Amphibian concentrations in desiccating mud may determine the breeding season of the White-shouldered Ibis (Pseudibis davisoni)

Habitat selection, foraging ecology and conservation of Eastern Curlews on their non-breeding grounds

Utilizing optimal foraging theory and laboratory estimates of foraging costs, we predict the choice of foods and use of habitats by fish in the field. These predictions are tested with the bluegill sunfish (Lepomis macrochirus) foraging in three habitats (open water, sediments, and vegetation) in a pond. Relations describing prey encounter rates in each habitat as a function of prey size, prey density, and fish size were derived from laboratory experiments. These relations permitted us to estimate prey encounter rates based on weekly prey samples in each habitat of the pond. We can then determined the optimal diet and profitability (net energy return) for each habitat through time. Predictions of optimal diet exhibited good qualitative correspondence to the actual diet of the fish in the open water and vegetation, although we consistently predicted a slightly narrower diet than the fish were choosing. The model correctly predicted the magnitude of the change in size selection on Daphnia pulex with fish size and with decline in prey density. Predictions of optimal diet in the sediments were considerably in error apparently due to a tendency for late—instar midges to burrow deep in the sediments, thereby becoming unavailable to the fish. In this case habitat profitabilities were computed simply on the basis of the actual observed diet. Predictions of optimal habitat use i.e., when the fish should switch habitats to maximize feeding rates, showed striking correspondence to the actual habitat use of the fish; the bluegills switched from feeding in the open water column to feeding from the sediments within a few days of our predictions. The actual habitat use pattern differs dramatically from a null model of random habitat use. We indicate how this approach may be useful in studying intra— and interspecific exploitative interactions.

Foraging bouts of male Swainson's Hawks (Buteo swainsoni) were observed using radio-telemetry to determine the effect of plant cover on the selection of foraging sites. Home ranges consisted of varying amounts of cultivated and uncultivated habitats. Cultivated fields were the most abundant and they supported large amounts of prey, but were not hunted until crop harvest reduced the density of their plant canopy. A negative correlation between estimates of plant cover and foraging suggested that habitat differences such as vegetative cover were of greater importance than prey density in the selection of hunting sites. A correlation between foraging and prey biomass after it had been adjusted for vegetative concealment indicated that models relating prey abundance with raptor foraging should consider the effect of such a habitat difference on the availability of a hunting site's food supply. The efficient selection of hunting sites is critical for foraging birds. Because physical features vary among habitats, it is logical to assume that these differences would influence the availability of food and a bird's hunting success. Discussions of avian foraging have begun to emphasize the importance of such factors as plant cover, distance of travel, and weather in the selection process (Goss-Custard 1970, Royama 1970, Hassell 1971, Smith and Dawkins 1971, Zach and Falls 1976a, b, Kushlan 1979, Eiserer 1980, Fitzpatrick 1980). Although descriptions of habitat use by foraging raptors are limited, they indicate that, for species such as hawks and owls, plant cover and weather may have a greater effect than prey abundance on the suitability of their foraging habitats (Southern and Lowe 1968, Wakeley 1978, Stinson 1980). If this is true, predator-prey models that assume a direct relationship between hunting and prey density (MacArthur and Pianka 1966, Emlen 1968, Simons and Alcock 1971, Alcock 1973, Poole 1974) would need to be reconsidered when applied to raptors. I report here the results of a study I conducted to determine the effect of plant cover on the availability of Swainson's Hawk (Buteo swainsoni) prey. I predicted that, if plant cover limited the productivity of potential hunting sites, a reduction in a habitat's cover would increase its use by foraging hawks. Birds nesting in farmland of southeastern Washington were ideal to study because crop harvest reduced plant cover in large portions of their nesting habitat. Using estimates of plant cover together with estimates of prey density, I compared the distribution of foraging efforts before and after harvest for a possible correlation between a habitat's use and its vegetative cover.

We investigated foraging behaviour of larval dragonflies Aeshna juncea in order to examine the significance of prey density and body size in predator-prey dynamics. A. juncea were offered separately three size-classes of Daphnia magna at low and high densities. The data were collected with direct observations of the foraging individuals. We found that large A. juncea larvae could better enhance their intake of prey biomass as prey size and prey density increased than their smaller conspecifics. However, increasing feeding efficiency of both larval instars was constrained by declining attack success and search rate with increasing prey size and density. With small D. magna, in contrast to large A. juncea, small A. juncea increased their searching efficiency as prey density increased keeping D. magna mortality rate at a constant level. In a predator-prey relationship this indicates stabilizing potential and feeding thresholds set by both prey density and prey-predator size ratio. Attack success dropped with prey size and density, but did not change in the course of the foraging bout. For both A. juncea sizes prey handling times increased as more medium and large prey were eaten. The slope of the increase became steeper with increasing prey-predator size ratio. These observations indicate that components of the predator-prey relationship vary with prey density, contrary to the basic assumptions of functional response equations. Moreover, the results suggest that the effects of prey density change during the ontogeny of predators and prey.

