Prey susceptibilities,prey utilization and variable attack efficiencies of Ural owls

Summary To investigate the factors that influence prey utilization among predators with active prey, three series of experiments were performed in which Ural owls (Strix uralensis) searched for and attacked three prey species of wild mice, Microtus montebelli, Apodemus speciosus, and A. argenteus, in a large flight cage. Over the whole study, owls attacked mice about ten times a night. The number of attacks on each prey species did not differ from that predicted by a random attack model. M. montebelli was taken more than either Apodemus species. Prey utilization appears to be influenced by prey susceptibilities only and it is unlikely that prey selection by the owls affected prey utilization patterns. Under the experimental conditions, random attack is predicted by optimal foraging theory. However, random attack may be explained just as well by the inability of the owl to discriminate prey type. The owls, energy gain was adjusted not by alteration in the number of attacks on a prey species but rather by alteration in the capture success between experiments. Capture success increased in poor food conditions for the same prey species. This flexibility in capture success has not been considered in the assumptions of optimal foraging theory. In conventional optimal foraging theory, the probability of capture success is implicitly assumed as constant and unity. We suggest that this assumption is inadequate to understand the foraging behavior of owls.     

Foraging behavior of Ural owls (Strix uralensis) in a patchy environment: The importance of acquired information

How the allocation of searching investiment by Ural owls (Strix uralensis) in a patchy and fluctuating environment is affected by prior information was experimentally studied. The owls searched among four patches, two with prey and two without prey. During the five days, the positions of two prey patches were randomly assigned daily (random treatment) and for the other five days, they were fixed (fixed treatment). In experiment I, the sequence of treatments was from random to fixed. In experiment II, the sequence of treatments was reversed. The choice of prey present patch was affected by acquired information which owls acquired during one night (short term), a treatment period (medium term), and throughout the experiment run (long term). They predicted the positions of the prey patches, to some extent, in the first choice in one night in the fixed treatment by the experience of previous days. The searching time per visit was also affected by previous information. Owls searched prey patches longer than empty patches. In the random treatment, the giving up time in prey patches was variable, while in the fixed treatment, it was longer and fixed. Although owls could use information on the environment where they had searched to make a decision of foraging behavior, they were not able to choose prey patches accurately. The information which was acquired during the treatment period and throughout the experiment run affected the gain from prey. The changes in gain were not caused by the encounter rate but by the proportion of attack success.     

To dare or not to dare? Risk management by owls in a predator–prey foraging game

In a foraging game, predators must catch elusive prey while avoiding injury. Predators manage their hunting success with behavioral tools such as habitat selection, time allocation, and perhaps daring—the willingness to risk injury to increase hunting success. A predator’s level of daring should be state dependent: the hungrier it is, the more it should be willing to risk injury to better capture prey. We ask, in a foraging game, will a hungry predator be more willing to risk injury while hunting? We performed an experiment in an outdoor vivarium in which barn owls (Tyto alba) were allowed to hunt Allenby’s gerbils (Gerbillus andersoni allenbyi) from a choice of safe and risky patches. Owls were either well fed or hungry, representing the high and low state, respectively. We quantified the owls’ patch use behavior. We predicted that hungry owls would be more daring and allocate more time to the risky patches. Owls preferred to hunt in the safe patches. This indicates that owls manage risk of injury by avoiding the risky patches. Hungry owls doubled their attacks on gerbils, but directed the added effort mostly toward the safe patch and the safer, open areas in the risky patch. Thus, owls dared by performing a risky action—the attack maneuver—more times, but only in the safest places—the open areas. We conclude that daring can be used to manage risk of injury and owls implement it strategically, in ways we did not foresee, to minimize risk of injury while maximizing hunting success.     

