Great tits (Parus major) in a west European temperate forest show little seasonal variation in metabolic energy requirements

Understanding how birds annually allocate energy to cope with changing environmental conditions and physiological states is a crucial question in avian ecology. There are several hypotheses to explain species' energy allocation. One prominent hypothesis suggests higher energy expenditure in winter due to increased thermoregulatory costs. The "reallocation" hypothesis suggests no net difference in seasonal energy requirements, while the "increased demand" hypothesis predicts higher energy requirements during the breeding season. Birds are expected to adjust their mass and/or metabolic intensity in ways that are consistent with their energy requirements. Here, we look for metabolic signatures of seasonal variation in energy requirements of a resident passerine of a temperate-zone (great tit, Parus major). To do so, we measured whole-body and mass-independent basal (BMR), summit (Msum), and field (FMR) metabolic rates during late winter and during breeding in Belgian great tits. During the breeding season, birds had on average 10% higher whole-body BMR and FMR compared to winter, while their Msum decreased by 7% from winter to breeding. Mass-independent metabolic rates showed a 10% increase in BMR and a 7% decrease in Msum from winter to breeding. Whole-body BMR was correlated with Msum, but this relationship did not hold for mass-independent metabolic rates. The modest seasonal change we observed suggests that great tits in our temperature study area maintain a largely stable energy budget throughout the year, which appears mostly consistent with the reallocation hypothesis.

Adaptive and non-adaptive convergent evolution in feather reflectance of California Channel Islands songbirds

Convergent evolution is widely regarded as a signature of adaptation. However, testing the adaptive consequences of convergent phenotypes is challenging, making it difficult to exclude non-adaptive explanations for convergence. Here, we combined feather reflectance spectra and phenotypic trajectory analyses with visual and thermoregulatory modelling to test the adaptive significance of dark plumage in songbirds of the California Channel Islands. By evolving dark dorsal plumage, island birds are generally less conspicuous to visual-hunting raptors in the island environment than mainland birds. Dark dorsal plumage also reduces the energetic demands associated with maintaining homeothermy in the cool island climate. We also found an unexpected pattern of convergence, wherein the most divergent island populations evolved greater reflectance of near-infrared radiation. However, our heat flux models indicate that elevated near-infrared reflectance is not adaptive. Analysis of feather microstructure suggests that mainland-island differences are related to coloration of feather barbs and barbules rather than their structure. Our results indicate that adaptive and non-adaptive mechanisms interact to drive plumage evolution in this system. This study sheds light on the mechanisms driving the association between dark colour and wet, cold environments across the tree of life, especially in island birds.

Seasonal Changes in Hematological Parameters in House Sparrows of Subtropical Pakistan

House sparrow is a globally adaptive bird. The way this creature adapted to all areas of the world, having different selection pressures, is interesting to understand. The present study is focused on seasonal changes, having different selection pressures and how it is adapted to these changes and whether hematological flexibility plays a role in this success. House sparrow's adaptations in the same area, during different seasons, have been studied in a sub-tropical area, Potohar, Pakistan. We used hematological parameter analysis for this purpose. Blood samples were collected from Sparrows in winter, spring, and summer and analyzed for some hematological parameters. White blood cells (WBCs) were higher in spring and summer which may relate to mating promiscuity. Sparrows were more stressed in summer. The Red blood cells (RBCs) and hematocrit (Hct) were greater in summer. Mean corpuscular volume (MCV) is lower in summer. This may have an adaptation to cope with high stress in summer as small-size RBCs increase gaseous exchange. Platelets were not affected by season or gender. Mean corpuscular volume and Mean corpuscular hemoglobin (MCH) are positively correlated with each other. Red blood cells, hemoglobin (Hb) and MCV were higher in males during the spring season perhaps as an adaptation to energetic activities during spring like mating calls and search for nesting sites. White blood cells remained the same in both genders in summer and winter, and effected in spring may be related to the mating system. Behavioral state is linked with physiological states that shows tradeoff and life history traits. This study is a small effort to know this incredible species. We can work further in different parts of the world to explore different aspects of it.

