The power of feeder-mask respirometry as a method for examining hummingbird energetics

Reporting guidelines for terrestrial respirometry: Building openness, transparency of metabolic rate and evaporative water loss data

Respirometry is an important tool for understanding whole-animal energy and water balance in relation to the environment. Consequently, the growing number of studies using respirometry over the last decade warrants reliable reporting and data sharing for effective dissemination and research synthesis. We provide a checklist guideline on five key sections to facilitate the transparency, reproducibility, and replicability of respirometry studies: 1) materials, set up, plumbing, 2) subject conditions/maintenance, 3) measurement conditions, 4) data processing, and 5) data reporting and statistics, each with explanations and example studies. Transparency in reporting and data availability has benefits on multiple fronts. Authors can use this checklist to design and report on their study, and reviewers and editors can use the checklist to assess the reporting quality of the manuscripts they review. Improved standards for reporting will enhance the value of primary studies and will greatly facilitate the ability to carry out higher quality research syntheses to address ecological and evolutionary theories.

Anna's hummingbird (Calypte anna) physiological response to novel thermal and hypoxic conditions at high elevations

Many species have not tracked their thermal niches upslope as predicted by climate change, potentially because higher elevations are associated with abiotic challenges beyond temperature. To better predict whether organisms can continue to move upslope with rising temperatures, we need to understand their physiological performance when subjected to novel high-elevation conditions. Here, we captured Anna's hummingbirds - a species expanding their elevational distribution in concordance with rising temperatures - from across their current elevational distribution and tested their physiological response to novel abiotic conditions. First, at a central aviary within their current elevational range, we measured hovering metabolic rate to assess their response to oxygen conditions and torpor use to assess their response to thermal conditions. Second, we transported the hummingbirds to a location 1200 m above their current elevational range limit to test for an acute response to novel oxygen and thermal conditions. Hummingbirds exhibited lower hovering metabolic rates above their current elevational range limit, suggesting lower oxygen availability may reduce performance after an acute exposure. Alternatively, hummingbirds showed a facultative response to thermal conditions by using torpor more frequently and for longer. Finally, post-experimental dissection found that hummingbirds originating from higher elevations within their range had larger hearts, a potential plastic response to hypoxic environments. Overall, our results suggest lower oxygen availability and low air pressure may be difficult challenges to overcome for hummingbirds shifting upslope as a consequence of rising temperatures, especially if there is little to no long-term acclimatization. Future studies should investigate how chronic exposure and acclimatization to novel conditions, as opposed to acute experiments, may result in alternative outcomes that help organisms better respond to abiotic challenges associated with climate-induced range shifts.

Locomotor energetics in the Indonesian blue-tongued skink (Tiliqua gigas) with implications for the cost of belly-dragging in early tetrapods

During the last decade, biomechanical and kinematic studies have suggested that a belly-dragging gait may have represented a critical locomotor stage during tetrapod evolution. This form of locomotion is hypothesized to facilitate animals to move on land with relatively weaker pectoral muscles. The Indonesian blue-tongued skink (Tiliqua gigas) is known for its belly-dragging locomotion and is thought to employ many of the same spatiotemporal gait characteristics of stem tetrapods. Conversely, the savannah monitor (Varanus exanthematicus) employs a raised quadrupedal gait. Thus, differences in the energetic efficiency of locomotion between these taxa may elucidate the role of energetic optimization in driving gait shifts in early tetrapods. Five Tiliqua and four Varanus were custom-fitted for 3D printed helmets that, combined with a Field Metabolic System, were used to collect open-flow respirometry data including O₂ consumption, CO₂ production, water vapor pressure, barometric pressure, room temperature, and airflow rates. Energetic data were collected for each species at rest, and when walking at three different speeds. Energetic consumption in each taxon increased at greater speeds. On a per-stride basis, energetic costs appear similar between taxa. However, significant differences were observed interspecifically in terms of net cost of transport. Overall, energy expenditure was ~20% higher in Tiliqua at equivalent speeds, suggesting that belly-dragging does impart a tangible energetic cost during quadrupedal locomotion. This cost, coupled with the other practical constraints of belly-dragging (e.g., restricting top-end speed and reducing maneuverability in complex terrains) may have contributed to the adoption of upright quadrupedal walking throughout tetrapod locomotor evolution.

