Net Rate Of Energy Gain Research Articles

Test of theory of foraging mode: Goldcrests, Regulus regulus, forage by high-yield, energy-expensive hovering flight when food is abundant but use low-yield, low-cost methods when food is scarce.

Here, I describe foraging behavior of goldcrests, Regulus regulus, based on eight years of field observation in a coniferous forest dominated by Norway spruce Picea abies in southwestern Sweden. The aim was to test predictions from theory on the choice of optimal foraging modes in relation to food availability.Mortality from early November to early March amounts to 70–86% among goldcrests in the resident population, suggesting they are food‐limited in winter. Food‐limitation manifests itself as a shortage of time for foraging. It promotes the use of foraging methods that minimize the daily foraging time by maximizing the rate of net energy gain. It increases both individual survival and competitiveness. Elimination of competitors by exploitation occurs when an individual is able to support itself, while food density in the habitat is reduced to levels at which others cannot.Theory shows that when food is abundant, high‐efficiency energy‐expensive search and capture methods give shorter daily foraging times than low‐efficiency low‐cost methods, whereas the latter gives shorter daily foraging times at food shortages (Norberg 2021). Hovering flight is extremely expensive in energy but results in high foraging efficiency. Hover‐foraging should therefore be used when food is abundant.In autumn, there were 85.3 arthropods per kilogram of branch mass, as opposed to 12.9 in spring. The numerical decline of arthropods, their fat metabolism, and size‐biased predation by birds reduced the spring density of food for goldcrests to less than 15.1% of the autumn density.Hover‐foraging occurred 5.29 times per minute in autumn but only 0.23 times per minute in spring, which is 4.4% of the autumn frequency.Foraging conditions are favorable at midsummer because of long days, high temperatures, and an abundance of arthropod prey. Parent birds that were feeding fledglings gathered food at a high rate and hovered 5.42 times per minute. But adults with no young to feed were not compelled to maximize the rate of net energy gain and only hover‐foraged 0.52 times per minute, which is 10% of that of providers.These results are highly consistent from year to year and in qualitative agreement with theory.

To minimize foraging time, use high-efficiency, energy-expensive search and capture methods when food is abundant but low-efficiency, low-cost methods during food shortages.

Based on a mathematical model, I show that the amount of food in the habitat determines which among alternative methods for search of prey, respectively, for pursuit‐and‐capture give the shortest daily foraging time. The higher the locomotor activity, the higher the rate of energy expenditure and the larger the habitat space a predator can search for prey per time unit. Therefore, I assume that the more efficient a foraging method is, the higher its rate of energy expenditure. Survival selection favors individuals that use foraging methods that cover their energy needs in the shortest possible time. Therefore, I take the optimization criterion to be minimization of the daily foraging time or, equivalently, maximization of the rate of net energy gain. When time is limiting and food is in short supply, as during food bottleneck periods, low‐efficiency, low‐cost foraging methods give shorter daily foraging times than high‐efficiency, energy‐expensive foraging methods. When time is limiting, food is abundant and energy needs are large, as during reproduction, high‐efficiency high‐cost foraging methods give shorter daily foraging times than low‐efficiency low‐cost foraging methods. When time is not limiting, food is abundant, and energy needs are small, the choice of foraging method is not critical. Small animals have lower rates of energy expenditure for locomotion than large animals. At a given food density and with similar diet, small animals are therefore more likely than large ones to minimize foraging time by using high‐efficiency energy‐expansive foraging methods and to exploit patches and sites that require energy‐demanding locomotion modes. Survival selection takes place at food shortages, while low‐efficiency low‐cost foraging methods are used, whereas reproduction selection occurs when food is abundant and high‐efficiency energy‐expensive foraging methods do better. In seasonal environments, selection therefore acts on different foraging methods at different times. Morphological adaptation to one method may oppose adaptation to another. Such conflicts select against foraging and morphological specialization and tend to give species‐poor communities of year‐round resident generalists. But a stable year‐round food supply favors specialization, niche narrowing, and dense species packing.

