Estimating piscine prey size from partial remains: testing for shifts in foraging mode by juvenile bluefish

Knowledge of prey sizes consumed by a predator aids in the estimation of predation impact. Young-of-the-year bluefish, Pomatomus saltatrix, attack their prey tail-first and often bite their prey in half; this poses a unique problem in determining prey sizes from stomach content analysis. We developed a series of linear regressions to estimate original prey lengths from measurements of eye diameter and caudal peduncle depth for striped bass, Morone saxatilis, bay anchovy, Anchoa mitchilli, American shad, Alosa sapidissima, blueback herring, Alosa aestivalis, Atlantic silverside, Menidia menidia, and white perch, Morone americana. We then used these regressions to estimate original prey sizes from pieces of prey found in stomachs of bluefish collected in the Hudson River estuary from 1990–1993. Lengths of prey that were swallowed whole were compared to estimated lengths of prey that were consumed in pieces. Lengths of prey that were consumed in pieces were larger than prey that were consumed whole. We determined the prey length/predator length ratio at which bluefish began shifting from swallowing their prey whole to partial consumption. Shifting occurred at a ratio of approximately 0.35 irrespective of prey species, suggesting that prey length plays an important role in predator foraging decisions and may contribute to gape limitations. Shifts in foraging mode effectively reduce gape limitation and allow bluefish to consume larger prey sizes which may increase their effect on prey populations.     

Implications of a fluctuating fish predator guild on behavior, distribution, and abundance of a shared prey species: the grass shrimp Palaemonetes pugio

In some systems, the identity of a prey species' dominant predator(s) may not be constant over time. In cases in which a prey species exhibits different responses to various predator species, such changes in predator identity may have population-wide consequences. Our goals were to determine (1) whether mortality of and refuge use by the grass shrimp, Palaemonetes pugio, were predator-specific, and (2) how effects of prey size and habitat interacted with predator type. Striped bass (Morone saxatilis) exerted twice as much predation pressure as mummichog (Fundulus heteroclitus), although not equally as great on large (female) and small (male) shrimp. Mummichog, which fed preferentially on large shrimp, forced a partitioning of habitat between the two shrimp size classes. In contrast, large and small shrimp occupied similar habitats when subjected to striped bass, which fed on both size classes equally. Refuge use of grass shrimp depended on predator type. In the presence of mummichog, which occupied shallower depths in the water column than striped bass, shrimp stayed deep and close to structural habitat. Striped bass, which were deeper, caused shrimp to move high in the water column away from structural habitat. When both predators were present, shrimp distribution was similar to that when only striped bass were present, striped bass predation rate was enhanced, and overall mortality was higher than with either predator alone. Results suggest that at times when mummichogs are the dominant predators, large (female) shrimp experience higher predation than small (male) shrimp and are physically separated from their potential mates. When striped bass are more abundant, male and female shrimp may share a similar, shallow, less structure-oriented distribution and be subjected to higher mortality. When both predators are present, mortality rates may be higher still. This predator-, size-, and habitat-specificity of grass shrimp behavior suggests significant population and distribution consequences of fluctuating predator guilds and fluctuating cover of structural habitats in the field.     

Role partitioning of swimming musculature of striped Bass Morone saxatilis Walbaum and Bluefish, Pomatomus saltatrix L.

Electromyography was used to determine the functional roles of the axial musculature in striped bass and bluefish at imposed swimming velocities. The lateral red muscle powers propulsive movements at all sustainable swimming speeds in both species. The amplitude and frequency of EMG's from the red muscle grade with increasing swimming velocity. The white muscle, forming the main mass of the myotome, is reserved for high-speed burst swimming above maximum sustainable speeds. The proportion of the myotome occupied by the red muscle at the level of the caudal peduncle is 10.9% and 18.6% for the striped bass and the bluefish respectively.     