Neonicotinoid pesticides exert metabolic effects on avian pollinators

Neonicotinoids are neurotoxic systemic insecticides applied extensively worldwide. The impacts of common neonicotinoids like imidacloprid on non-target invertebrate pollinators have been widely studied, however effects on vertebrate pollinators have received little attention. Here, we describe the first study evaluating the effects of short-term (3 d) exposure to a range of environmentally relevant concentrations ([Formula: see text] to [Formula: see text]Body Weight) of imidacloprid on wild-caught ruby-throated hummingbirds. Within 2 h of exposure, hummingbirds exhibited a significant depression in energy expenditure (up to [Formula: see text]). We did not observe significant effects on foraging behaviour measured in the subsequent 2 h to 4 h, although the effect size estimate was large (0.29). We also analyzed tissues collected 24 h after the final dose and did not observe significant effects on immune response or cholinesterase activity, although this may be related to our small sample size. We determined that hummingbirds excrete imidacloprid quickly (elimination half-life of [Formula: see text]) relative to other bird species. Hummingbirds have high energetic demands and store relatively little energy, especially during migration and breeding seasons. Therefore, changes in their metabolism following exposures to imidacloprid observed herein could bear important survivorship consequences for hummingbirds.

Low ambient temperature reduces the time for fuel switching in the ruby-throated hummingbird (Archilochus colubris)

Physiological adaptations that enhance flux through the sugar oxidation cascade permit hummingbirds to rapidly switch between burning lipids when fasted to burning ingested sugars when fed. Hummingbirds may be able to exert control over the timing and extent of use of ingested sugars by varying digestive rates when under pressure to accumulate energy stores or acquire energy in response to heightened energy demands. We hypothesized that hummingbirds would modulate the timing of a switch to reliance on ingested sugars differently when facing distinct energetic demands (cool versus warm ambient temperatures). The timing of the oxidation of a single nectar meal to fuel metabolism was assessed by open-flow respirometry, while the time to first excretion following the meal was used as a proxy for digestive throughput time. As predicted, birds showed a more rapid switch in respiratory exchange ratio (RER = rate of O₂ consumption/CO₂ production) and excreted earlier when held at cool temperatures compared to warm. In both cases, RER peaked barely above 1.0 indicating ingested sugar fueled ≈100% of resting metabolism. Our findings suggest that energetic demands modulate the rate of fuel switching through shifts of the sugar oxidation cascade. The speed of this shift may involve decreases in gut passage times which have previously been thought to be inflexible, or may be caused by changes in circulation as a result of low ambient temperature.

Integrating morphology and kinematics in the scaling of hummingbird hovering metabolic rate and efficiency

Wing kinematics and morphology are influential upon the aerodynamics of flight. However, there is a lack of studies linking these variables to metabolic costs, particularly in the context of morphological adaptation to body size. Furthermore, the conversion efficiency from chemical energy into movement by the muscles (mechanochemical efficiency) scales with mass in terrestrial quadrupeds, but this scaling relationship has not been demonstrated within flying vertebrates. Positive scaling of efficiency with body size may reduce the metabolic costs of flight for relatively larger species. Here, we assembled a dataset of morphological, kinematic, and metabolic data on hovering hummingbirds to explore the influence of wing morphology, efficiency, and mass on hovering metabolic rate (HMR). We hypothesize that HMR would decline with increasing wing size, after accounting for mass. Furthermore, we hypothesize that efficiency will increase with mass, similarly to other forms of locomotion. We do not find a relationship between relative wing size and HMR, and instead find that the cost of each wingbeat increases hyperallometrically while wingbeat frequency declines with increasing mass. This suggests that increasing wing size is metabolically favourable over cycle frequency with increasing mass. Further benefits are offered to larger hummingbirds owing to the positive scaling of efficiency.

Sugar Metabolism in Hummingbirds and Nectar Bats

Hummingbirds and nectar bats coevolved with the plants they visit to feed on floral nectars rich in sugars. The extremely high metabolic costs imposed by small size and hovering flight in combination with reliance upon sugars as their main source of dietary calories resulted in convergent evolution of a suite of structural and functional traits. These allow high rates of aerobic energy metabolism in the flight muscles, fueled almost entirely by the oxidation of dietary sugars, during flight. High intestinal sucrase activities enable high rates of sucrose hydrolysis. Intestinal absorption of glucose and fructose occurs mainly through a paracellular pathway. In the fasted state, energy metabolism during flight relies on the oxidation of fat synthesized from previously-ingested sugar. During repeated bouts of hover-feeding, the enhanced digestive capacities, in combination with high capacities for sugar transport and oxidation in the flight muscles, allow the operation of the “sugar oxidation cascade”, the pathway by which dietary sugars are directly oxidized by flight muscles during exercise. It is suggested that the potentially harmful effects of nectar diets are prevented by locomotory exercise, just as in human hunter-gatherers who consume large quantities of honey.

