Noninvasive methods for measuring and manipulating corticosterone in hummingbirds

Illuminating the Mysteries of the Smallest Birds: Hummingbird Population Health, Disease Ecology, and Genomics

Hummingbirds share biologically distinctive traits: sustained hovering flight, the smallest bird body size, and high metabolic rates fueled partially by nectar feeding that provides pollination to plant species. Being insectivorous and sometimes serving as prey to larger birds, they fulfill additional important ecological roles. Hummingbird species evolved and radiated into nearly every habitat in the Americas, with a core of species diversity in South America. Population declines of some of their species are increasing their risk of extinction. Threats to population health and genetic diversity are just beginning to be identified, including diseases and hazards caused by humans. We review the disciplines of population health, disease ecology, and genomics as they relate to hummingbirds. We appraise knowledge gaps, causes of morbidity and mortality including disease, and threats to population viability. Finally, we highlight areas of research need and provide ideas for future studies aimed at facilitating hummingbird conservation.

Habitat urbanization and stress response are primary predictors of personality variation in northern cardinals (Cardinalis cardinalis)

Behavioral traits that vary consistently among individuals across different contexts are often termed as ‘personality traits,’ while the correlated suite formed by those traits is called a ‘behavioral syndrome’. Both personality trait and behavioral syndrome are potentially responsive to animal ‘states’, defined as strategically relevant individual features affecting the cost-and-benefit trade-offs of behavioral actions. Both extrinsic ‘states’ (e.g. urban versus rural habitats), and intrinsic ‘states’ (e.g. sex), may shape among-individual variation in personality traits, as well as behavioral syndromes. Here, we used northern cardinals sampled from four locations to examine the effect of habitat type (urban versus rural, an extrinsic state), stress hormone corticosterone (CORT) parameters, body weight and sex (intrinsic states) on personality traits and behavioral syndrome variation. We used behavioral trials to measure five personality traits. Using principal component analysis to quantify personality traits first, followed by general linear mixed models, we found that habitat type, CORT at capture and 2-day CORT response affected some personality traits, while body weight and sex did not. Cardinals inhabiting more urbanized areas had lower CORT metabolite levels at capture and were more neophilic, less neophobic and also less aggressive than their rural conspecifics. Using structural equation modeling to construct behavioral syndromes formed by our selected personality traits, we found that urban and rural cardinals varied in the models representing syndrome structure. When utilizing the shared syndrome structural model to examine the effects of states, habitat type and 2-day CORT response appear to affect syndrome variation in a coordinated, not hierarchical, manner.

Circadian Rhythm and Stress Response in Droppings of Serinus canaria

Serinus canaria is a widespread domestic ornamental songbird, whose limited knowledge of biology make compelling studies aimed to monitor stress. Here, a commercial enzyme immunoassay was adopted to measure immunoreactive corticosterone (CORT) in single Serinus canaria dropping sample, to monitor the daily fecal excretion of CORT in birds bred singly or in-group and to detect the effect promoted by aviary or small transport cage restraint. A robust daily rhythm of CORT was recorded in animals held on short-day light cycle, independent of bred conditions (single or group), which persisted when space availability was modified in single bred animal (transfer in aviary and transport cages). By contrast, a significant change in CORT excretion was recorded when group bred animals are restrained in a smaller cage. The daily rhythm in CORT excretion in response to manipulation showed the greatest response at the beginning of the light period, followed by the absence of the peak usually recorded at the end of the dark phase. These data indicated that EIA could be used as a reliable noninvasive approach to monitor the stress induced by restraint conditions in Serinus canaria.