Sex-specific energy management strategies in response to training for increased foraging effort prior to reproduction in captive zebra finches

Free-living animals often engage in behaviour that involves high rates of workload and results in high daily energy expenditure (DEE), such as reproduction. However, the evidence for elevated DEE accompanying reproduction remains equivocal. In fact, many studies have found no difference in DEE between reproducing and non-reproducing females. One of the hypotheses explaining the lack of difference is the concept of an 'energetic ceiling'. However, it is unclear whether the lack of increase in energy expenditure is due to the existence of an energetic ceiling and/or compensation by males during parental care. To investigate whether an energetic ceiling exists, we experimentally manipulated foraging effort in captive zebra finches, Taeniopygia guttata, creating two groups with high and low foraging efforts followed by both groups breeding in the low foraging effort common garden condition. DEE was measured in both sexes throughout the experiment. We show sex-specific energy management strategies in response to training for increased foraging effort prior to reproduction. Specifically, males and females responded differently to the high foraging effort treatment and subsequently to chick rearing in terms of energy expenditure. Our results also suggest that there is an energetic ceiling in females and that energetic costs incurred prior to reproduction can be carried over into subsequent stages of reproduction in a sex-specific manner.

Seasonal temperature acclimatization in a semi-fossorial mammal and the role of burrows as thermal refuges

Small mammals in habitats with strong seasonal variation in the thermal environment often exhibit physiological and behavioral adaptations for coping with thermal extremes and reducing thermoregulatory costs. Burrows are especially important for providing thermal refuge when above-ground temperatures require high regulatory costs (e.g., water or energy) or exceed the physiological tolerances of an organism. Our objective was to explore the role of burrows as thermal refuges for a small endotherm, the pygmy rabbit (Brachylagus idahoensis), during the summer and winter by quantifying energetic costs associated with resting above and below ground. We used indirect calorimetry to determine the relationship between energy expenditure and ambient temperature over a range of temperatures that pygmy rabbits experience in their natural habitat. We also measured the temperature of above- and below-ground rest sites used by pygmy rabbits in eastern Idaho, USA, during summer and winter and estimated the seasonal thermoregulatory costs of resting in the two microsites. Although pygmy rabbits demonstrated seasonal physiological acclimatization, the burrow was an important thermal refuge, especially in winter. Thermoregulatory costs were lower inside the burrow than in above-ground rest sites for more than 50% of the winter season. In contrast, thermal heterogeneity provided by above-ground rest sites during summer reduced the role of burrows as a thermal refuge during all but the hottest periods of the afternoon. Our findings contribute to an understanding of the ecology of small mammals in seasonal environments and demonstrate the importance of burrows as thermal refuge for pygmy rabbits.

Free-living greylag geese adjust their heart rates and body core temperatures to season and reproductive context

Animals adaptively regulate their metabolic rate and hence energy expenditure over the annual cycle to cope with energetic challenges. We studied energy management in greylag geese. In all geese, profound seasonal changes of heart rate (f_H) and body temperature (T_b) showed peaks in summer and troughs during winter, and also daily modulation of f_H and T_b. Daily mean f_H was on average 22% lower at the winter trough than at the summer peak, whereas daily mean T_b at the winter trough was only about 1 °C below the summer peak. Daily means of T_b together with those of air temperature and day length were the most important predictors of daily mean f_H, which was further modulated by precipitation, reproductive state, and, to a minor degree, social rank. Peaks of f_H and T_b occurred earlier in incubating females compared to males. Leading goslings increased daily mean f_H. Our results suggest that in greylag geese, pronounced changes of f_H over the year are caused by photoperiod-induced changes of endogenous heat production. Similar to large non-hibernating mammals, tolerance of lower T_b during winter seems the major factor permitting this. On top of these major seasonal changes, f_H and T_b are elevated in incubating females.

