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

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

Avian prey-dropping behavior. I. The effects of prey characteristics and prey loss

Numerous species of birds break hard-shelled prey items by droppingthemfrom a height. This intriguing prey-extraction method providesan excellentopportunity for studying foraging behavior becausea single, easily measurablequantity—height of drop—maybe influenced by a wide variety ofidentifiable characteristicsof the prey (e.g., breakability, weight) andsocial environment(e.g., alone or in the presence of kleptoparasites). Usingadynamic, state variable modeling approach, this paper presentsthe firsttheoretical framework for avian prey-dropping systemsthat incorporates thediversity of prey characteristics andsocial situations. The model yielded aseries of qualitativepredictions about prey-dropping behavior that can betestedreadily in any prey-dropping system. In particular, the resultsindicatedthat quantitative and qualitative differences in item breakabilityandpotential kleptoparasitism should have a significant effecton the heightand pattern of prey dropping.     

Do foraging methods in winter affect morphology during growth in juvenile snow geese?

Physical exertion during growth can affect ultimate size and density of skeletal structures. Such changes from different exercise regimes may explain morphological differences between groups, such as those exhibited by lesser snow geese (Chen caerulescens caerulescens; hereafter snow geese) foraging in southwest Louisiana. In rice‐prairie habitats (hereafter rice‐prairies), snow geese bite off or graze aboveground vegetation, whereas they dig or grub for subterranean plant parts in adjacent coastal marshes. Grubbing involves considerably more muscular exertion than does grazing. Thus, we hypothesized that rates of bone formation and growth would be lower for juveniles wintering in rice‐prairies than those in coastal marshes, resulting in smaller bill and skull features at adulthood. First, we tested this exertion hypothesis by measuring bills, skulls, and associated musculature from arrival to departure (November–February) in both habitats in southwest Louisiana, using both banded birds and collected specimens. Second, we used the morphological data to test an alternative hypothesis, which states that smaller bill dimensions in rice‐prairies evolved because of hybridization with Ross's geese (C. rossii). Under the exertion hypothesis, we predicted that bill and skull bones of juveniles would grow at different rates between habitats. However, we found that bill and skull bones of juveniles grew similarly between habitats, thus failing to support the exertion hypothesis. Morphometrics were more likely to differ by sex or change with sampling date than to differ by habitat. We predicted that significant, consistent skewness toward smaller birds could indicate hybridization with Ross's geese, but no skewness was observed in our morphological data, which fails to support the hybridization hypothesis. Further research is needed to clarify whether snow geese wintering in Louisiana represent a single polymorphic population that segregates into individually preferred habitats, which we believe at present to be more likely as an explanation than two ecologically and spatially distinct morphotypes.     

Prey abundance and the strength of interference in a foraging shorebird

Interference is an important component of food competition but is often difficult to detect and measure in natural animal populations. Although interference has been shown to occur between oystercatchers Haematopus ostralegus L. feeding on mussels Mytilus edulis L., four previous studies have not detected interference between oystercatchers feeding on cockles Cerastoderma edule L. In contrast, this study detected interference between cockle-feeding oystercatchers in the Baie de Somme, France. Prey stealing (kleptoparasitism), one of the main causes of interference between mussel-feeders, also occurred between oystercatchers in the Baie de Somme. The kleptoparasitism rate was related to the natural variation in the food supply, tending to be higher when cockles were rare. Feeding rate was negatively related to competitor density, so providing evidence for interference, but, as in mussel-feeders, only above a threshold density of about 50–100 birds ha⁻¹. The strength of interference at a fixed competitor density was related to the cockle food supply, usually being greater when cockles were rare. Previous studies probably failed to detect interference between cockle-feeders because competitor densities were too low, or cockles were too abundant, or because they were not conducted during late winter when interference is most intense. The study shows that natural variation in the food supply can influence the strength of interference within an animal population and provides support for those behaviour-based interference models which predict that the strength of interference will be greatest when competitor densities are high and prey scarce.     