Locomotor Economy and Foraging Ecology in Hominins

The ratio of net energy gained over time expended while foraging (Net Rate of Energy Gain or NREG) or the ratio of net energy gained over energy expended (Foraging Energy Efficiency or FEE) are com...

Sea otter population collapse in southwest Alaska: assessing ecological covariates, consequences, and causal factors

AbstractSea otter (Enhydra lutris) populations in southwest Alaska declined substantially between about 1990 and the most recent set of surveys in 2015. Here we report changes in the distribution and abundance of sea otters, and covarying patterns in reproduction, mortality, body size and condition, diet and foraging behavior, food availability, health profiles, and exposure to environmental contaminants over this 25‐yr period. The population decline, which resulted in densities on the order of 5% of environmental carrying capacity, ranged from Attu Island in the west to about Castle Cape (on the south side of the Alaska Peninsula) in the east. Remaining sea otters moved closer to shore and into shallow, protected habitats. Reproductive rates appeared unchanged with the decline. Although the demographic cause of the decline was clearly elevated mortality, stranded carcasses were rare or absent. The net rate of energy gain by foraging sea otters, body length and condition, and prey biomass density, all increased after the decline and varied inversely with sea otter population density beyond the area of decline. Sea otters within the area of decline showed no increases in health anomalies, disease, contaminant exposure, or abnormal gene transcription patterns as compared to animals outside the area of decline. These collective findings are inconsistent with nutritional limitation, disease, or environmental contaminants, and consistent with predation (or possibly some other density‐independent factor) as the reason for the sea otter population decline. Our approach and analyses provide a broad conceptual template for thinking about and assessing the causes of wildlife population declines.

The diving behavior of African clawless and spotted-necked otters in freshwater environments

Abstract Understanding the diving behavior of semiaquatic mammals, particularly in relation to estimated aerobic dive limits and diet, is important to understand their adaptability and potential vulnerability to changes in prey type and distribution. The diving behavior of African clawless otters, Aonyx capensis, and spotted-necked otters, Hydrictis maculicollis, is poorly known, and no estimates of their dive performance in relation to targeted prey and calculated dive limits have been reported previously for freshwater environments. We investigated the diving behavior of both these otter species in freshwater environments within South Africa through video recordings of direct observations and subsequent video analyses where dive and recovery durations and dive function were recorded. African clawless otters were found to perform longer dives (mean ± SD = 26.9 ± 12.2 s), compared to spotted-necked otters (8.5 ± 7.6 s). African clawless otters showed substantial variation in dive durations, with the shortest dives sometimes lasting < 5 s, and the longest recorded dive being 70 s. The majority of spotted-necked otter dives lasted < 10 s, with the shortest recorded dive lasting 0.66 s and the longest recorded dive lasting 50.9 s. Spotted-necked otters performed different dive types that were evidently dependent on prey targeted, with dives targeting crabs (16.10 ± 1.91 s) being longer than dives targeting rainbow trout, Oncorhynchus mykiss (5.58 ± 0.17 s). The theoretical dive durations of African clawless otters were exceeded during play dives, while spotted-necked otters sometimes exceeded their theoretical dive durations when performing successful foraging dives. The results of this study suggest that spotted-necked otters can vary behavior in relation to prey and exceed theoretical dive duration during successful foraging dives to maximize the net rate of energy gain. Furthermore, when considering known individual-level dietary specialization and plasticity in these species, it may be predicted that dive behaviors are likely to vary substantially among individuals, and in relation to prey availability and localized habitat conditions.