Effects of water clarity and other environmental factors on trophic niches of two sympatric piscivores

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Effects of alternative prey on cannibalism in age-1 bluefish

Age-1 bluefish Pomatomus saltatrix were offered both age-0 bluefish and age-1 Atlantic silversides Menidia menidia in the following proportions: 0:1·0, 0·25:0·75, 0·5:0·5, 0·75:0·25, 1·0:0. Atlantic silversides were consumed in greater proportion than offered throughout all treatments suggesting strong selection for silversides. There was no evidence of prey-switching. The behavioural processes leading to age-1 bluefish selection of Atlantic silversides was investigated. The proportion of attacks on each prey species was distributed randomly, or slightly higher on age-0 bluefish; however, capture success was greater on silversides than on age-0 bluefish prey. Therefore, age-1 bluefish did not actively select for silversides and the observed selection pattern is a result of passive processes. These findings suggest that prey switching between age-0 bluefish and an alternative prey probably does not act as a stabilizing mechanism in the bluefish population and confirm the importance of capture success in determining piscivore selectivity patterns.     

Vulnerability of age-0 pallid sturgeon Scaphirhynchus albus to predation; effects of predator type,turbidity, body size,and prey density

Predation can play an important role in the recruitment dynamics of fishes with intensity regulated by behavioral (i.e., prey selectivity) and/or environmental conditions that may be especially important for rare or endangered fishes. We conducted laboratory experiments to quantify prey selection and capture efficiency by three predators employing distinct foraging strategies: pelagic piscivore (walleye Sander vitreus); benthic piscivore (flathead catfish Pylodictis olivaris) and generalist predator (smallmouth bass Micropterus dolomieu) foraging on two size classes of age-0 pallid sturgeon: large (75–100 mm fork length [FL]) and small (40–50 mm FL). Experiments at high (> 70 nephalometric turbidity units [NTU]) and low (< 5 NTU) turbidity for each predator were conducted with high and low densities of pallid sturgeon and contrasting densities of an alternative prey, fathead minnow Pimephales promelas. Predator behaviors (strikes, captures, and consumed prey) were also quantified for each prey type. Walleye and smallmouth bass negatively selected pallid sturgeon (Chesson’s α?=?0.04–0.1) across all treatments, indicating low relative vulnerability to predation. Relative vulnerability to predation by flathead catfish was moderate for small pallid sturgeon (α?=?0.44, neutral selection), but low for large pallid sturgeon (α?=?0.11, negative selection). Turbidity (up to 100 NTU) did not affect pallid sturgeon vulnerability, even at low density of alternative prey. Age-0 pallid sturgeon were easily captured by all predators, but were rarely consumed, suggesting mechanisms other than predator capture efficiency govern sturgeon predation vulnerability.     

Spatiotemporal patterns and habitat associations of smallmouth bass (Micropterus dolomieu) invading salmon‐rearing habitat

1. Smallmouth bass (Micropterus dolomieu) have been widely introduced to fresh waters throughout the world to promote recreational fishing opportunities. In the Pacific Northwest (U.S.A.), upstream range expansions of predatory bass, especially into subyearling salmon‐rearing grounds, are of increasing conservation concern, yet have received little scientific inquiry. Understanding the habitat characteristics that influence bass distribution and the timing and extent of bass and salmon overlap will facilitate the development of management strategies that mitigate potential ecological impacts of bass. 2. We employed a spatially continuous sampling design to determine the extent of bass and subyearling Chinook salmon (Oncorhynchus tshawytscha) sympatry in the North Fork John Day River (NFJDR), a free‐flowing river system in the Columbia River Basin that contains an upstream expanding population of non‐native bass. Extensive (i.e. 53 km) surveys were conducted over 2 years and during an early and late summer period of each year, because these seasons provide a strong contrast in the river’s water temperature and flow condition. Classification and regression trees were applied to determine the primary habitat correlates of bass abundance at reach and channel‐unit scales. 3. Our study revealed that bass seasonally occupy up to 22% of the length of the mainstem NFJDR where subyearling Chinook salmon occur, and the primary period of sympatry between these species was in the early summer and not during peak water temperatures in late summer. Where these species co‐occurred, bass occupied 60–76% of channel units used by subyearling Chinook salmon in the early summer and 28–46% of the channel units they occupied in the late summer. Because these rearing salmon were well below the gape limitation of bass, this overlap could result in either direct predation or sublethal effects of bass on subyearling Chinook salmon. The upstream extent of bass increased 10–23 km (2009 and 2010, respectively) as stream temperatures seasonally warmed, but subyearling Chinook salmon were also found farther upstream during this time. 4. Our multiscale analysis suggests that bass were selecting habitat based on antecedent thermal history at a broad scale, and if satisfactory temperature conditions were met, mesoscale habitat features (i.e. channel‐unit type and depth) played an additional role in determining bass abundance. The upstream extent of bass in the late summer corresponded to a high‐gradient geomorphic discontinuity in the NFJDR, which probably hindered further upstream movements of bass. The habitat determinants and upstream extent of bass were largely consistent across years, despite marked differences in the magnitude and timing of spring peak flows prior to bass spawning. 5. The overriding influence of water temperature on smallmouth bass distribution suggests that managers may be able limit future upstream range expansions of bass into salmon‐rearing habitat by concentrating on restoration activities that mitigate climate‐ or land‐use‐related stream warming. These management activities could be prioritised to capitalise on survival bottlenecks in the life history of bass and spatially focused on landscape knick points such as high‐gradient discontinuities to discourage further upstream movements of bass.     