Relative effects of climate change, isolation and competition on body-size evolution in the Japanese field mouse, Apodemus argenteus

Aim This paper examines body size variation in both recent and Quaternary populations of the Japanese field mouse Apodemus argenteus in order to assess the relative effects on body size of climate change, isolation and competitive interactions with its congeneric A. speciosus. Both temporal (since the Last Glacial Maximum, LGM) and spatial (over the Japanese archipelago) scales are considered. Location The small field mouse is widespread in Japan, and the specimens examined were collected from 10 localities on islands of widely differing area (from 4 km² to 230,510 km²) and at latitudes ranging from 30.3° N to 45.1° N. Methods The effects of geographical factors such as latitude and island area on the size variation of A. argenteus were investigated, using the lower incisor size. In addition, the size of some specimens from two Quaternary localities was compared with the size of the extant specimens. Evolutionary rates of size change since the LGM were calculated in darwins. Hutchinson size ratios were used to examine the pattern of variation of the size segregation between the two Japanese field mice, A. argenteus and A. speciosus, in relation to time and space. Results There was a negative relationship between size and latitude among living A. argenteus populations. In addition, there was no effect of island area on body size, especially at higher latitudes. At lower latitudes, A. argenteus were larger on smaller islands, although this trend was not statistically significant. Quaternary specimens of A. argenteus were smaller in size than their living representatives. The interspecific size ratio between the two Japanese Apodemus was larger on smaller islands and at higher latitudes, and there has been a decrease in the size ratio between the two Apodemus since the LGM. Lastly, in accordance with the theory of character displacement, the small A. argenteus was larger in allopatry than in sympatry, whereas the large A. speciosus was smaller in allopatry than in sympatry. Main conclusions These results indicate that A. argenteus does not conform to Bergmann's rule or to the island rule. The variation in size for the small Japanese field mouse at both spatial and temporal scales may be related to climate change, with an additional effect of competition with the large field mouse, especially on smaller islands. The size convergence between the two Japanese Apodemus observed over the last 21,000 years may be explained by the diminution of available food resources due to the reduction of land mass areas following the LGM. It may also be the result of an evolution towards an optimal body size; a hypothesis previously proposed to explain the evolution of body size in island mammals. Lastly, the evolutionary rates of body size calculated for A. argenteus since the LGM are typical of rates calculated for other Quaternary mainland mammals, thus suggesting that the evolution in this species was not particularly rapid, as is often thought for island mammals.     

Seasonal dynamic shifts in patch exploitation by parasitic wasps

We developed and tested predictions of a dynamic life historymodel that is concerned with how temperate-zone parasitic waspsadjust patch residence time and tendency to superparasitizewhen expectation of life and habitat quality varies. The theorypredicts that wasps with short life expectancy should continueto search longer and superparasitize more frequently than similarwasps with long life expectancy. Similarly, wasps with longlife expectancy that forage in habitats where patches are alreadyheavily exploited should continue to search longer and superparasitizemore frequently than similar wasps foraging in habitats wherepatches are relatively unexploited. In contrast, the theorypredicts that wasps with short life expectancy will be insensitiveto habitat quality. We tested the predictions on Drosophilaparasitoids (Lep-topilina heterotoma) by (1) rearing wasps underfall and summer photoperiod (i.e., short versus long life expectancy)and (2) giving wasps foraging experience on different qualitypatches (i.e., exploited versus unexploited habitats). Resultsof the experiments corroborated our predictions. We discusshow parasitic wasp behavior can be shaped by globally predictableand locally unpredictable events.     

Home range and mating system of two sympatric field mouse species,Apodemus speciosus andApodemus argenteus

The size and spatial distribution of home ranges in two sympatric field mouse species,Apodemus speciosus andApodemus argenteus, were revealed by the capture-recapture method in a temperate deciduous forest from June 1987 to June 1990. InA. speciosus, the home ranges of males were significantly larger tha those of females, and overlapped with those of other males and females during the breeding season. InA. argenteus, the home range of each male overlapped only that of a single female throughout the year. These results suggested thatA. speciosus was promiscuous or polygynous andA. argenteus was monogamous.     