Microhabitat variables explain patch switching by wintering Bewick’s swans through giving-up net energy intake rates

Microhabitat variables are determinants for animals to select forage patches and evaluate the cost/benefit tradeoffs of habitat switching. Optimal foragers would weigh habitat quality by giving-up net energy intake rate (GUN), which includes the energy intake rate and cost rate. The GUNs, energy intake rate, and cost rate can be influenced by variations in different microhabitat factors and interactive effects. In this study, we assessed the GUN patterns of wintering Bewick's swans and the effects of microhabitat factors on their foraging strategy in three different habitats: foxnut ponds, paddy fields, and shallow lake. The foraging behaviors and microhabitat variables of the swans were investigated during the winters of 2016-2018 and 2017/2018 at Huangpi and Shengjin Lakes in Anhui Province, southeastern China. The results showed that the percentage of disturbance time and the giving-up food density in shallow water had significant negative effects on GUNs. In contrast, water depth and the giving-up food density in deep water showed positive effects on GUNs. GUNs were significantly different among the three habitats. GUNs also decreased as winter progressed. Swans would decrease their GUNs under unfavorable foraging conditions such as more disturbances; however, GUNs would increase with water depth and food availability in patches with deep water. The swans demonstrated diverse GUN patterns in different foraging habitats, driven by the tradeoffs between energy intake rates and energy cost rates that were influenced by microhabitat variables. It implies that waterbirds exposed to decreasing GUNs may mitigate energy demand and environmental stress by switching foraging habitat while in subprime foraging habitat if alternative habitats that offered higher net rates of energy gain were available.

Seasonal variation in energy gain explains patterns of resource use by avian herbivores in an agricultural landscape: Insights from a mechanistic model

Our understanding of how energy shapes animal behavioural decisions has been limited by the difficulty of measuring directly the energy gain and expenditure in free-living animals. Mechanistic models that simulate energy gain and expenditure from estimable parameters can overcome these limitations and hence could help scientists to gain a predictive understanding of animal behaviour. Such models could be used to test mechanistic explanations of observed patterns of resource use within a landscape, such as behavioural decisions to switch among food resources. Here, we developed mechanistic models of the instantaneous and daily rates of net energy gain for two species of migratory swans, the Bewick’s swan (Cygnus columbianus bewickii) and whooper swan (Cygnus cygnus), that feed on root and cereal crops within an agricultural landscape in eastern England. Field data show that both species shift from using predominantly root crops (e.g. sugar beet and potatoes) in early winter to using mostly cereals (e.g. wheat) in late winter. Our models correspondingly predicted that swans could achieve the greatest rates of net energy gain on root crops in early winter and on cereal crops in late winter. The change from root crops to cereal crops providing the greatest net rates of energy gain was predicted to occur at the same time as the birds’ switch from feeding predominantly on root crops to predominantly cereal crops (between December and January). We used Monte Carlo simulations to account for variance in model parameters on predictions of energy gain and profitability. A sensitivity analysis indicated that predictions of net energy gain were most sensitive to variance in the intake rate and food quantity parameters. The agreement between our model estimates of energy gain and the observed shifts in resource use observed among the overwintering swans suggests that maximising net rates of energy gain is an important resource selection strategy among overwintering birds. A mechanistic understanding of where and when birds will use food resources can inform the conservation management of key feeding areas for species of conservation concern, as well as the deployment of crop protection strategies.

Wintering Swan Geese maximize energy intake through substrate foraging depth when feeding on buried Vallisneria natans tubers

BackgroundForaging theory predicts that animals select patches that offer the highest net rate of energy gain. Hence, prey distribution patterns and spatiotemporal heterogeneity play important roles in determining animal feeding patch selection. For waterfowl foraging on buried aquatic plant tubers, the distribution and biomass of these plant organs vary with depth in the substrate. Since excavation costs also increase with depth, the energy intake of the animals foraging on these plants is highly sediment depth dependent.MethodsHere, using observations of Swan Geese (Anser cygnoides) foraging on Vallisneria natans tubers, we test our hypothesis that geese feeding on tubers buried at intermediate sediment depth maximize their daily energy intake because of the interaction between tuber size and abundance with depth. To do this, we measured the distribution patterns of buried Vallisneria tubers under both undisturbed conditions and post-exploitation by geese (i.e. giving-up conditions). We investigated the relationship between tuber size and burial depth, and total tuber biomass within each sediment layer in undisturbed and exploited plots. Finally, we compared modelled Swan Goose daily energy intake feeding on Vallisneria tubers buried at different sediment layers (1–10, 11–20 and 21–30 cm below the surface).ResultsDry weight of Vallisneria tubers linearly increased with burial depth, while average total dry weight density of tubers showed a unimodal relationship, peaking at intermediate levels. Not surprisingly, Swan Geese foraged most intensively on tubers buried at intermediate sediment depths, where they maximize their daily energy intake. Our results support our hypothesis that Swan Geese feeding on tubers at intermediate depths maximize their daily energy intake.ConclusionsOur study is the first to quantify foraging strategies of Swan Geese during the wintering period, emphasizing the importance of plant traits on foraging selection of belowground foragers.