Foraging among cannibals and kleptoparasites: effects of prey size on pike behavior

The northern pike (Esox lucius) is an important and selective piscivorethat chooses smaller prey than predicted from energy / timebudgets. In a laboratory experiment, we investigated pike predatorybehavior to explain this selectivity. Northern pike feedingon different prey sizes in aquaria were observed when foragingalone, when in the presence of chemical cues from similar-sizedor larger conspecifics, and when in the presence of conspecifics thatwere allowed to interact with the focal pike. The results showthat prey handling time increases with prey size and that theduration of manipulating and handling prey inflicts a risk ofexposure to cannibals and kleptoparasites on the pike. Therefore,the risk of falling victim to cannibals or kleptoparasites increaseswith prey size. Attracting and experiencing intraspecific interactorscan be regarded as major fitness costs. Chemical cues from foragingconspecifics have only minor effects on pike foraging behavior.Furthermore, the ability to strike and swallow prey head first improvespike predatory performance because failing to do so increases handlingtime. Our findings emphasize the increasing potential costswith large prey and explain previous contradictory suggestionson the underlying mechanisms of behavior, selectivity, and trophiceffects of northern pike predation.     

Kleptoparasitism as an explanation for paradoxical oviposition decisions of the parasitoid Asobara tabida

It is known that the braconid Asobara tabida, a parasitoid of Drosophila larvae, takes oviposition decisions in accordance with survival probabilities in several host species. Nevertheless, larvae of D. simulans, in which the survival probability is virtually zero, are readily accepted for oviposition by searching females. This even happens when they are offered together with D. melanogaster larvae, in which the parasitoid can develop. Here I show that A. tabida can act as a kleptoparasitoid in D. simulans larvae: it can develop in D. simulans larvae, once these larvae are parasitized by another parasitoid, the eucoilid Leptopilina boulardi. Analysis with an optimal foraging model suggests that the increase in survival probability and the occurrence of L. boulardi in the field are high enough to make this kleptoparasitic behaviour of A. tabida pay.     

The adaptive significance of stealing in a marine bird and its relationship to parental quality

Kleptoparasitism (food theft) is a tactic used opportunisticallyby many foraging birds, but little is known about its fitnessbenefits. Here we show that habitual kleptoparasitism by individualparent roseate terns (Sterna dougallii) is associated with consistentlysuperior reproductive performance relative to nonkleptoparasitic("honest") parents, as measured by growth and survival to fledgingamong their offspring. In broods of two, both chicks of kleptoparasiticparents exhibited superior growth performance during the middleand later stages of the rearing period, relative to chicks ofhonest parents. This difference was especially pronounced insecond-hatched chicks, whose survival is highly variable amongyears and dependent on food availability. Over a 10-year period,average productivity (number of chicks fledged per pair) wassignificantly higher among kleptoparasites than among honestparents, with a larger relative difference during years of foodshortage. Our study indicates that kleptoparasitism in roseateterns is an important component of parental quality and providesthe first evidence that food stealing is associated with enhancedfitness in a facultatively kleptoparasitic seabird.     

Wild Carib grackles play a producer scrounger game

Producer–scrounger (PS) game-theoretical foraging modelsmake predictions about the decision of group-feeding animalseither to look for food (produce) or for opportunities to exploitthe discoveries of other foragers (scrounge). We report themost complete demonstration to date of the applicability ofthe PS foraging game in a free-living animal, the Carib grackle(Quiscalus lugubris) of Barbados. As assumed by PS games, thepayoffs obtained by scroungers were negatively frequency dependent.Experimentally, increasing the cost of scrounging led to a decreasein the observed proportion of scroungers, whereas raising thecost of producing increased the proportion of scroungers. Observationsof marked birds revealed that group-level changes could be broughtabout by individual flexibility in tactic use. Despite consistentindividual differences in tactic use, most birds used both tacticsand could alter their use of producing and scrounging when conditionschanged. We found no difference in the payoffs obtained by producersand scroungers, suggesting a symmetrical game equilibrium. Ourresults call for testing the PS foraging game in a broader rangeof biological systems that include different types of scroungingbehavior (e.g., scramble, stealthful, or aggressive scrounging)as well as the exploitation of different phases of food production(e.g., searching, handling).     