Energetics of communal roosting in chestnut-crowned babblers: implications for group dynamics and breeding phenology

For many endotherms, communal roosting saves energy in cold conditions, but how this might affect social dynamics or breeding phenology is not well understood. Using chestnut-crowned babblers (Pomatostomus ruficeps), we studied the effects of nest use and group size on roosting energy costs. These 50 g cooperatively breeding passerine birds of outback Australia breed from late winter to early summer and roost in huddles of up to 20 in single-chambered nests. We measured babbler metabolism at three ecologically relevant temperatures: 5°C (similar to minimum nighttime temperatures during early breeding), 15°C (similar to nighttime temperatures during late breeding) and 28°C (thermal neutrality). Nest use alone had modest effects: even for solitary babblers at 5°C, it reduced nighttime energy expenditures by <15%. However, group-size effects were substantial, with savings of up to 60% in large groups at low temperatures. Babblers roosting in groups of seven or more at 5°C, and five or more at 15°C, did not need to elevate metabolic rates above basal levels. Furthermore, even at 28°C (thermoneutral for solitary babblers), individuals in groups of four or more had 15% lower basal metabolic rate than single birds, hinting that roosting in small groups is stressful. We suggest that the substantial energy savings of communal roosting at low temperatures help explain why early breeding is initiated in large groups and why breeding females, which roost alone and consequently expend 120% more energy overnight than other group members, suffer relatively higher mortality than communally roosting group mates.

Thermoregulation in endotherms: physiological principles and ecological consequences

In a seminal study published nearly 70 years ago, Scholander et al. (Biol Bull 99:259-271, 1950) employed Newton's law of cooling to describe how metabolic rates (MR) in birds and mammals vary predictably with ambient temperature (T a). Here, we explore the theoretical consequences of Newton's law of cooling and show that a thermoregulatory polygon provides an intuitively simple and yet useful description of thermoregulatory responses in endothermic organisms. This polygon encapsulates the region in which heat production and dissipation are in equilibrium and, therefore, the range of conditions in which thermoregulation is possible. Whereas the typical U-shaped curve describes the relationship between T a and MR at rest, thermoregulatory polygons expand this framework to incorporate the impact of activity, other behaviors and environmental conditions on thermoregulation and energy balance. We discuss how this framework can be employed to study the limits to effective thermoregulation and their ecological repercussions, allometric effects and residual variation in MR and thermal insulation, and how thermoregulatory requirements might constrain locomotor or reproductive performance (as proposed, for instance, by the heat dissipation limit theory). In many systems the limited empirical knowledge on how organismal traits may respond to environmental changes prevents physiological ecology from becoming a fully developed predictive science. In endotherms, however, we contend that the lack of theoretical developments that translate current physiological understanding into formal mechanistic models remains the main impediment to study the ecological and evolutionary repercussions of thermoregulation. In spite of the inherent limitations of Newton's law of cooling as an oversimplified description of the mechanics of heat transfer, we argue that understanding how systems that obey this approximation work can be enlightening on conceptual grounds and relevant as an analytical and predictive tool to study ecological phenomena. As such, the proposed approach may constitute a powerful tool to study the impact of thermoregulatory constraints on variables related to fitness, such as survival and reproductive output, and help elucidating how species will be affected by ongoing climate change.

Three dimensional printing as an effective method of producing anatomically accurate models for studies in thermal ecology

Hollow copper models painted to match the reflectance of the animal subject are standard in thermal ecology research. While the copper electroplating process results in accurate models, it is relatively time consuming, uses caustic chemicals, and the models are often anatomically imprecise. Although the decreasing cost of 3D printing can potentially allow the reproduction of highly accurate models, the thermal performance of 3D printed models has not been evaluated. We compared the cost, accuracy, and performance of both copper and 3D printed lizard models and found that the performance of the models were statistically identical in both open and closed habitats. We also find that 3D models are more standard, lighter, durable, and inexpensive, than the copper electroformed models.

Acclimatization of seasonal energetics in northern cardinals (Cardinalis cardinalis) through plasticity of metabolic rates and ceilings