Diet composition in San Francisco Estuary striped bass: does trophic adaptability have its limits?

Trophic adaptability is a term used to describe feeding flexibility in fishes. Though a useful conceptual starting point, fishes often face constraints on their ability to switch prey that could limit feeding success even when prey switching is observed. We compared striped bass diet compositions summarized from previously published studies in California’s Sacramento-San Joaquin Delta during two time periods (1963–1964 and 2001–2003), which allowed us to evaluate trophic adaptability in San Francisco Estuary striped bass at multiple time scales, ranging from intra-annual to multidecadal. The Delta is the landward region of the San Francisco Estuary; over time between the study periods, the Delta underwent substantial changes in potential prey availability for striped bass. We found evidence for trophic adaptability in San Francisco Estuary (SFE) striped bass at all temporal scales examined. Despite this ability to adapt to changes in prey availability, the relative abundance and carrying capacity of young striped bass have declined. This decline has previously been associated with substantial declines in their dominant historical prey—mysid shrimp. Our results, coupled with these previous findings, indicate that trophic adaptability may have limited usefulness as a conceptual model to predict foraging success when other food web constraints are not considered. We speculate that this is particularly true in highly invaded ecosystems like the San Francisco Estuary because invading species often introduce substantial and permanent changes into food webs, decreasing the likelihood that a predator will find prey assemblages that fully replace historical prey assemblages.     

广西扶绥破碎化生境中黑叶猴的食物种类多样性和季节性重叠

动物使用生境中食物种类的方式对其适应性有重要的影响。为了研究黑叶猴在破碎化栖息地中利用食物的方式,2006年1—12月,采用焦点动物取样法和连续记录技术于每月初连续观察8 d,记录破碎化栖息地中黑叶猴取食的种类和时间,分析其食物多样性和月份间种类的重叠度。结果表明,黑叶猴年均食物多样性指数为3.03(范围1.93—2.62),且多样性指数在各月间没有显著的差异(One-sample K-S Test,P=0.99),说明黑叶猴在各月份中取食的食物种数相似,不同季节利用食物种数的变异程度不大。在所有的食物种类中,仅两种食物青檀Pteroceltis tatarinowii和潺槁树Litsea glutinosa在12个月份都被取食,其他种类则表现了月份间明显的季节性替代。各月份间食物种类的相似性在0.42(3—4月)和0.75(9—10月)之间变化。黑叶猴对破碎化生境中食物种类的利用方式是:首先利用常年均有的食物种类以保证食物来源稳定,同时选择性地利用新出现的种类,使食物种类多样化,满足不同的营养需求。     