Turbulence induces metabolically costly behaviors and inhibits food capture in oyster larvae, causing net energy loss.

Planktotrophic invertebrate larvae require energy to develop, disperse and settle successfully, and it is unknown how their energetics are impacted by turbulence. Ciliated larvae gain metabolic energy from their phytoplankton food to offset the energetic costs of growth, development and ciliary activity for swimming and feeding. Turbulence may affect the energetic balance by inducing behaviors that alter the metabolic costs and efficiency of swimming, by raising the encounter rate with food particles and by inhibiting food capture. We used experiments and an empirical model to quantify the net rate of energy gain, swimming efficiency and food capture efficiency for eyed oyster larvae (Crassostrea virginica) in turbulence. At dissipation rates representative of coastal waters, larvae lost energy even when food concentrations were very high. Both feeding activity and turbulence-induced behaviors incurred high metabolic costs. Swimming efficiency was concave up versus dissipation rate, suggesting that ciliary activity for food handling became more costly while swimming became more efficient with turbulence intensity. Though counter-intuitive, swimming may have become more efficient in turbulence because vorticity-induced rotation caused larvae to swim more horizontally, which requires less effort than swimming vertically against the pull of gravity. Overall, however, larvae failed to offset high activity costs with food energy gains because turbulence reduced food capture efficiency more than it enhanced food encounter rates. Younger, smaller larvae may have some energetic advantages, but competent larvae would lose energy at turbulence intensities they experience frequently, suggesting that turbulence-induced starvation may account for much of oysters' high larval mortality.

Diet of the feral cat, Felis catus, in central Australian grassland habitats during population cycles of its principal prey

Foraging theory predicts that animals should forage so as to maximize their net rate of energy gain or to minimize their risk of starvation. In situations where prey numbers fluctuate dramatically, theory predicts further that foragers will eat ‘optimal’ prey when it is abundant but expand their diet to include other prey types when the optimal prey is scarce; this is the alternative prey hypothesis. Here, we test this prediction by analyzing the diet of a mammalian predator, the feral house cat Felis catus, during periods of scarcity and abundance of the long-haired rat Rattus villosissimus. We also investigate whether the body condition of feral cats differs during different stages of the prey population cycle. Feral cats were shot during culling operations in semi-arid grassland habitats in central Queensland, Australia, and the stomach contents later identified. We found that the body condition of feral cats did not differ between phases of the prey population cycle, but the diets of cats culled when long-haired rats were scarce were significantly more diverse than when this rodent was abundant. Rats comprised about 80 % of cats’ diet by volume and frequency of occurrence when they were present, whereas birds, reptiles and invertebrates comprised the bulk of the diet when rats were not available. We conclude that, whilst feral cats are often thought to be specialist predators, they may be better considered as facultative specialists that will shift their diet in predictable ways in response to changes in the abundance of primary prey.