Northern cardinals (Cardinalis cardinalis) are faced with energetically expensive seasonal challenges that must be met to ensure survival, including thermoregulation in winter and reproductive activities in summer. Contrary to predictions of life history theory that suggest breeding metabolic rate should be the apex of energetic effort, winter metabolism exceeds that during breeding in several temperate resident bird species. By examining whole-animal, tissue and cellular function, we ask whether seasonal acclimatization is accomplished by coordinated phenotypic plasticity of metabolic systems. We measured summit metabolism (V(O(2),sum)), daily energy expenditure (DEE) and muscle oxidative capacity under both winter (December to January) and breeding (May to June) conditions. We hypothesize that: (1) rates of energy utilization will be highest in the winter, contrary to predictions based on life history theory, and (2) acclimatization of metabolism will occur at multiple levels of organization such that birds operate with a similar metabolic ceiling during different seasons. We measured field metabolic rates using heart rate telemetry and report the first daily patterns in avian field metabolic rate. Patterns of daily energy use differed seasonally, primarily as birds maintain high metabolic rates throughout the winter daylight hours. We found that DEE and V(O(2),sum) were significantly greater and DEE occurred at a higher fraction of maximum metabolic capacity during winter, indicating an elevation of the metabolic ceiling. Surprisingly, there were no significant differences in mass or oxidative capacity of skeletal muscle. These data, highlighting the importance of examining energetic responses to seasonal challenges at multiple levels, clearly reject life history predictions that breeding is the primary energetic challenge for temperate zone residents. Further, they indicate that metabolic ceilings are seasonally flexible as metabolic effort during winter thermoregulation exceeds that of breeding.

Seasonal variation in energy expenditure is not related to activity level or water temperature in a large diving bird

There is considerable interest in understanding how the energy budget of an endotherm is modulated from a physiological and ecological point of view. In this paper, we used the heart rate method and daily heart rate (DHR), as a proxy of DEE across seasons, to test the effect of locomotion activity and water temperature on the energy budget of a large diving bird. DHR was monitored continuously in common eiders (Somateria mollissima) during seven months together with measures of time spent flying and time spent feeding. DHR varied substantially during the recording period with numerous increases and decreases that occurred across seasons although we could not find any relationship between DHR and the time spent active (feeding and flying). However, inactive heart rate (IHR) decreased as locomotion activity increases suggesting common eiders were using behavioural compensation when under a high work load. We were also unable to detect a negative relationship between water temperature and resting heart rate, a proxy of resting metabolic rate. This was unexpected based on the assumption that high thermoregulation costs would be associated with cold waters. We showed that high level of energy expenditure coincided with feather moult and warm waters, which suggest that the observed variable pattern of seasonal DEE was driven by feather growth and possibly by other productive costs. Nevertheless, our results indicate that behavioural compensation and possibly the timing of moult may be used as mechanisms to reduce seasonal variation in energy expenditure.

Heat for nothing or activity for free? Evidence and implications of activity-thermoregulatory heat substitution

If heat generated through activity can substitute for heat required for thermoregulation, then activity in cold environments may be energetically free for endotherms. Although the possibility of activity-thermoregulatory heat substitution has been long recognized, its empirical generality and ecological implications remain unclear. We combine a review of the literature and a model of heat exchange to explore the generality of activity-thermoregulatory heat substitution, to assess the extent to which substitution is likely to vary with body size and ambient temperature, and to examine some potential macroecological implications. A majority of the 51 studies we located showed evidence of activity-thermoregulatory heat substitution (35 of 51 studies), with 28 of 32 species examined characterized by substitution in one or more study. Among studies that did detect substitution, the average magnitude of substitution was 57%, but its occurrence and extent varied taxonomically, allometrically, and with ambient temperature. Modeling of heat production and dissipation suggests that large birds and mammals, engaged in intense activity and exposed to relatively warm conditions, have more scope for substitution than do smaller endotherms engaged in less intense activity and experiencing cooler conditions. However, ambient temperature has to be less than the lower critical temperature (the lower bound of the thermal neutral zone) for activity-thermoregulatory heat substitution to occur and this threshold is lower in large endotherms than in small endotherms. Thus, in nature, substitution is most likely to be observed in intermediate-sized birds and mammals experiencing intermediate ambient temperatures. Activity-thermoregulatory heat substitution may be an important determinant of the activity patterns and metabolic ecology of endotherms. For example, a pattern of widely varying field metabolic rates (FMR) at low latitudes that converges to higher and less variable FMR at high latitudes has been interpreted as suggesting that warm environments at low latitudes allow a greater variety of feasible metabolic niches than do cool, high-latitude environments. However, activity-thermoregulatory heat substitution will generate this pattern of latitudinal FMR variation even if endotherms from cold and warm climates are metabolically and behaviorally identical, because the metabolic rates of resting and active animals are more similar in cold than in warm environments. Activity-thermoregulatory heat substitution is an understudied aspect of endotherm thermal biology that is apt to be a major influence on the physiological, behavioral and ecological responses of free-ranging endotherms to variation in temperature.