Our understanding of the dynamics of predator–prey systems has relied heavily on the use of models based on the standard Lotka–Volterra (LV) framework, dating back over 80 years. Although these models have been repeatedly analysed and refined since their initial inception, the way they describe the predator's growth rate has received surprisingly little attention; typically it is simply assumed that the predator's growth rate is linearly related to its ingestion rate according to a constant assimilation efficiency, e. However, for many consumers e is known to decrease at high prey densities. Models that ignore variable assimilation efficiencies overlook potentially important non-linearities, affecting the validity of predictions relating to conservation, invasion biology and pest control. Directly quantifying the relationship between e and prey abundance is, however, difficult. An alternative approach (the independent-response, IR, approach) is to not assume any direct link between the predator's functional response (the relationship between ingestion rate and prey abundance) and its growth response. This flexibility is invaluable when parameterising models from data; providing the model-fitting process is constrained to ensure that e never exceeds 1, this approach allows considerable insight into whether, and how, e varies with prey density. Here we examine the synergistic value of combining the IR and LV approaches. We illustrate these concepts through analysis of published functional and growth response data and show that, in many cases, e does vary with prey abundance. This paper is the first recognition that these two complementary approaches can be combined into a single framework that allows the relationship between a predator's functional and growth responses to emerge during the parameterisation process, thereby acting as a compromise between restrictive models that require this relationship to be defined a priori, and completely unrestrained models that allow assimilation efficiencies to exceed 1.

Losses of suitable foraging and nesting habitats are key drivers for ongoing declines of farmland birds. For Eurasian Skylarks (Alauda arvensis), this is reflected by the lack of sparse and short vegetation structure (i.e., crop architecture). Existing countermeasures outside cultivated areas seem unable to offset declines, indicating the need for in‐field measures, ideally in cereal fields which cover large shares of agricultural land and ideally without compromising yields.We experimentally tested whether unsown Skylark strips in intensive winter cereal fields (treatment) provided sparse and short crop architecture throughout the breeding season and whether this improved habitat quality by comparing key parameters of Skylark populations on treatment fields and fields without Skylark strips (control). Further, we tested whether the size and direction of effects of the measure depended on surrounding landscape features and seasonality.Breeding performance was improved in treatment fields: the odds for nests being successful were 12.5 times that of control fields. During important phases of the breeding season, prey density and parental foraging were increased in treatment fields. Territorial behavior was higher in treatment fields throughout the breeding season, which was also less dependent on other attractants (i.e., proximity to ecotones) compared to control fields. Habitat use was increasingly higher in treatment fields as the breeding season progressed. Further, the availability of small‐parceled fields in the surrounding area was related to decreased habitat use in control fields but not in treatment fields. In general, habitat use in cereal fields depended on adjacent habitat types, with the presence of spring sown crops leading to the largest declines.We conclude that Skylark strips can improve habitat quality for Skylark nesting and foraging in intensive winter cereal fields. Given the previously shown yield stability, this encourages large‐scale implementation of this measure for Skylark conservation, which might be most beneficial on large winter cereal fields in landscapes where there are few spring‐sown crops.

Environmental factors and prey availability determine wintering habitat use by shorebirds along the west coast of India.

ABSTRACT. Ingestion rate of Paraphysomonas imperforata was found to be a hyperbolic function of prey density. But the same flagellate clone had multiple ingestion responses to prey density, depending on its physiological state and physical stress it suffers. The flagellates in a physiological state of higher growth tended to have higher maximum ingestion and clearance rates than ones in a physiological state of lower growth. The same trend was observed for volume‐specific maximum ingestion and volume‐specific clearance rate. In response to changing prey density, the growth rate did not change as quickly as the ingestion rate, suggesting imbalance between the two. The tested physical stresses, including shaking, centrifugation, and filtration, also resulted in reduction of ingestion parameters of the flagellates. But half‐saturation constants did not show any trend in response to either physiological state or physical stress. In light of the dynamic nature of protistan ingestion response to prey abundance, short incubation, which minimizes the physiological change, and careful handling, which prevents the possible physical stress, should be employed in order to avoid underestimation of in situ ingestion rates. Previously reported ingestion parameters of lab‐cultured protists, which are thought to be unrealistic in natural conditions, may represent only one of multiple ingestion responses, probably prey‐rich condition.