Wingbeat kinematics and energetics during weightlifting in hovering hummingbirds across an elevational gradient

Hummingbirds differentially modify flight kinematics in response to the type of challenge imposed. Weightlifting is associated with increases in stroke amplitude (the angle swept by the wings) to increase the angular velocity of the wings and generate the requisite lift, but only up to 160°. Conversely, flight in hypodense air is accomplished by increasing the angular velocity of the wing through increases in wingbeat frequency and stroke amplitudes, with larger increases in amplitude than seen in weightlifting flight. The kinematic differences between these two challenges may be facilitated by the lower energetic costs associated with overcoming drag and inertial forces over the wing during hypodense flight. Thus, we hypothesized that energetic expenditure is what limits the kinematics of weightlifting flight, with lower air densities permitting increases in angular velocity at comparatively lower costs. To explore the kinematic and energetic effects of air density and weightlifting on hovering flight performance, video and respirometric recordings of weightlifting were performed on four species of hummingbirds across an elevational gradient. Contrary to our hypothesis, wingbeat frequency did not vary due to elevation. Instead, wingbeat frequency seems to increase depending on the power requirements for sustaining hovering flight. Furthermore, metabolic rates during hovering increased with angular velocity alone, independent of elevation. Thus, it appears that the differential responses to flight challenges are not driven by variation in the flight media.

Measurement of Whole-Body CO2 Production in Birds Using Real-Time Laser-Derived Measurements of Hydrogen (δ(2)H) and Oxygen (δ(18)O) Isotope Concentrations in Water Vapor from Breath

The doubly labeled water (DLW) method is commonly used to measure energy expenditure in free-living wildlife and humans. However, DLW studies involving animals typically require three blood samples, which can affect behavior and well-being. Moreover, measurement of H (δ(2)H) and O (δ(18)O) isotope concentrations in H2O derived from blood using conventional isotope ratio mass spectrometry is technically demanding, time-consuming, and often expensive. A novel technique that would avoid these constraints is the real-time measurement of δ(2)H and δ(18)O in the H2O vapor of exhaled breath using cavity ring-down (CRD) spectrometry, provided that δ(2)H and δ(18)O from body H2O and breath were well correlated. Here, we conducted a validation study with CRD spectrometry involving five zebra finches (Taeniopygia guttata), five brown-headed cowbirds (Molothrus ater), and five European starlings (Sturnus vulgaris), where we compared δ(2)H, δ(18)O, and rCO2 (rate of CO2 production) estimates from breath with those from blood. Isotope concentrations from blood were validated by comparing dilution-space estimates with measurements of total body water (TBW) obtained from quantitative magnetic resonance. Isotope dilution-space estimates from δ(2)H and δ(18)O values in the blood were similar to and strongly correlated with TBW measurements (R(2) = 0.99). The (2)H and (18)O (ppm) in breath and blood were also highly correlated (R(2) = 0.99 and 0.98, respectively); however, isotope concentrations in breath were always less enriched than those in blood and slightly higher than expected, given assumed fractionation values between blood and breath. Overall, rCO2 measurements from breath were strongly correlated with those from the blood (R(2) = 0.90). We suggest that this technique will find wide application in studies of animal and human energetics in the field and laboratory. We also provide suggestions for ways this technique could be further improved.

Carbon stable-isotope tracking in breath for comparative studies of fuel use

Almost half a century ago, researchers demonstrated that the ratio of stable carbon isotopes in exhaled breath of rats and humans could reveal the oxidation of labeled substrates in vivo, opening a new chapter in the study of fuel use, the fate of ingested substrates, and aerobic metabolism. Until recently, the combined use of respirometry and stable-isotope tracer techniques had not been broadly employed to study fuel use in other animal groups. In this review, we summarize the history of this approach in human and animal research and define best practices that maximize its utility. We also summarize several case studies that use stable-isotope measurements of breath to explore the limits of aerobic metabolism and substrate turnover among several species and various physiological states. We highlight the importance of a comparative approach in revealing the profound effects that phylogeny, ecology, and behavior can have in shaping aerobic metabolism and energetics as well as the fundamental biological principles that underlie fuel use and metabolic function across taxa. New analytical equipment and refinement of methodology make the combined use of respirometry and stable-isotope tracer techniques simpler to perform, less costly, and more field ready than ever before.