Application of the two-sample doubly labelled water method alters behaviour and affects estimates of energy expenditure in black-legged kittiwakes

Despite the widespread use of the doubly labelled water (DLW) method in energetic studies of free-ranging animals, effects of the method on study animals are rarely assessed. We studied behavioural effects of two alternative DLW protocols. During two consecutive breeding seasons, 42 parent black-legged kittiwakes received either the commonly used two-sample (TS) or the less invasive single-sample (SS) DLW treatment. A third group served as a non-treated control. We evaluated the effect of treatment with respect to the time birds took to return to their nest after treatment and recaptures, and the nest attendance during DLW measurement periods. We found that TS kittiwakes took on average 20 times longer to return to their nest than SS kittiwakes after initial treatment, and nest attendance was reduced by about 40% relative to control birds. In contrast, nest attendance did not differ between control and SS kittiwakes. Estimates of energy expenditure of SS kittiwakes exceeded those of TS kittiwakes by 15%. This difference was probably caused by TS birds remaining inactive for extended time periods while at sea. Our results demonstrate that the common assumption that the TS DLW method has little impact on the behaviour of study subjects is in some circumstances fallacious. Estimates of energy expenditure derived by the SS approach may thus more accurately reflect unbiased rates of energy expenditure. However, the choice of protocol may be a trade-off between their impact on behaviour, and hence accuracy, and their differences in precision. Adopting procedures that minimize the impact of TS protocols may be useful.

Thermal substitution and aerobic efficiency: measuring and predicting effects of heat balance on endotherm diving energetics

For diving endotherms, modelling costs of locomotion as a function of prey dispersion requires estimates of the costs of diving to different depths. One approach is to estimate the physical costs of locomotion (Pmech) with biomechanical models and to convert those estimates to chemical energy needs by an aerobic efficiency (eta=Pmech/Vo2) based on oxygen consumption (Vo2) in captive animals. Variations in eta with temperature depend partly on thermal substitution, whereby heat from the inefficiency of exercising muscles or the heat increment of feeding (HIF) can substitute for thermogenesis. However, measurements of substitution have ranged from lack of detection to nearly complete use of exercise heat or HIF. This inconsistency may reflect (i) problems in methods of calculating substitution, (ii) confounding mechanisms of thermoregulatory control, or (iii) varying conditions that affect heat balance and allow substitution to be expressed. At present, understanding of how heat generation is regulated, and how heat is transported among tissues during exercise, digestion, thermal challenge and breath holding, is inadequate for predicting substitution and aerobic efficiencies without direct measurements for conditions of interest. Confirming that work rates during exercise are generally conserved, and identifying temperatures at those work rates below which shivering begins, may allow better prediction of aerobic efficiencies for ecological models.

Phenotypic flexibility in basal metabolic rate and the changing view of avian physiological diversity: a review

Comparative analyses of avian energetics often involve the implicit assumption that basal metabolic rate (BMR) is a fixed, taxon-specific trait. However, in most species that have been investigated, BMR exhibits phenotypic flexibility and can be reversibly adjusted over short time scales. Many non-migrants adjust BMR seasonally, with the winter BMR usually higher than the summer BMR. The data that are currently available do not, however, support the idea that the magnitude and direction of these adjustments varies consistently with body mass. Long-distance migrants often exhibit large intra-annual changes in BMR, reflecting the physiological adjustments associated with different stages of their migratory cycles. Phenotypic flexibility in BMR also represents an important component of short-term thermal acclimation under laboratory conditions, with captive birds increasing BMR when acclimated to low air temperatures and vice versa. The emerging view of avian BMR is of a highly flexible physiological trait that is continually adjusted in response to environmental factors such as temperature. The within-individual variation observed in avian BMR demands a critical re-examination of approaches used for comparisons across taxa. Several key questions concerning the shapes and other properties of avian BMR reaction norms urgently need to be addressed, and hypotheses concerning metabolic adaptation should explicitly account for phenotypic flexibility.

