed density, seed size or substrate firmness. The greatest seed removal and lowest GUDs in experimental patches occurred when food resources in alternative foraging locations outside of plots (i.e. opportunity costs) appeared to be near or below a critical food density (i.e. 119–181 kg ha^–1). Giving‐up densities varied substantially from a critical food density across a range of food densities in alternative foraging locations suggesting that fixed GUDs should not be used as surrogates for critical food densities in energetic carrying capacity models. Foraging and resting rates in and near experimental foraging patches did not reflect patterns of seed removal and were poor predictors of GUD and foraging habitat quality. Our results demonstrated the usefulness of GUDs as indicators of habitat quality for subsurface, benthic foragers relative to other available foraging patches and suggested that food may be limited for dabbling ducks during spring migration in some years in the midwestern USA.