Hovering performance of Anna's hummingbirds (Calypte anna) in ground effect

Aerodynamic performance and energetic savings for flight in ground effect are theoretically maximized during hovering, but have never been directly measured for flying animals. We evaluated flight kinematics, metabolic rates and induced flow velocities for Anna's hummingbirds hovering at heights (relative to wing length R = 5.5 cm) of 0.7R, 0.9R, 1.1R, 1.7R, 2.2R and 8R above a solid surface. Flight at heights less than or equal to 1.1R resulted in significant reductions in the body angle, tail angle, anatomical stroke plane angle, wake-induced velocity, and mechanical and metabolic power expenditures when compared with flight at the control height of 8R. By contrast, stroke plane angle relative to horizontal, wingbeat amplitude and wingbeat frequency were unexpectedly independent of height from ground. Qualitative smoke visualizations suggest that each wing generates a vortex ring during both down- and upstroke. These rings expand upon reaching the ground and present a complex turbulent interaction below the bird's body. Nonetheless, hovering near surfaces results in substantial energetic benefits for hummingbirds, and by inference for all volant taxa that either feed at flowers or otherwise fly close to plant or other surfaces.

Metabolic scaling in animals: methods, empirical results, and theoretical explanations

Life on earth spans a size range of around 21 orders of magnitude across species and can span a range of more than 6 orders of magnitude within species of animal. The effect of size on physiology is, therefore, enormous and is typically expressed by how physiological phenomena scale with mass(b). When b ≠ 1 a trait does not vary in direct proportion to mass and is said to scale allometrically. The study of allometric scaling goes back to at least the time of Galileo Galilei, and published scaling relationships are now available for hundreds of traits. Here, the methods of scaling analysis are reviewed, using examples for a range of traits with an emphasis on those related to metabolism in animals. Where necessary, new relationships have been generated from published data using modern phylogenetically informed techniques. During recent decades one of the most controversial scaling relationships has been that between metabolic rate and body mass and a number of explanations have been proposed for the scaling of this trait. Examples of these mechanistic explanations for metabolic scaling are reviewed, and suggestions made for comparing between them. Finally, the conceptual links between metabolic scaling and ecological patterns are examined, emphasizing the distinction between (1) the hypothesis that size- and temperature-dependent variation among species and individuals in metabolic rate influences ecological processes at levels of organization from individuals to the biosphere and (2) mechanistic explanations for metabolic rate that may explain the size- and temperature-dependence of this trait.

Into turbulent air: size-dependent effects of von Kármán vortex streets on hummingbird flight kinematics and energetics

Animal fliers frequently move through a variety of perturbed flows during their daily aerial routines. However, the extent to which these perturbations influence flight control and energetic expenditure is essentially unknown. Here, we evaluate the kinematic and metabolic consequences of flight within variably sized vortex shedding flows using five Anna's hummingbirds feeding from an artificial flower in steady control flow and within vortex wakes produced behind vertical cylinders. Tests were conducted at three horizontal airspeeds (3, 6 and 9 m s(-1)) and using three different wake-generating cylinders (with diameters equal to 38, 77 and 173% of birds' wing length). Only minimal effects on wing and body kinematics were demonstrated for flight behind the smallest cylinder, whereas flight behind the medium-sized cylinder resulted in significant increases in the variances of wingbeat frequency, and variances of body orientation, especially at higher airspeeds. Metabolic rate was, however, unchanged relative to that of unperturbed flight. Hummingbirds flying within the vortex street behind the largest cylinder exhibited highest increases in variances of wingbeat frequency, and of body roll, pitch and yaw amplitudes at all measured airspeeds. Impressively, metabolic rate under this last condition increased by up to 25% compared with control flights. Cylinder wakes sufficiently large to interact with both wings can thus strongly affect stability in flight, eliciting compensatory kinematic changes with a consequent increase in flight metabolic costs. Our findings suggest that vortical flows frequently encountered by aerial taxa in diverse environments may impose substantial energetic costs.