Chronic stress in free-living European starlings reduces corticosterone concentrations and reproductive success

Chronic increases in stress hormones such as glucocorticoids are maladaptive, yet studies demonstrating a causal relationship among chronic stress, increases in glucocorticoid concentrations, and subsequent fitness costs in free-living animals are lacking. We experimentally induced chronic psychological stress in female European starlings (Sturnus vulgaris) by subjecting half of the females at our study site to a chronic stress protocol consisting of 4, 30 min stressors (loud radio, predator calls, a novel object, or predator decoys including a snake, rat, and owl) administered in random order daily for 8 days after clutch completion. Experimental females were captured at the end of the chronic stress protocol (9 days after the onset of the chronic stress protocol), and unstressed control females were captured at the same stage of the nesting cycle. Chronically stressed females had lower baseline corticosterone (CORT, the avian glucocorticoid) concentrations and lower reproductive success than unstressed females. Furthermore, surviving nestlings in experimentally stressed broods showed sensitization of the CORT response to acute stress, which is a physiological change that could persist to adulthood. Attenuation of baseline CORT concentrations in adult females is contrary to the general assumption that elevated CORT concentrations indicate stress, suggesting that more research is necessary before CORT concentrations can be used to accurately assess chronic stress in field studies.

Substitution of heat from exercise and digestion by ducks diving for mussels at varying depths and temperatures

Diving birds can lose significant body heat to cold water, but costs can be reduced if heat from exercising muscles or the heat increment of feeding (HIF) can substitute for thermogenesis. Potential for substitution depends jointly on the rate of heat loss, the rate of heat produced by exercise, and the level of HIF. To explore these interactions, we measured oxygen consumption by lesser scaup ducks (Aythya affinis) diving to depths of 1.2 and 2 m at thermoneutral (23 degrees C) and sub-thermoneutral (18 and 8 degrees C) temperatures. Birds dove while fasted and when feeding on blue mussels (Mytilus edulis). Substitution occurred if HIF or costs of diving above resting metabolic rate (RMR) were lower at 18 or 8 degrees C than at 23 degrees C, indicating reduction in the thermoregulatory part of RMR. For fasted scaup diving to 1.2 m, substitution from exercise heat was not apparent at either 18 or 8 degrees C. At 2 m depth, dive costs above RMR were reduced by 5% at 18 degrees C and by 40% at 8 degrees C, indicating substitution. At 1.2 m depth (with voluntary intake of only 14-17% of maintenance requirements), HIF did not differ between temperatures, indicating no substitution. However, at 2 m (intake 13-25% of maintenance), substitution from HIF was 23% of metabolizable energy intake at 18 degrees C and 22% at 8 degrees C. These results show that even with low HIF due to low intake rates, substitution from HIF can add to substitution from the heat of exercise.

Direct calorimetry reveals large errors in respirometric estimates of energy expenditure

Knowledge of animal energetics is based largely upon indirect calorimetry, which is estimation of metabolic heat production by an organism from measurement of indices such as oxygen consumption or carbon dioxide production. Remarkably, indirect calorimetry has been validated by comparison to direct measurements of metabolic heat production (direct calorimetry) only for highly restricted conditions, primarily with a few species of medium-to-large mammals. Taxa with differing physiologies are little studied. For birds, for example, validations are limited to chickens and waterfowl exposed to mild environmental conditions and typically fasted for prolonged periods. Workers rely upon these restricted validations when studying animals ranging greatly in activity, phylogeny, body size and nutritional status. We tested the accuracy of respirometric estimates of energy expenditure by simultaneous indirect and direct calorimetry in a small mammal (the kangaroo rat Dipodomys merriami Mearns), a small bird (the dove Columbina inca Lesson) and a medium-sized bird (the quail Coturnix communis Linnaeus). We find that conventional respirometric estimates of energy expenditure may incorporate large errors (up to 38%) that are sufficient to call into question generalizations regarding patterns of animal energy use in many studies.