Variation in food availability influences prey‐capture method in antlion larvae

Abstract 1. Larvae of a Myrmecaelurus sp. are unique among antlions because they have two prey‐capture methods; they either ambush prey at the surface, or dig pit traps that prey fall in to. It was hypothesised that larvae will use the capture method that maximises their net rate of energy gain, which will be influenced by food availability (encounter rate) and by past energy inputs (body condition).2. Costs were estimated by measuring resting and activity metabolic rates and determining the duration of pit maintenance at various encounter rates with ants that served as prey. Benefits were estimated from the energy gained per ant captured at different encounter rates.3. Net energy gained was higher with a pit than without one, and was influenced more by the differences in prey capture rate between the two capture methods, and less by the differences in energy costs associated with each method. The proportion of larvae that constructed pits was higher when they were in intermediate body condition than when in good or in poor body condition.4. Thus, the use of one capture method or the other depends on a combination of the influences of past net energy gain and the antlion’s most recent change in encounter rate with prey. Ambushing without a pit may serve as a default when physiological constraints limit the larvae’s ability to invest in pit construction and maintenance, or when larvae are sated, and saving the energy of pit construction and maintenance is worthwhile.

Floral Temperature and Optimal Foraging: Is Heat a Feasible Floral Reward for Pollinators?

As well as nutritional rewards, some plants also reward ectothermic pollinators with warmth. Bumble bees have some control over their temperature, but have been shown to forage at warmer flowers when given a choice, suggesting that there is some advantage to them of foraging at warm flowers (such as reducing the energy required to raise their body to flight temperature before leaving the flower). We describe a model that considers how a heat reward affects the foraging behaviour in a thermogenic central-place forager (such as a bumble bee). We show that although the pollinator should spend a longer time on individual flowers if they are warm, the increase in total visit time is likely to be small. The pollinator's net rate of energy gain will be increased by landing on warmer flowers. Therefore, if a plant provides a heat reward, it could reduce the amount of nectar it produces, whilst still providing its pollinator with the same net rate of gain. We suggest how heat rewards may link with plant life history strategies.

How long does a dive last? Foraging decisions by breath-hold divers in a patchy environment: a test of a simple model

Many theoretical models have been proposed to explain and predict the behaviour of air-breathing divers exploiting a food resource underwater. Many field observations of the behaviour of divers do not fit with the prediction that to maximize energetic gain divers should dive close to their aerobic diving limits. In an attempt to explain this paradox, Thompson & Fedak (2001, Animal Behaviour, 61, 286–297) proposed a model of diving behaviour that takes into account patchily distributed prey patches of varying quality. We tested this model experimentally in a simulated foraging set-up. We measured the diving behaviour of grey seals, Halichoerus grypus, diving to patches of varying prey density and distance from the surface. Our results were equivocal with respect to the model predictions. Seals responded to prey density, leaving low-quality patches earlier. However, this pattern was still evident at long dive distances, contrary to the prediction that during deep dives seals should stay at a patch regardless of prey density. While seals maximized dive durations at high prey densities and long distances, they did not do so at short distances. The apparent quitting strategy of the seals always produced higher net rates of energy gain than would have been achieved if they had remained at the foraging site up to their aerobic dive limit on every dive. These results indicate that seals' diving behaviour, particularly bottom duration, may indicate the relative prey availability in their environment.

Effect of Ambient Temperature on Crop Loading in the Honey Bee, Apis mellifera (Hymenoptera: Apidae)

The effect of ambient temperature on crop loading and foraging behaviour of two genotypes of cofostered honey bees (Apis mellifera Linnaeus 1758) was observed using a set of artificial flowers in a temperature-controlled flight room. When bees had to fly between flowers to collect sucrose solution they returned to the hive before fully filling their crops. Bees of the Buckfast genotype carried heavier loads than those of the Italian genotype at all ambient temperatures. Crop loading in both genotypes was similarly affected by ambient temperature, and peaked at 32 °C. When foraging from a feeder that provided sucrose solution ad lib, bees collected greater loads than when flying between flowers. Loads also increased with ambient temperature. The crop-loading decisions are better explained by maximizing energetic efficiency than net rate of energy gain and support the physiological findings that show a decrease in flight metabolic rate with increasing ambient temperatures. Ambient temperature and genotype also affected the time flying in circles above the patch of flowers before feeding, which may reflect motivational state. The effect mirrored that on crop loading. The study shows how detailed physiological and behavioural data are necessary for understanding foraging decisions, and in turn, how optimal foraging theory can strengthen the confidence in physiological measurements. Furthermore, these findings imply that global temperature changes may affect pollinators decision making and thereby affect ecosystem structure.