Auditory prey location in a pause--travel predator: search height, search time, and attack range of Tengmalm's owls (Aegolius funereus)

We studied the nocturnal hunting and diurnal roosting behaviorof 17 radio-equipped Tengmalm's owls (Aegolius funereus), 12males and 5 females, in coniferous forest during their nestingseason. The owls perched lower when hunting than when roosting,probably because hunting perches were selected to minimize thepredator-prey distance or to obtain unobstructed access to theground-dwelling small mammal prey, whereas roosting percheswere selected to minimize the probability of being detectedby an avian predator. There was no difference between perchingheights associated with giving up and prey attack, nor werethere any differences between perching heights, perching times,and attack distances associated with successful and unsuccessfulattacks. There were no sexual differences in perching heightduring hunting or roosting. However, giving-up times tendedto be longer for females than for their mates, which is expectedbecause females are larger than males, and the relative costof flight increases with body mass. The instantaneous attackrate was independent of perching time. The owls gave up theirperches at a constant rate and independently of the amount oftime already spent on the perch in an exponentially decayingpattern. The owls perched longer, however, before launchingan attack than before giving up, probably in order to observedetected prey until the right moment for an attack. Attack distancewas independent of both perching height and perching time. Perchingtime was inversely related to perching height, which fits thetheoretical expectation that the search area will decrease withincreasing height in birds that locate prey auditorily.     

Search mechanism of a stream grazer in patchy environments: the role of food abundance

Summary The search behavior of the grazing stream insect Baetis tricaudatus (Ephemeroptera: Baetidae) was examined in field and laboratory experiments. Regardless of food abundance in experimental habitats, nymphs spent significantly more time in food patches than predicted if they had moved randomly with respect to patches. A significant reduction in movement rate within patches relative to movement rate between patches largely accounted for these results. The movement pattern within patches was highly systematic and in agreement with predictions of optimal foraging theory since food was uniformly distributed within patches. Between-patch search movements were affected by food abundance in the most recently grazed patch. Search intensity after departure from a patch was positively related to food abundance in the patch while movement rate after patch departure was inversely related to patch food level. These effects produced between-patch movement patterns that were suboptimal in the experimental habitats because they resulted in revisitation of previously depleted patches. However, differences between experimental and natural habitats in the spatial occurrence of patch types suggest that Baetis between-patch search behavior may be adaptive in natural habitats.     

Hoarding behaviors of two wood mouse species: Different preference for acorns of two Fagaceae species

Two species of wood mouse, Apodemus argenteus and A. speciosus, were observed consuming and hoarding acorns of Quercus serrata and Castanopsis cuspidata. When each species of acorn was supplied individually, both species of mice used each species of acorn for eating and hoarding. When both species of acorn were supplied, A. argenteus consumed or hoarded only C. cuspidata, whereas A. speciosus tended to eat C. cuspidata acorns at the feeding site, and disperse or hoard Q. serrata acorns. Apodemus speciosus is unlikely to disperse C. cuspidata acorns (their utilization was biased towards consumption) when Q. serrata acorns are also available. Apodemus argenteus will make almost no contribution to the dispersal of Q. serrata when the two acorn species coexist.     

Role of interspecific competition in the coexistence ofApodemus argenteus andA. speciosus (Rodentia: Muridae)

Reciprocal-removal experiments with two replicates were conducted to test for the role of interspecific competition in the coexistence ofApodemus argenteus andA. speciosus. Population density, rate of appearance of new (unmarked) individuals, reproduction, survival rate and habitat use were monitored during pre- and removal periods. In both removal experiments, the removal ofA. argenteus had little effect onA. speciosus, while that ofA. speciosus affected several population characteristics ofA. argenteus. Namely, the removal ofA. speciosus shifted the distribution ofA. argenteus to the habitat with a denser shrub cover in one experiment. Also, the removal increased the population densities and appearance rates of new individuals ofA. argenteus in another experiment. Interspecific interactions between the two species appeared to be a one-way action fromA. speciosus toA. argenteus. In removal periods in both experiments, the rates of appearance of new individuals in each species were the highest on the grid where that species was removed. These results suggest that, though interspecific competition occurred between the two species, intraspecific competition had greater effects than interspecific competition on the abundance and habitat use ofA. argenteus andA. speciosus. This implies that the fundamental niches ofA. argenteus andA. speciosus differ potentially, which may play an important role in the coexistence of the two species.     