Effects of surface activity patterns and dive depth on thermal substitution in fasted and fed lesser scaup (Aythya affinis) ducks

High thermoregulation costs incurred by aquatic endotherms can be reduced if heat from digestion and exercising muscles can substitute for thermogenesis. Costs (VO2) of surface activities and diving were measured in lesser scaup (Aythya affinis (Eyton, 1838)) ducks while fasted and when feeding on mixed grain at depths of 1.2 and 2 m at thermoneutral (23 °C) and sub-thermoneutral (8 °C) water temperatures. Substitution occurred if the heat increment of feeding (HIF) or costs of activities above resting metabolic rate (RMR) were lower at 8 °C than at 23 °C, indicating that the thermoregulatory part of RMR at 8 °C had been reduced. Changes in body core temperature were not significant, and there was no substitution of HIF. At the surface at 8 °C, fasted birds that were active (nonresting) >75% of the time had lower RMR, and lower cost of activities above RMR, than less active birds. Dive recovery cost after dive bouts (DRC) constituted 33%–35% of total dive costs at 23 °C and 48%–55% at 8 °C. Dive costs above RMR (including DRC) were lower at 8 °C than at 23 °C by 14% at 1.2 m and 25% at 2 m, suggesting appreciable substitution that increased with longer, deeper dives; however, results varied widely and were not statistically significant. Our experiments indicate that thermal substitution varies in importance depending on levels of activity and heat loss.

The Allometry of Avian Basal Metabolic Rate: Good Predictions Need Good Data

Basal metabolic rate (BMR) is often predicted by allometric interpolation, but such predictions are critically dependent on the quality of the data used to derive allometric equations relating BMR to body mass (Mb). An examination of the metabolic rates used to produce conventional and phylogenetically independent allometries for avian BMR in a recent analysis revealed that only 67 of 248 data unambiguously met the criteria for BMR and had sample sizes with n>/=3. The metabolic rates that represented BMR were significantly lower than those that did not meet the criteria for BMR or were measured under unspecified conditions. Moreover, our conventional allometric estimates of BMR (W; logBMR=-1.461+0.669logMb) using a more constrained data set that met the conditions that define BMR and had n>/=3 were 10%-12% lower than those obtained in the earlier analysis. The inclusion of data that do not represent BMR results in the overestimation of predicted BMR and can potentially lead to incorrect conclusions concerning metabolic adaptation. Our analyses using a data set that included only BMR with n>/=3 were consistent with the conclusion that BMR does not differ between passerine and nonpasserine birds after taking phylogeny into account. With an increased focus on data mining and synthetic analyses, our study suggests that a thorough knowledge of how data sets are generated and the underlying constraints on their interpretation is a necessary prerequisite for such exercises.

Seasonal acclimatization to extreme climatic conditions by black-capped chickadees (Poecile atricapilla) in interior Alaska (64 degrees N)

Winters in interior Alaska (64 degrees N) are characterized by short photoperiod (5L : 19D) and chronic subfreezing temperatures. To determine if seasonal acclimatization of black-capped chickadees (Poecile atricapilla) at high latitude differs from that of conspecifics at lower latitudes, standard metabolic rates (SMR), metabolic response to low temperature (-30 degrees C), nocturnal hypothermia, body mass, fat reserves, and conductance were measured over two winters and one summer in three groups of seasonally acclimatized birds. Body mass and conductance did not vary with season, although furcular fat levels were higher in winter. Birds used nocturnal hypothermia when exposed to -30 degrees C in summer or winter. Although SMR did not vary seasonally, winter SMRs differed between the two winters of the study. Nocturnal hypothermia in summer and decreased SMR in response to extreme conditions may either reflect plasticity inherent to all populations of black-capped chickadees or may result from individual variation within this northern population.

The adjustment of avian metabolic rates and water fluxes to desert environments

We tested the hypothesis that birds in arid environments, where primary productivity is low and surface water is scarce, have reduced energy expenditure and water loss compared with their mesic counterparts. Using both conventional least squares regression and regression based on phylogenetically independent contrasts, we showed that birds from desert habitats have reduced basal and field metabolic rates compared with species from mesic areas. Previous work showed that desert birds have reduced rates of total evaporative water loss when exposed to moderate environmental temperatures in the laboratory. We tested whether reduced rates of total evaporative water loss translate into low field water fluxes. Conventional ANCOVA indicated that desert birds have reduced water fluxes, but an analysis based on phylogenetically independent contrasts did not support this finding, despite the wide array of taxonomic affiliations of species in the data set. We conclude that the high ambient temperatures, the low primary productivity, and the water scarcity in desert environments have selected for or resulted in reduced rates of energy expenditure and evaporative water loss in birds that live in these climes.