Estimating nest locations of bumblebee Bombus ardens from flower quality and distribution

AbstractThe central‐place forager in a social‐insect colony, e.g., the bumblebee, has been expected to maximize its net rate of energy gain to increase the success of its colony. In addition to foraging behavior, the nest location is an important factor for the success of the colony. The bumblebee's nest location would be affected by the spatial distribution of flowers and their food quality. In this study, we constructed a model to estimate bumblebee nest sites, using the net energy intake rate at available food sites for workers foraging from the nest site. We hypothesized that the probability of colony establishment at a site in coordinates (x, y) was high as the sum of the net energy intake rate I(x, y) increased. To obtain I(x, y), nectar standing crop, sugar concentration, and foraging time were measured for ten plant species in the study site covering 6.25 km2. As available flowers changed seasonally, I(x, y) was calculated for three periods: the end of April, the beginning of May, and the middle of May. To verify our hypothesis, we compared the estimations in our model with the actual nest sites of Bombus ardens found in the beginning of May and June by means of tracking bumblebees. From the results, we considered that the net energy intake rate at mid‐May might represent the probability of colony establishment, because it could affect colony persistence and reproductive success.

Morphological correlates of nectar production used by honeybees

Abstract 1. Honeybees foraging on lavender have been shown to choose inflorescences that are larger and have more flowers. If they are selecting optimally then these inflorescences should yield higher net rates of energy gain. The number and distribution of flowers within inflorescences is a complex function of age, however, which might itself influence nectar quality and availability.2. Sampling of the overnight nectar secretion of visited and unvisited inflorescences showed that younger inflorescences with more flowers produced more sugar per flower and had fewer unproductive flowers than other inflorescences, but the size of the inflorescence had no effects.3. Overall display size attracted bees to inspect inflorescences, as inflorescences that were inspected but rejected were larger and/or had larger or more bracts than those that were ignored. Bees, however, accepted more productive inflorescences based on different cues: inflorescence age and number of flowers.4. Inflorescence choice thus appeared to reflect a two‐stage decision process based on different morphological criteria at each stage.

Ecological mechanisms favouring behavioural diversification in the absence of morphological diversification: a theoretical examination using brook charr (Salvelinus fontinalis)

1. Behavioural diversification is thought to be an important initial step in the origin of resource polymorphisms. We developed a model for young brook charr (Salvelinus fontinalis Mitchill) to examine four mechanisms that could generate a U-shaped relationship between growth rate (fitness) and the proportion of time spent moving that would favour alternative foraging tactics in the absence of obvious differences in body size and shape. 2. Recently emerged brook charr of similar size and shape inhabit still-water pools along the sides of streams. Some individuals tend to sit and wait for crustacean prey at the pool substrate near the bank, while others tend to search actively for insect prey at the pool surface away from the bank. 3. The ecological mechanisms modelled were (i) the relationship between the rate of prey capture and the proportion of time spent moving is curvilinear, such that net rate of energy gain is maximized at two different levels of activity; (ii) switching between foraging locations and, hence, tactics involves lost opportunity and travel costs; (iii) switching between prey types and, hence, tactics involves a learning cost; and (iv) foraging success is status-dependent with individuals switching between tactics having a lower status than those specializing at a tactic. 4. Singly, no mechanism predicted the U-shaped relationship between growth rate and the proportion of time spent moving. Together, a U-shaped relationship was obtained, indicating that the behavioural diversification and diversifying selection observed in the field may be a consequence of multiple, subtle mechanisms.

Does the Flower Constancy of Bumble Bees Reflect Foraging Economics?