Assessing Fishers' Support of Striped Bass Management Strategies

Incorporating the perspectives and insights of stakeholders is an essential component of ecosystem-based fisheries management, such that policy strategies should account for the diverse interests of various groups of anglers to enhance their efficacy. Here we assessed fishing stakeholders’ perceptions on the management of Atlantic striped bass (Morone saxatilis) and receptiveness to potential future regulations using an online survey of recreational and commercial fishers in Massachusetts and Connecticut (USA). Our results indicate that most fishers harbored adequate to positive perceptions of current striped bass management policies when asked to grade their state’s management regime. Yet, subtle differences in perceptions existed between recreational and commercial fishers, as well as across individuals with differing levels of fishing experience, resource dependency, and tournament participation. Recreational fishers in both states were generally supportive or neutral towards potential management actions including slot limits (71%) and mandated circle hooks to reduce mortality of released fish (74%), but less supportive of reduced recreational bag limits (51%). Although commercial anglers were typically less supportive of management changes than their recreational counterparts, the majority were still supportive of slot limits (54%) and mandated use of circle hooks (56%). Our study suggests that both recreational and commercial fishers are generally supportive of additional management strategies aimed at sustaining healthy striped bass populations and agree on a variety of strategies. However, both stakeholder groups were less supportive of harvest reductions, which is the most direct measure of reducing mortality available to fisheries managers. By revealing factors that influence stakeholders’ support or willingness to comply with management strategies, studies such as ours can help managers identify potential stakeholder support for or conflicts that may result from regulation changes.     

Maturation and fecundity of a stock-enhanced population of striped bass in the Savannah River Estuary, U.S.A

The striped bass Morone saxatilis population in the Savannah River (south-eastern U.S.A.) collapsed in the 1980s, and recent eorts to restore the population have resulted in increased catch-per-unit-eort (CPUE) of striped bass in the Savannah River Estuary (SRE). The abundance of eggs and larvae, however, remain well below historic levels. The primary cause of the population decline was remedied, and environmental conditions seem suitable for striped bass spawning. Regression analysis of data derived from ultrasonic imaging of 31 striped bass resulted in a statistical model that predicted ovary volume well (r²=0.95). The enumeration of oocytes from ovarian tissue samples and the prediction of ovary volume allowed fecundity to be estimated without sacrificing the fish. Oocyte maturation in Savannah River striped bass seemed to progress normally, with oocytes developing to final stages of maturity in larger fish (>750 mm L _T). Additionally, fecundity estimates were comparable to a neighbouring striped bass population. The environmental cues needed to trigger development and release of striped bass oocytes into the SRE appeared to be present. If most of the striped bass females in the SRE are still young (<7 years), the ability to produce large numbers of eggs will be limited. As these young fish mature, egg production probably will increase and the density of striped bass eggs eventually will approach historic levels, provided suitable habitat and water quality are maintained.     

黑龙江省胜山林场冬季驼鹿、马鹿和狍的种间关系

1987—1989年的两个冬季在黑龙江省黑河市胜山林场,对驼鹿、马鹿和狍的食性、采食高度和栖息地进行了研究。驼鹿的主要食物是柳(22.4％)、榛(16.0％)、红松(14.3％)和桦(13.8％);马鹿为杨(36.0％)、桦(22.7％)和红松(17.3％);狍为桦(26.5％)、柳(19.0％)、杨(12.7％)。三种动物采食高度的频次为正态分布。驼鹿的采食高度(标准差)为137.4(±39.0)厘米;马鹿为136.2(±37.0)厘米;狍为84.2(±29.2)厘米。驼鹿和马鹿对红松林的利用程度最高,而狍主要利用沼泽植物和红松林。驼鹿与狍在食物维和栖息地维上的生态位重叠最大,驼鹿与马鹿在采食高度维上的重叠最大。三种动物在食物、采食高度和栖息地三维生态位上的重叠大小的顺序为:驼鹿和马鹿>驼鹿和狍>马鹿和狍。     

Comparative severity of experimentally induced mycobacteriosis in striped bass Morone saxatilis and hybrid tilapia Oreochromis spp