Sexual dimorphism in baseline urinary corticosterone metabolites and their association with body-condition indices in a peri-urban population of the common Asian toad (Duttaphrynus melanostictus)

Field endocrinology research through the quantification of glucocorticoids or stress hormones in free-living wildlife is crucial for assessing their physiological responses towards pervasive environmental changes. Urinary corticosterone metabolite (UCM) enzyme-immunoassay (EIA) has been validated for numerous amphibian species as a non-invasive measure of physiological stress. Body-condition indices (BCIs) have also been widely used in amphibians as an indirect measure of animal health. Field endocrinology research on amphibian species in Asia is limited. In this study, we validated a UCM EIA in a peri-urban sub-population of the common Asian toad (Duttaphrynus melanostictus) in Pune, Maharashtra, India. We determined the baseline levels of UCMs in male (n=39) and female (n=19) toads. Secondly, we used a standard capture handling protocol to quantify changes in UCMs during short-term captivity. We also determined BCIs in the male and female toads using Fulton's index (K) and residual condition index (RCI). The results showed that mean baseline levels of UCMs were significantly higher in male toads than in females. There was no significant change in mean levels of UCMs of males and females between capture and captivity (0-12h). This highlights plausible habituation of the species to the peri-urban environment. Associations between UCMs with BCIs (K and R) were positive in male toads but negative in females. In conclusion, our UCMs EIA can be applied with BCIs to assess health of the Asian toads. We also suggest that direct fitness parameters such as sperm and oocyte quality, reproductive ecology and immunocompetence measurements should be applied in combination with these conservation physiology tools to quantify the fitness consequences of pervasive environmental changes on native amphibians.

Validation of a fecal glucocorticoid metabolite assay to assess stress in the budgerigar (Melopsittacus undulatus)

The budgerigar (Melopsittacus undulatus) is a small parrot native to Australia that is commonly held in zoos, laboratories, and private homes. Assessment of budgerigar stress levels would aid welfare monitoring and improve our understanding of their biology. Analyzing fecal glucocorticoid metabolites provides a noninvasive method to measure stress levels in birds. For this method to be reliable, the antibody to be used in an immunoassay must be carefully selected for each species, and validation must be performed. A common limitation in many existing assays is the inability to accurately detect variable fecal glucocorticoid metabolites in minute quantities of feces, requiring small samples to be combined. We have developed a double antibody radioimmunoassay protocol based on a commercially available (125) I-corticosterone radioimmunoassay kit for use in detecting fecal glucocorticoid metabolites in small quantities (<20 mg) of budgerigar droppings. The assay was validated pharmacologically with an adrenocorticotropic hormone challenge and with oral administration of corticosterone. Our validation has demonstrated our assay is both sensitive and a reliable approach to noninvasive monitoring of stress in budgerigars.

Effects of corticosterone pellets on baseline and stress-induced corticosterone and corticosteroid-binding-globulin

Exogenous administration of glucocorticoids is a widely used and efficient tool to investigate the effects of elevated concentrations of these hormones in field studies. Because the effects of corticosterone are dose and duration-dependent, the exact course of plasma corticosterone levels after exogenous administration needs to be known. We tested the performance of self-degradable corticosterone pellets (implanted under the skin) in elevating plasma corticosterone levels. We monitored baseline (sampled within 3min after capture) total corticosterone levels and investigated potential interactions with corticosteroid-binding-globulin (CBG) capacity and the endogenous corticosterone response to handling in Eurasian kestrel Falco tinnunculus and barn owl Tyto alba nestlings. Corticosterone pellets designed for a 7-day-release in rodents elevated circulating baseline total corticosterone during only 2-3 days compared to placebo-nestlings. Highest levels occurred 1-2days after implantation and levels decreased strongly thereafter. CBG capacity was also increased, resulting in a smaller, but still significant, increase in baseline free corticosterone levels. The release of endogenous corticosterone as a response to handling was strong in placebo-nestlings, but absent 2 and 8 days after corticosterone pellet implantation. This indicates a potential shut-down of the hypothalamo-pituitary-adrenal axis after the 2-3 days of elevated baseline corticosterone levels. 20 days after pellet implantation, the endogenous corticosterone response to handling of nestlings implanted with corticosterone pellets attained similar levels as in placebo-nestlings. Self-degradable pellets proved to be an efficient tool to artificially elevate circulating baseline corticosterone especially in field studies, requiring only one intervention. The resulting peak-like elevation of circulating corticosterone, the concomitant elevation of CBG capacity, and the absence of an endogenous corticosterone response to an acute stressor have to be taken into account.