Host frass as arrestant chemicals in locating host Mythimna separata by the tachinid fly Exorista japonica

We investigated the response of the tachinid fly, Exorista japonica (Townsend), to host frass or its extracts in order to clarify the host location mechanisms of female flies in a potential host habitat. Host searching time in a patch and the number of patch visits were analyzed by using a frass-containing patch which was excreted by host larvae, Mythimna separata (Walker) (Lepidoptera: Noctuidae), and patches to which host frass extracts were applied. E. japonica females were arrested in response to the host-frass-containing patch after contacting the frass with their front tarsi, thereby spending most of the time to search the patch and to revisit the host-frass patch. While host-searching time in the patch by the females was longest at their first visit of a patch with host frass, searching time decreased with successive visits. The female flies also exhibited area-restricted searching with methanol extracts of the host frass. Area-restricted searching activity increased with the concentration of host-frass extract, i.e., total searching time in the patch and the number of patch visits varied in a dose-dependent manner. E. japonica females likely employ chemicals in host frass as arrestants in host location.     

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.     

Sex differences in foraging behaviour and oviposition site preference in an insect predator, Orius sauteri

The effects of patch quality on the foraging behaviour of an anthocorid predator Orius sauteri (Poppius) were compared between sexes. Prior experience in patches was also studied to determine whether this was a factor affecting oviposition decisions. Patch quality affected patch residence time differently for the two sexes; females stayed much longer in a patch with prey (60 Thrips palmi larvae) than a patch without prey, while males did not remain in any patch for extended periods. Most of the females remained in or moved to patches with prey, whereas males dispersed, irrespective of patch quality. Both females released in patches with prey and females released in patches without prey deposited more eggs per hour in patches with prey than in patches without prey. Females released in patches without prey laid eggs in patches with prey at higher rates than did females released in patches with prey. Causes for the sex difference in patch residence time and allocation are discussed in relation to optimal foraging theory. The significance of selective oviposition and the role of experience in oviposition decisions within heterogeneous environments are also discussed.     

Central Die-back of Monoclonal Stands of Reynoutria japonica in an Early Stage of Primary Succession on Mount Fuji

Reynoutria japonica is a common perennial pioneer species onJapanese volcanoes. In a volcanic desert (1500m above sea level)on Mount Fuji (3776m), central Japan, this species forms circularstands (patches). As a patch develops, shoot density decreasesin its centre (‘central die-back’). The centraldie-back has been considered a key process in the early stagesof primary succession, though its mechanism has been unknown. The pattern of patch development, population dynamics of aerialshoots, and growth patterns of below-ground organs were analysedin order to investigate the mechanism of die-back, and the followingtraits are clarified: (1) central die-back areas occur in mostsmall patches (approx. 1m²) without later successional species;(2) shoot characteristics are dependent both on their positionwithin a patch and on patch size; (3) despite the large differencesin shoot density, neither time course of shoot growth nor theirmortality differs between the centre and periphery of patches;and (4) rhizomes ofR. japonicagrow outwards with regular sympodialbranching. From these results, it is concluded that neither interspecificnor intraspecific competition is likely to be a primary causeof the die-back phenomenon, but that central die-back is broughtabout intrinsically by the growth pattern of the rhizome systems.We also discuss the importance of the central die-back in facilitatingestablishment of later successional species in the early stagesof primary succession. Clonal plant; central die-back; competition; facilitation; Japanese knotweed; Mount Fuji; primary succession;Reynoutria japonica ; rhizome growth; volcanic desert     