AbstractWe examined the effects of floral reward level and spatial arrangement on the propensity of bumble bees to exhibit flower constancy. In three separate experiments, we compared the flower constancy of bees on dimorphic arrays of blue and yellow flowers that differed either in reward concentration, reward volume, or inter‐flower distance. Overall, flower choice patterns varied among bees, ranging from random selection to complete constancy. When flowers contained greater reward volumes and were spaced farther apart, bees showed less flower constancy and more moves to closely neighbouring flowers. Changes in reward concentration had no effect on flower constancy; however, more dilute rewards produced shorter flight times between flowers. In addition, there was a strong positive relationship between degree of flower constancy and net rate of energy gain when flowers were spaced farther apart, indicating that constant bees were more economic foragers than inconstant bees. Together, these results support the view that the flower constancy of pollinators reflects an economic foraging decision.

Damage to saplings by red deer ( Cervus elaphus): effect of foliage height and structure

Red deer cause heavy damage to forest production, hence the need to better understand what determines their dietary choices. Deer are selective herbivores. According to the optimal foraging theory (OFT), they should select foods that maximise their net rate of energy gain. For a given nutritional quality, we predict that plant architecture and height should affect deer feeding preferences through intake rate variations. In the first experiment, 12 tame red deer with shoulder height (SH) ranging from 82 to 105 cm were individually offered fresh bunches of Salix caprea at seven heights regularly spaced from 25 to 205 cm. Animals preferred bunches at 85 and 115 cm height (29 and 32% of feeding time). There was a positive relationship between average consumption height (CH) and animal shoulder height (CH=0.99×SH; r 2=0.43; P<0.05) but intake rate did not change with consumption height. In the second experiment, 10 fresh bunches of Quercus pedunculata were offered on three wooden structures, resembling a bushy topped (BT), a normal ( N) and a five-shoot (FS) oak sapling. For each structure, average bunch height was set at preferred consumption height. Five tame hinds were observed during individual cafeteria trials (double- then triple-choice). Intake rate was slightly higher for FS than for the other structures (FS=23.7 g DM/min, N=22.3 g DM/min and BT=22.0 g DM/min). Preferences were expressed as the proportion of feeding time during the first min of test. In the double-choice trials, preference was higher for FS relative to BT (61%–39%), and for N relative to BT (62%–38%), but similar between FS and N (57%–43%). In the triple-choice trials, preference for FS (44%), N (24%) and BT (32%) differed from 33%–33%–33%. In both trials, there was thus a slight preference for FS, i.e. for the structure where all bunches were widely spaced and at the hinds’ preferred height. Foliage height and, to a lesser extent, structure of saplings thus affect the feeding preferences of red deer. These results can contribute to improving management practices to limit deer damages to commercial plantations.

Interplay between the energetics of foraging and thermoregulatory costs in the green‐backed firecrown hummingbird Sephanoides sephaniodes

AbstractIn response to the interplay between variation in food quality and energetic demands, the foraging behaviour of captive green‐backed firecrown hummingbirds Sephanoides sephaniodes was studied. Hummingbirds were exposed to two temperatures (25°C vs 15°C), two food qualities (0.5 vs 0.75 m sucrose solutions), and two costs of feeding (birds were provided with feeders with and without a perch). Food selection and consumption were measured, as well as time budgets and metabolic rate while feeding. We predicted that when given a choice, birds would minimize the cost of feeding by selecting feeders with a perch and with a high sugar concentration. However, rather than increasing energy consumption when energy availability was low and thermoregulatory demands were high, hummingbirds remained perched. They reduced feeding and spent most of their time perching. Our results identify a novel behavioural and physiological strategy in hummingbirds. These birds seem to shift their foraging behaviour depending on thermoregulatory and feeding costs. When these costs are high, rather than matching them with increased energy consumption, hummingbirds reduce energy costs by reducing activity. They seemed to adopt the following strategy: when food quality was high and thermoregulatory demands were low, they adopted a high‐expense lifestyle. In contrast, when thermoregulatory costs were high, they adopted an energy conserving strategy even when food quality was high. We hypothesize that limitations imposed by physiological processes may explain why animals do not forage during all available time and why under some circumstances they choose foraging behaviours with lower rates of net energy gain.