Twenty striped bass Morone saxatilis and 20 hybrid tilapia Oreochromis niloticus x O. mossambicus x O. aureus each received a single intramuscular injection of 1.6 x 10(6) colony forming units per gram body weight of Mycobacterium marinum. Striped bass manifested significantly greater clinical and microscopic disease compared to tilapia. Whereas all the striped bass had died or were clinically ill by Day 8 post-infection, there was no apparent disruption of normal behaviour, physical appearance, or growth in any of the sacrificed or surviving tilapia. Histologically, granulomas in striped bass were generally larger and less discrete, with a higher proportion of heavily vacuolated macrophages, and large cores of necrotic cells. Visceral granulomas in tilapia were smaller, with a higher proportion of epithelioid macrophages, more pigment-containing cells, more peripheral lymphocytes, and virtually no central necrosis. Visceral granulomas were 18-fold more numerous in striped bass than in tilapia. Based upon histomorphometric data, mean proportions of acid-fast bacteria within pronephros granulomas were 4-fold greater in striped bass than tilapia, and striped bass granulomas averaged more than twice as large as tilapia granulomas. In the anterior kidney of striped bass, a positive correlation existed between mean mycobacterial proportions and mean necrosis scores. In tilapia, mean mycobacterial proportions correlated negatively with mean granuloma numbers, whereas there was no correlation between these parameters in striped bass. Results suggest that intrinsic functional differences in the immunologic systems of striped bass and hybrid tilapia may contribute to inter-species variation in mycobacteriosis susceptibility.     

Fish assemblages across a vegetation gradient in a restoring tidal freshwater wetland: diets and potential for resource competition

Marsh habitats have been the focus of recent worldwide restoration efforts due to their degradation and destruction as a result of human development. We assessed fish resource use at a naturally restoring marsh (Liberty Island, California, USA) by comparing diet composition, stomach fullness, normalized stomach weight, and diet overlap across a vegetation gradient. Fish were collected using gill nets and fyke nets at six sites during spring 2010, summer 2011 and winter 2012. We analyzed 392 stomachs from the eleven most abundant species collected. Prey composition and biomass varied seasonally for all fish species, but there were no notable differences across sites or seasons for stomach fullness or normalized stomach weight for most fish species. Results from non-metric multidimensional scaling (NMDS) and two-way analysis of similarities (ANOSIM) indicated minimal diet overlap between species (R?=?0.633, p?=?0.001) and seasons (R?=?0.413, p?=?0.001). Seasonal habitat and resource use across the vegetation gradient was species-specific. Small but significant spatial diet differences were detected for inland silverside, striped bass and bigscale logperch. Delta smelt exhibited seasonal diet differences by shifting from chironomids and zooplankton during spring, to amphipods and zooplankton during winter. More generally, fish maintained stomach fullness across all sites and seasons while maintaining minimal dietary overlap. Our study emphasizes the importance of tidal marshes as feeding habitat for several fish species, including the endangered delta smelt.     

FEEDING BEHAVIOUR AND ECOLOGY OF NEW GUINEA RAINFOREST INSECTIVOROUS PASSERINES

Fifty species of insectivorous warblers Sylviidae, flycatchers Muscicapidae and whistlers Pachycephalidae were studied in primary rainforest at various localities in New Guinea. The structure of the various forest types is described and the birds' feeding ecology and behaviour analysed by recognizing three main foraging techniques and five horizontal and three vertical basic structural divisions of the habitats. Altitudinal ranges of the species are assessed to determine potential co-existence and they are divided into lowland and lower montane groups (either side of the main avifaunal discontinuity at 1500 m) with a third small group occurring in both areas and a fourth group of 12 lower montane species that occur also in the structurally much simpler Upper Montane forest. The feeding behaviour and ecology of the species within each major habitat are compared, with particular attention to taxonomically related and ecologically similar species. Other important considerations—additional behavioural differences, notable morphological distinctions, altitudinal separation of ranges within the habitat—are also noted. The likely importance of differences in foraging behaviour and feeding sites for reducing competition between related species is amply demonstrated, members of several pairs and groups of species have nearly mutually exclusive preferences. The overall pattern of habitat utilization is, however, extremely complex with nearly all stations used, in a variety of ways, by several species and there are many instances of substantial similarity between pairs of species, often involving congeners. The calculation of information theory derived indices of foraging diversity and overlap enables more general comparisons between the altitudinally graded habitats to be made and differences related to current ideas on tropical species diversity. Between Lowland and Lower Montane forest there is a fairly general trend of reduction in foraging diversity and decrease in the mean overlap between species in many genera and groups. The 12 species that continue into the simpler Upper Montane forest show very significantly reduced foraging diversity (compared with their values in lower montane forest) and also less overlap, indicating a different relationship between these species in the absence of the other Lower Montane forest birds. Together these results suggest that the most tropical (i.e., lowland) species show greatest overlap but do not necessarily have smaller niches. In progressively higher habitats there is a bias to the disappearance of generalist (high diversity index) species. These mainly use flycatcher-gleaning techniques supporting suggestions that the increase in insectivorous species in the tropics is partly due to exploitation of feeding strategies related to hovering. Habitat and ecological factors influencing this are assessed. The importance of altitudinal isolating mechanisms is also discussed and, amongst the species studied, both on average and in specific cases, those with the greatest similarities in foraging behaviour and ecology are segregated altitudinally and do not co-exist. It is suggested, however, that substantial overlap between many co-existing tropical species may not be abnormal, but rather an adaptation for ensuring maximum efficiency of habitat utilization in the prevailing environmental conditions of tropical rainforest.     