Because pelagic prey concentrations are patchy in both space and time, predators such as marine mammals require high degrees of flexibility in their habitat use. We tested the hypothesis that minke whales Balaenoptera acutorostrata adjust their habitat use during the feed- ing season at different spatial scales: their overall distribution should be determined by broad- scale oceanographic features, while foraging activity at finer scales should be dictated by short- term changes in habitat conditions. Results from generalized additive models indicate that minke whale distribution off the west coast of Scotland is dependent largely on temporally variable para- meters (sea surface temperature in spring, chlorophyll concentration in autumn), in addition to depth and topography. However, fine-scale foraging behaviour was dictated by the strength and direction of tidal currents. Seasonal distribution patterns according to environmental parameters were largely consistent between 2 different spatial scales, and over a time period of 15 yr. Signif- icantly higher sighting rates occurred in areas of predicted sandeel Ammodytes marinus presence in spring, but not during the rest of the summer, while in August and September, prey samples from the core study area consisted almost entirely of sprat Sprattus sprattus. The low energetic cost of swimming in minke whales and their ability to switch between different prey according to their seasonal availability thus appears to allow them to readily respond to temporal changes in pelagic prey concentrations at different scales. This occurs through a distribution influenced by temporally variable parameters (temperature and chlorophyll concentration), combined with adjustments in foraging activity dependent on variable conditions at fine spatial scales (tides).

Seasonal habitat use and breeding performance of the Eurasian Skylark (Alauda arvensis) in Central European farmland

Simple SummaryGulls are known for their adaptability and can navigate various environments from coastal and inland areas to marine spaces. Despite extensive studies using GPS tracking devices to explore the habitat use and flight patterns of numerous gull and other bird species, little such research has focused on the widely-distributed black-tailed gulls (Larus crassirostris) inhabiting the Korean Peninsula. To fill this gap in our understanding, our study employed GPS trackers to analyze the flight behavior and habitat use of black-tailed gulls. Our results unveiled distinct ranges of activity and flight altitudes that were influenced by both season and region. These findings suggest that the flight behavior of black-tailed gulls is responsive to environmental changes. Our study contributes fundamental data on the flight behavior of black-tailed gulls.In this study, GPS trackers were attached to black-tailed gulls (Larus crassirostris) breeding on five islands in Republic of Korea during April and May 2021, and their flight frequency, behavioral range, and flight altitude were compared during and after the breeding season. During the breeding season, the flight frequency was lowest on Dongman Island (28.7%), where mudflats were distributed nearby, and the range of activity was narrow. In contrast, it tended to be high on Gungsi Island (52%), where the breeding colony was far from land, resulting in a wider range of activity. Although the flight frequency on Dongman Island increased post-breeding season (42.7%), it decreased on other islands. The mean flight altitude during the breeding season was lowest on Dongman Island and highest on Napdaegi Island. In most breeding areas, the mean flight altitude during the post-breeding season was higher than that during the breeding season. However, the lead flight altitude was lower during the non-breeding season compared to that in the breeding season. The home range expanded after the breeding season, with no significant difference in lead time between the breeding and non-breeding seasons. Our findings reveal that black-tailed gulls exhibit varying home ranges and flight altitudes depending on season and geographical location. As generalists, gulls display flexible responses to environmental changes, indicating that flight behavior adapts to the evolving environment over time and across regions.

Understanding the variations of the functional response of an organism, i.e. the predation rate in relation to prey density, is necessary to understand the interactions between the animal and its food supply. This has received little attention in dabbling ducks so we investigated experimentally the shape of the functional response of mallard feeding on poultry pellets, and assessed the influence of several factors such as the size of food items, sex or individual performance on this functional response. Individual differences in intake rate are of crucial importance in group or gregarious foraging species. We used two approaches of the functional response: 1) the relation between feeding rate (pellets/s) and pellet densities (pellets/m2), and 2) the relationship between instantaneous intake rate (g/s) and biomass density (g/m2). For both approaches, we found that the Type II functional response gave better estimates than a Type I linear functional response but explained only a third of the variance. Our results show that pellet size has a large effect on instantaneous intake rate. The comparison of the functional response parameters suggest that handling time per prey may not reflect the real constraints on intake rate, but that handling time per gram ingested may be more appropriate to integrate the effect of item size in the functional response. We then discuss the possible mechanisms involved. We also found individual variations in the functional response for each of the experiments, with some consistency in the hierarchy regarding feeding efficiency. We did not find any differences between males and females. Our results provide an evaluation of individual variations in intake rate in interference‐free conditions, which has rarely been done, and call for more controlled experiments to allow a finer understanding of the mechanisms of food acquisition in dabbling ducks.