Habitat assessment by parasitoids: mechanisms for patch use behavior

Animals foraging for patchily distributed resources may optimizetheir foraging decisions concerning the patches they encounter,provided that they base these decisions on reliable informationabout the profitability of the habitat as a whole. Females ofthe parasitoid Lysiphlebus testaceipes exploit aphid hosts,which typically aggregate in discrete colonies. We show herehow between-colony travel time and the number of aphids in previouslyvisited colonies affect parasitoid foraging behavior. We firstassumed that parasitoids use travel time and previous colonysize to estimate a mean rate of fitness gain in the habitatand derived quantitative predictions concerning the effect ofthese two variables on patch residence time and patch-leavingrate of attack. We then tested these theoretical predictionsin laboratory experiments in which female parasitoids were allowedto visit two successive colonies. As predicted, the observedresidence time in the second colony increased with increasingtravel time and decreasing size of the first colony. Patch-leavingrate of attack decreased with increasing travel time but wasnot affected by previous colony size. These results suggestthat parasitoids use these two variables to assess habitat quality.However, discrepancies between the data obtained and quantitativepredictions show that the effect of travel time on patch usemay be more complex than assumed in our model.     

Knowing your habitat: linking patch-encounter rate and patch exploitation in parasitoids

According to optimal foraging theory, animals should decidewhether or not to leave a resource patch by comparing the currentprofitability of the patch with the expected profitability ofsearching elsewhere in the habitat. Although there is abundantevidence in the literature that foragers in general are wellable to estimate the value of a single resource patch, theirdecision making has rarely been investigated with respect tohabitat quality. This is especially true for invertebrates.We have conducted experiments to test whether parasitic waspsadjust patch residence time and exploitation in relation tothe abundance of patches within the environment. We used thebraconid Asobara tabida, a parasitoid of Drosophila larvae,as our model species. Our experiments show that these waspsreduce both the residence time and the degree of patch exploitationwhen patches become abundant in their environment, as predictedby optimal foraging models. Based upon a detailed analysis ofwasp foraging behavior, we discuss proximate mechanisms thatmight lead to the observed response. We suggest that parasitoidsuse a mechanism of sensitization and desensitization to chemicalsassociated with hosts and patches, in order to respond adaptivelyto the abundance of patches within their environment.     

The effect of patch size and competitor number on aggression among foraging house sparrows

We examined the effect of patch size and competitor number onaggression among house sparrows, Passer domesticus, foragingat patches of seven different sizes in a doubling series (0.014,0.029, 0.058, 0.116, 0.230, 0.462, and 0.922 m²). Contrary toour expectations, the birds did not defend an entire patch,even when it was small as 0.014 m². The frequency of aggressionamong the birds decreased gradually with increasing patch size,in contrast to the step decline predicted by resource defensetheory. Moreover, the birds fought more frequently and moreintensely as competitor density increased. Both results areconsistent with the predictions of a modified hawk-dove modelfor shared patches. Females were more aggressive and fed ata higher rate than did males. The proportion of females increasedas patch size decreased, and aggression became more frequentand intense. Even when patches are shared, patch size has animportant effect on the frequency and intensity of foragingcompetition and the size and composition of foraging groups.     

Habitat displacement effect between two competing owl species in fragmented forests

Many owl species use the same nesting and food resources, which causes strong interspecific competition and spatio-temporal niche separation. We made use of a recent colonisation of Ural Owls (Strix uralensis) in southern Poland to compare habitat preferences of Tawny Owls (Strix aluco) allopatry and sympatry with Ural Owls. We investigated spatial niche segregation of Ural Owl and the Tawny Owl in sympatry and compared habitat preferences of Tawny Owls breeding in allopatry and sympatry. Tawny Owls breeding in sympatry with Ural Owls occupied forests with higher canopy compactness, sites located closer to forest border and to built-up areas, as well as stands with a higher share of fir and spruce and a lower share of beech as compared to sites occupied by Ural Owls. Allopatric Tawny Owls occupied sites with lower canopy compactness and bred at sites located further from forest borders and in stands with lower share of fir and spruce and a higher share of deciduous as compared to sympatric Tawny Owls. As Ural owls are dominant in relation to Tawny Owls, this indicates that the presence of Ural Owls prevents Tawny Owls from occupying deciduous-dominated and old stands located in forest interior areas, far from buildings and forest edges. The results support habitat displacement between the two species when breeding in sympatry. We also show that protection of large forest patches is crucial for the Ural Owl, a species still rare in central Europe, while small patches are occupied by the abundant Tawny Owl.     

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.