Influence of cruising and ambush predators on 3-dimensional habitat use in age 0 juvenile Atlantic cod Gadus morhua

Predators and prey often co-exist at high densities within the same habitat, yet the behavioural and spatial dynamics underlying this co-existence are not well known. To better understand small-scale, predator-prey co-occurrence, the spatial patterns and behaviour of age 0 juvenile cod Gadus morhua 75-88 mm SL and two of their known predators, age 2+ cod and short-horn sculpin Myoxocephalus scorpinus, were examined in two habitats (i.e., sand and eelgrass) using three-dimensional video analysis. Both habitat and predator type interacted to result in unique spatial patterns of prey. Spatial overlap between predators and prey was highest in open habitat in the presence of the cruising predator but lowest in the presence of sculpin in the same habitat. In eelgrass, age 0 cod avoided predators primarily along the vertical axis (i.e., distance off bottom). Age 0 cod stayed above eelgrass in the presence of sculpin but lowered themselves into the eelgrass while in the presence of predator cod. Anti-predator behaviour (i.e., predator-prey distance, prey cohesion and freezing) was significantly reduced over eelgrass compared to sand, suggesting eelgrass has lower ‘inherent risk’ than open habitats. However, predator consumption was similar across all treatments, suggesting that, 1) complex habitat also impairs the visual cues needed for anti-predator behaviour (e.g., schooling) and assessing the location of predators, and 2) predators change their behaviour with habitat to enhance their opportunities for finding and capturing prey.     

Foraging in space and time structure an African small mammal community

We used live-trapping and foraging to test for the effect of habitat selection and diet on structuring a community of six small mammals and one bird within the Soutpansberg, South Africa. We established grids that straddled adjacent habitats: woodland, rocky hillside, and grassland. Trapping and foraging were used to estimate abundance, habitat use, and species-specific foraging costs. The species with the highest abundance and foraging activity in a habitat, activity time, or food was considered the most efficient and presumed to have a competitive advantage. All species exhibited distinct patterns of spatial and temporal habitat preference which provided the main mechanism of coexistence, followed by diet selection. The study species were organized into three assemblages (α diversity): grassland, Rhabdomys pumilio, Dendromus melanotis, and Mus minutoides.; woodland, Aethomys ineptus and Micaelamys namaquensis; and rock-dwelling, M. namaquensis and Elephantulus myurus. Francolinus natalensis foraged in open rocky areas and under wooded islands within the grassland. Species organization across the habitats suggested that feeding opportunities are available within all habitats; however, distinct habitat preferences resulted from differing foraging aptitudes and efficiencies of the competing species. At Lajuma, species distribution and coexistence are promoted through distinct habitat preferences that were shaped by competition and species-specific foraging costs. The combination of trapping and foraging provided a mechanistic approach that integrates behavior into community ecology by ‘asking’ the animal to reveal its perspective of the environment. Using spatial and temporal foraging decisions—as behavioral indicators—enables us to guide our understanding for across-taxa species coexistence.     