Backward flight in hummingbirds employs unique kinematic adjustments and entails low metabolic cost

Backward flight is a frequently used transient flight behavior among members of the species-rich hummingbird family (Trochilidae) when retreating from flowers, and is known from a variety of other avian and hexapod taxa, but the biomechanics of this intriguing locomotor mode have not been described. We measured rates of oxygen uptake (V(O2)) and flight kinematics of Anna's hummingbirds, Calypte anna (Lesson), within a wind tunnel using mask respirometry and high-speed videography, respectively, during backward, forward and hovering flight. We unexpectedly found that in sustained backward flight is similar to that in forward flight at equivalent airspeed, and is about 20% lower than hovering V(O2). For a bird that was measured throughout a range of backward airspeeds up to a speed of 4.5 m s(-1), the power curve resembled that of forward flight at equivalent airspeeds. Backward flight was facilitated by steep body angles coupled with substantial head flexion, and was also characterized by a higher wingbeat frequency, a flat stroke plane angle relative to horizontal, a high stroke plane angle relative to the longitudinal body axis, a high ratio of maximum:minimum wing positional angle, and a high upstroke:downstroke duration ratio. Because of the convergent evolution of hummingbird and some hexapod flight styles, flying insects may employ similar kinematics while engaged in backward flight, for example during station keeping or load lifting. We propose that backward flight behavior in retreat from flowers, together with other anatomical, physiological, morphological and behavioral adaptations, enables hummingbirds to maintain strictly aerial nectarivory.

The heart rate method for estimating metabolic rate: review and recommendations

Under most circumstances heart rate (f(H)) is correlated with the rate of oxygen consumption (VO(2)) and hence the rate of energy expenditure or metabolic rate (MR). For over 60 years this simple principle has underpinned the use of heart rate to estimate metabolic rate in a range of animal species and to answer questions about their physiology, behaviour and ecology. The heart rate method can be applied both quantitatively and qualitatively. The quantitative approach is a two-stage process where firstly f(H) and MR are measured simultaneously under controlled conditions and a predictive calibration relationship derived. Secondly, measurements of heart rate are made and converted to estimates of MR using the calibration relationship. The qualitative approach jumps directly to the second stage, comparing estimates of f(H) under different circumstances and drawing conclusions about MR under the assumption that a relationship exists. This review describes the range of studies which have adopted either the quantitative or qualitative approach to estimating the MR of birds, mammals and reptiles. Studies have tended to focus on species, states and questions which are hard to measure, control or define using other techniques. For example, species studied include large, wide-ranging species such as ungulates, marine predators, and domestic livestock while research questions have concerned behaviours such as flight, diving and the effects of stress. In particular, the qualitative approach has applied to circumstances and/or species where it may be hard or impossible to derive a calibration relationship for practical reasons. The calibration process itself can be complex and a number of factors such as body mass, activity state and stress levels can affect the relationship between f(H) and VO(2). I recommend that a quantitative approach be adopted wherever possible but that this may entail deriving a calibration relationship which is practical and applicable, rather than the most accurate possible. I conclude with a series of recommendations for the application and development of this method.

Assessing the development and application of the accelerometry technique for estimating energy expenditure

A theoretically valid proxy of energy expenditure is the acceleration of an animal's mass due to the movement of its body parts. Acceleration can be measured by an accelerometer and recorded onto a data logging device. Relevant studies have usually derived a measure of acceleration from the raw data that represents acceleration purely due to movement of the animal. This is termed 'overall dynamic body acceleration' (ODBA) and to date has proved a robust derivation of acceleration for use as an energy expenditure proxy. Acceleration data loggers are generally easy to deploy and the measures recorded appear robust to slight variation in location and orientation. This review discusses important issues concerning the accelerometry technique for estimating energy expenditure and ODBA; deriving ODBA, calibrating ODBA, acceleration logger recording frequencies, scenarios where ODBA is less likely to be valid, and the power in recording acceleration and heart rate together. While present evidence suggests that ODBA may not quantify energy expenditure during diving by birds and mammals, several recent studies have assessed changes in mechanical work in such species qualitatively through variation in ODBA during periods of submergence. The use of ODBA in field metabolic studies is likely to continue growing, supported by its relative ease of use and range of applications.