Avian prey-dropping behavior. II. American crows and walnuts

Complex and energetically expensive foraging tasks should beshaped bynatural selection to be efficient. Many species ofbirds open hard-shelledprey by dropping the prey repeatedlyonto the ground from considerableheights. Urban-dwelling Americancrows (Corvus brachyrhynchos) foragein this way on two speciesof walnuts in central California, USA. As predictedfrom a theoreticalmodel, crows dropped nuts with harder shells from greaterheightsand dropped them from greater heights when over softer substrates.Theheight selected for dropping nuts decreased in the presenceof numerous nearbyconspecifics, indicating that crows weresensitive to the risk ofkleptoparasitism when selecting dropheights. Drop height decreased withrepeated drops of the samewalnut, suggesting that crows adjusted for theincreasing likelihoodthat a repeatedly-dropped nut would break on subsequentdrops.Crows did not alter height of drop in accordance with differencesinthe mass of the prey. When faced with multiple prey typesand droppingsubstrates, and high rates of attempted kleptoparasitism,crows adjusted theheight from which they dropped nuts in waysthat decreased the likelihood ofkleptoparasitism and increasedthe energy obtained from each nut.     

Wide home ranges for widely foraging lizards

Space usage by animals may be influenced by a range of factors. In this study we investigate whether foraging behaviour affects the home range size of lizards. Two distinct tactics of foraging have been recognized in predators: sit-and-wait foraging (SW) and active foraging (AF). Foraging activity level of a data set of lizard species, mainly compiled from literature, is compared with their home range sizes. Two opposite predictions can be made about foraging in connection with home range area: on the one hand, SW species may exhibit larger home ranges due to their mating system; on the other hand, AF species have higher metabolic energy and thus food requirements and can be expected to have larger home ranges that have to yield this food. This study shows that percentage of the time moving (as an index of foraging mode) correlates positively with home range, even after correcting for body mass, and these patterns remain when phylogenetic relationships are taken into account. We thus conclude that home range areas parallel activity levels in lizards.     

贵州草海越冬黑颈鹤觅食栖息地选择的初步研究

本文报道了利用Friedman非参数统计方法,研究越冬黑颈鹤(Grus nigricollis)在贵州草海对其觅食栖息地选择的结果。在草海的黑颈鹤越冬觅食栖息地可分为5种——莎草草甸、浅水沼泽、草地、玉米地和蔬菜地。黑颈鹤对莎草草甸的选择性最高,对玉米地的选择性最差,对其余类型栖息地的选择随地点的不同而不同。草海的人为活动很多,是影响黑颈鹤利用和选择栖息地的一个重要因素。本文也对改善黑颈鹤越冬栖息地的管理和保护提出了一些建议。     

Culicidae (Diptera) selection of humans,chickens and rabbits in three different environments in the province of Chaco, Argentina

Studies were conducted to determine the selection of humans, chickens and rabbits by Culicidae in three different environments in the province of Chaco, Argentina. Mosquitoes were collected fortnightly using cylindrical metal traps containing animal bait (chickens and rabbits). The mosquitoes were collected between June 2001-May 2002. During the same period and with the same frequency, mosquitoes biting the human operators of the traps were collected during the first 15 min of exposure within different time intervals: from 09:00 am-11:00 am, 01:00 pm-03:00 pm, 05:00 pm-07:00 pm and 09:00 pm-10:00 pm. A total of 19,430 mosquitoes of 49 species belonging to 10 genera were collected. Culex species mainly selected chicken bait and Wyeomyia species selected rabbit bait. Ochlerotatus and Psorophora species were more abundant in rabbit-baited traps. Anopheles triannulatus, Coquillettidia nigricans, Ochlerotatus scapularis, Mansonia titillans and Psorophora albigenu showed a strong attraction for human bait. The Anopheles, Coquillettidia, Culex and Mansonia species were more active between 05:00 pm-09:00 pm, while Ochlerotatus, Psorophora, Haemagogus and Wyeomyia were most active from 09:00 am-07:00 pm. This study provides additional information about the biology and ecology of arbovirus vectors in Chaco.