1. The foraging ecology of Brünnich's guillemots Uria lomvia was studied during the breeding season in south‐eastern Svalbard. In the region of Storfjorden there are two large breeding colonies comprising a total of about 540 000 individuals. These birds forage in the western part of Storfjorden and further to the south. Their main prey are polar cod Boreogadus saida, pelagic amphipods Parathemisto spp. and euphausiids Thysanoessa inermis. 2. A ship‐based transect survey was used to record bird abundance and the acoustically determined biomass of presumed prey. The five transects were divided into 33 segments, each 8–11 km in length. The resolution of the survey was 150 m, and analyses of correlations between predators and prey were performed at length scales from 150 m to 9 km. We differentiated acoustic signals into aggregated and dispersed categories according to the estimated horizontal distribution of presumed prey. 3. Foraging guillemots were consistently more strongly correlated with the aggregated prey than with dispersed prey over scales ranging from 150 m to 9 km. Correlations were weak at small scales (150 m – 1 km) and increased and stabilized at scales of 2–3 km. The spatial scale at which we obtained a shift from weak to strong correlations between guillemots and their prey was similar to the scale at which the spatial variances in both guillemot and prey abundance were high. 4. Guillemots showed low correlations with prey at low prey densities. Similarly, correlations between guillemots and prey were low at low bird densities. The data support the hypothesis that the birds associate with prey patches with densities above a certain threshold, and that ‘regional’ prey abundances affect local use of patches. 5. The numerical aggregative response curves between guillemot and prey density were classified as being neither hyperbolic (type II) or sigmoidal (type III) within the range of prey densities observed in this study. The aggregative response curves were sensitive to spatial scales, which suggest that studies of response curves should be conducted at a range of spatial scales.

Time and energy budgets were quantified for sandhill cranes (Grus canadensis) wintering in western Texas, at 3 spring migration staging areas, and during the prenesting period in western Alaska. Integration of habitat use, food habits, and physiology and condition data with concurrent time and energy budgets indicated that lipid dynamics covaried with reproductive status, habitat conditions, and time and energy allocations. Cereal grains provided >95% of the energy during winter and spring migration. Lipid reserves obtained during migration were used on nesting areas because of high energy demands and low food availability. Lipids were accumulated primarily in Nebraska where cranes exploited concentrated high energy food with minimum energy expenditures. Amounts of small grains necessary to meet daily food requirements were quantified for each study location to assist managers in determining availability of adequate food supplies based on crane-use days. J. WILDL. MANAGE. 51(2):440-448 Quantification of habitat selection can delineate resources important to wildlife (Iverson et al. 1985 a,b), whereas the time-activity budgets reveal how available habitats are used (Frederick and Klaas 1982). Food habits data reveal how nutrients are extracted from the environments (Iverson et al. 1982), whereas physiology and condition provide insights into nutritional requirements and how well animals are able to exploit an area to meet their needs (Iverson 1981). Energy budgets provide a common denominator (energy) to relate body condition to food resources (King 1974). Integration of information from these diverse studies is required to describe animal-habitat relationships adequately, identify requirements for survival and reproduction, and detect real or potential problems for management consideration (Tacha et al. 1987). This paper integrates information on habitat use, food habits, and physiology and condition with time and energy budgets to portray the resource needs of sandhill cranes on wintering and spring migration staging areas in mid-continental North America. This study was funded by Contract 14-160008-2133, Accelerated Res. Program for Migratory Shore and Upland Game Birds, administered by the Cent. Manage. Unit Tech. Comm. and the Migratory Bird and Habitat Res. Lab., U.S. Fish and Wildl. Serv. (USFWS); by the Oklahoma Coop. Wildl. Res. Unit (OCWRU), and the Natl. Wildl. Fed. The OCWRU has Oklahoma State Univ., Ok. Dep. Wildl. Conserv., USFWS, and the Wildl. Manage. Inst. cooperating. We thank D. C. Martin for field assistance, and G. L. Krapu and K. J. Reinecke for manuscript review. W. D. Warde aided in statistical analyses. C. L. Jorgenson and C. W. Strickland provided valuable tactical support in Can. and Alaska.

Negative density-dependence is generally studied within a single trophic level, thereby neglecting its effect on higher trophic levels. The 'functional response' couples a predator's intake rate to prey density. Most widespread is a type II functional response, where intake rate increases asymptotically with prey density; this predicts the highest predator densities at the highest prey densities. In one of the most stringent tests of this generality to date, we measured density and quality of bivalve prey (edible cockles Cerastoderma edule) across 50 km² of mudflat, and simultaneously, with a novel time-of-arrival methodology, tracked their avian predators (red knots Calidris canutus). Because of negative density-dependence in the individual quality of cockles, the predicted energy intake rates of red knots declined at high prey densities (a type IV, rather than a type II functional response). Resource-selection modelling revealed that red knots indeed selected areas of intermediate cockle densities where energy intake rates were maximized given their phenotype-specific digestive constraints (as indicated by gizzard mass). Because negative density-dependence is common, we question the current consensus and suggest that predators commonly maximize their energy intake rates at intermediate prey densities. Prey density alone may thus poorly predict intake rates, carrying capacity and spatial distributions of predators.