Energy costs of chick rearing in Black-legged Kittiwakes (Rissa tridactyla)

We studied energy expenditure in adult Black-legged Kittiwakes (Rissa tridoctyla) with doubly labeled water to measure energy costs of chick rearing. We removed eggs from randomly selected nests and compared energy expenditure late in the chick-rearing period between adults raising chicks and adults whose eggs had been removed. Adults raising chicks expended energy at a rate 21% higher than adults from manipulated nests, apparently owing to differences in activity patterns while away from the colony. No sex-specific differences were detected in energy costs of chick rearing or energy expenditure, although statistical power for these analyses was fairly low. Among the unmanipulated group, energy expenditure tended to be positively related to natural brood size. An ancillary goal of our study was to test hypotheses that describe how population-level field metabolic rates (FMRs) vary during chick rearing. We compared FMRs among kittiwakes raising chicks at a colony in Alaska (61°09'N) with those reported for a colony in Norway (76°30'N). FMRs of adults raising chicks were nearly identical at the two colonies, suggesting that adults may have preferred levels of energy expenditure during chick rearing that are relatively invariant with environmental conditions, and that are not adjusted according to adult survival probabilities.

Energetics of free-ranging mammals, reptiles, and birds

We summarize the recent information on field metabolic rates (FMR) of wild terrestrial vertebrates as determined by the doubly labeled water technique. Allometric (scaling) relationships are calculated for mammals (79 species), reptiles (55 species), and birds (95 species) and for various taxonomic, dietary, and habitat groups within these categories. Exponential equations based on body mass are offered for predicting rates of daily energy expenditure and daily food requirements of free-ranging mammals, reptiles, and birds. Significant scaling differences between various taxa, dietary, and habitat groups (detected by analysis of covariance with P < or = 0.05) include the following: (a) The allometric slope for reptiles (0.889) is greater than that for mammals (0.734), which is greater than that for birds (0.681); (b) the slope for eutherian mammals (0.772) is greater than that for marsupial mammals (0.590); (c) among families of birds, slopes do not differ but elevations (intercepts) do, with passerine and procellariid birds having relatively high FMRs and gallinaceous birds having low FMRs; (d) Scleroglossan lizards have a higher slope (0.949) than do Iguanian lizards (0.793); (e) desert mammals have a higher slope (0.785) than do nondesert mammals; (f) marine birds have relatively high FMRs and desert birds have low FMRs; and (g) carnivorous mammals have a relatively high slope and carnivorous, insectivorous, and nectarivorous birds have relatively higher FMRs than do omnivores and granivores. The difference detected between passerine and nonpasserine birds reported in earlier reviews is not evident in the larger data set analyzed here. When the results are adjusted for phylogenetic effects using independent contrasts analysis, the difference between allometric slopes for marsupials and eutherians is no longer significant and the slope difference between Scleroglossan and Iguanian lizards disappears as well, but other taxonomic differences remain significant. Possible causes of the unexplained variations in FMR that could improve our currently inaccurate FMR prediction capabilities should be evaluated, including many important groups of terrestrial vertebrates that remain under- or unstudied and such factors as reproductive, thermoregulatory, social, and predator-avoidance behavior.

The evolution of sexual dimorphism in the lizard Anolis polylepis (Iguania): evidence from intraspecific variation in foraging behavior and diet

Two main explanations, intraspecific niche divergence and sexual selection, have been proposed to explain the origin of sexual size dimorphism. To test these competing hypotheses I studied the ecology, feeding behavior, and diet of the lizard Anolis polylepis in a Costa Rican rain forest. Male A. polylepis were significantly larger and heavier than females but ate smaller food items and had lower stomach volumes, despite possessing longer and wider heads. Males were more sedentary than females or juveniles, chose higher perches, and were more likely to be involved in agonistic interactions. Diets of males, females, and juveniles were also significantly different taxonomically. These data are consistent with the sexual selection origin theory but not with an ecological one. Thus, observed dietary differences probably evolved once dimorphism had been attained through sexual selection.