Determination of Foraging Thresholds and Effects of Application on Energetic Carrying Capacity for Waterfowl

Energetic carrying capacity of habitats for wildlife is a fundamental concept used to better understand population ecology and prioritize conservation efforts. However, carrying capacity can be difficult to estimate accurately and simplified models often depend on many assumptions and few estimated parameters. We demonstrate the complex nature of parameterizing energetic carrying capacity models and use an experimental approach to describe a necessary parameter, a foraging threshold (i.e., density of food at which animals no longer can efficiently forage and acquire energy), for a guild of migratory birds. We created foraging patches with different fixed prey densities and monitored the numerical and behavioral responses of waterfowl (Anatidae) and depletion of foods during winter. Dabbling ducks (Anatini) fed extensively in plots and all initial densities of supplemented seed were rapidly reduced to 10 kg/ha and other natural seeds and tubers combined to 170 kg/ha, despite different starting densities. However, ducks did not abandon or stop foraging in wetlands when seed reduction ceased approximately two weeks into the winter-long experiment nor did they consistently distribute according to ideal-free predictions during this period. Dabbling duck use of experimental plots was not related to initial seed density, and residual seed and tuber densities varied among plant taxa and wetlands but not plots. Herein, we reached several conclusions: 1) foraging effort and numerical responses of dabbling ducks in winter were likely influenced by factors other than total food densities (e.g., predation risk, opportunity costs, forager condition), 2) foraging thresholds may vary among foraging locations, and 3) the numerical response of dabbling ducks may be an inconsistent predictor of habitat quality relative to seed and tuber density. We describe implications on habitat conservation objectives of using different foraging thresholds in energetic carrying capacity models and suggest scientists reevaluate assumptions of these models used to guide habitat conservation.     

Intersexual segregation in foraging microhabitat use by Magellanic Woodpeckers (Campephilus magellanicus): Seasonal and habitat effects at the world's southernmost forests

Animals facing seasonal food shortage and habitat degradation may adjust their foraging behaviour to reduce intraspecific competition. In the harsh environment of the world's southernmost forests in the Magellanic sub‐Antarctic ecoregion in Chile, we studied intersexual foraging differences in the largest South American woodpecker species, the Magellanic Woodpecker (Campephilus magellanicus). We assessed whether niche overlap between males and females decrease when food resources are less abundant or accessible, that is, during winter and in secondary forests, compared to summer and in old‐growth forests, respectively. We analysed 421 foraging microhabitat observations from six males and six females during 2011 and 2012. As predicted, the amount of niche overlap between males and females decreased during winter, when provisioning is more difficult. During winter, males and females (i) used trees with different diameter at breast height (DBH); (ii) fed in trunk sections with different diameters; and (iii) fed at different heights on tree trunks or branches. Vertical niche partitioning between sexes was found in both old‐growth and secondary forests. Such a niche partitioning during winter may be a seasonal strategy to avoid competition between sexes when prey resources are less abundant or accessible. Our results suggest that the conservation of this forest specialist, dimorphic and charismatic woodpecker species requires considering differences in habitat use between males and females.     

Relationships between food resources, foraging patterns, and reproductive success in the water pipit, Anthus sp. spinoletta

A basic but rarely tested assumption in optimal foraging theoryis that positive relationships exist between the foraging patternof an animal, its short-term benefits in feeding, and its long-termfitness. We present evidence for these relationships for a centralplace foraging situation. We studied the foraging behavior ofadult water pipits (Anthus sp. spinoletta) feeding nestlingsin an Alpine habitat near Davos, Switzerland, with the followingresults: (1) searching effort decreases with increasing distancefrom the nest, (2) the amount of prey and the proportion oflarge items brought to the nest increases with increasing foragingdistance, (3) water pipits do not forage according to habitatavailability, but prefer vegetation types with the highest fooddensity (mainly grass and herbs) and avoid those with the lowest,and (4) this selectivity is only expressed when the birds foragemore than 50 m from the nest, i.e., usually outside the territory.Among the several potential interpretations of these results,the most parsimonious is that foraging decisions are based onprofitability, i.e., on the net energy gain per time unit. Additionally,we found that food conditions translate into fitness: the numberof fledglings per nest is related positively to the averageprey biomass at the foraging place and negatively to the averagedistance between the foraging place and the nest. Maximum economicdistances, which were predicted from this food-fitness relationship,agreed well with the actual foraging distances observed. Thissuggests a dose connection between foraging decisions and fitness.In addition to the theoretical issues, some conservation issuesare also briefly discussed.