Effects of repeated, short-term, corticosterone administration on the hypothalamo-pituitary-adrenal axis of the white-crowned sparrow (Zonotrichia leucophrys gambelii)

Our knowledge of glucocorticoid actions in vertebrates comes primarily from laboratory studies, which are often conducted with little consideration of how animals experience changes in glucocorticoid secretion in natural contexts. Typically, free-living animals are exposed to acute perturbations of the environment, ranging from a few minutes to a few hours duration, with varying frequency. The cumulative effects of these perturbations and their resultant glucocorticoid surges are not well known. To investigate the possible cumulative effects of repeated, acute surges in glucocorticoid secretion, we developed an ecologically relevant methodology for treating captive white-crowned sparrows (Zonotrichia leucophrys gambelii) with corticosterone (CORT). We dissolved CORT in dimethylsulfoxide (DMSO) and administered this cocktail directly on the skin. Treatments resulted in small elevations of CORT within the physiological range. In our first experiment at the end of the breeding life stage, birds were treated three times a day (3x). Two control groups were used: one treated with DMSO 3x and one not handled nor treated. In a second study at the beginning of the breeding life stage, one group was treated once a day and a second group 3x. A DMSO-control group was used for each dosage regime. Repeated, acute administration of CORT resulted in higher baseline CORT levels and a down-regulation of the endogenous adrenocortical response to a standardized stress. Maximum CORT and plasma corticosterone binding globulin levels increased in response to the CORT treatments only at the end of the breeding season. CORT treatment did not alter adrenal size, adrenal response to ACTH, or hepatic CORT metabolism.

Fecal steroid monitoring for assessing gonadal and adrenal activity in the golden eagle and peregrine falcon

We examined the efficacy of noninvasive monitoring of endocrine function via fecal steroid immunoassays in the golden eagle and peregrine falcon. High-pressure liquid chromatography analyses of fecal glucocorticoid metabolites (fGCM) revealed that minor percentages of immunoreactive fGCM co-eluted with [(3)H]corticosterone in both sexes of the eagle (2.5-2.7%) and falcon (7.5-11.9%). In contrast, most fecal estrogen metabolites in eagle and falcon females co-eluted with radiolabeled estradiol-17beta ([(3)H]; 57.6, 64.6%, respectively) or estrone ([(3)H]; 26.9, 4.1%, respectively). Most fecal progestin metabolite immunoreactivity in the female eagle (24.8%) and falcon (21.7%) co-eluted with progesterone ([(14)C]). Most fecal androgen metabolite immunoreactivity in eagle (55.8%) and falcon (63.7%) males co-eluted with testosterone ([(14)C]). Exogenous adrenocorticotropin hormone induced increased fGCM excretion above pre-treatment in both species, but only significantly (P < 0.05) in the eagle. Both species showed increased fGCM after saline administration, suggesting the detection of 'handling stress.' Both species exhibited enterohepatic and renal recirculation of administered steroids as demonstrated by biphasic and triphasic excretion patterns. Thus, noninvasive fecal hormone monitoring is a valid and promising tool for assessing gonadal and adrenal status in rare and threatened birds-of-prey.

Synthesis of Measuring Steroid Metabolites in Goose Feces

The reliability of noninvasively measuring steroid hormones from feces in greylag geese (Anser anser) and domestic geese (A. domesticus), both qualitatively and quantitatively, was tested experimentally. Geese are mainly herbivorous birds with a short gut-passage time (2-3 h). Groups of eight outdoor-housed male domestic geese were subjected to two different experiments, injection of either GnRH or ACTH, which were replicated in three different seasons (spring, summer, and fall). GnRH stimulation resulted in significant increases of response fecal testosterone metabolites (TM; 17beta-OH-androgens) in spring and fall, but not during the summer photorefractoriness. Testosterone response patterns obtained from plasma samples paralleled those from feces; however, no direct correlation between individual immunoreactive plasma and feces contents was observed. To improve the sample handling during extraction and the assay sensitivity, we promote the use of a group-specific antibody against 17-oxoandrogens that does not require deconjugation prior to the analysis. ACTH robustly increased fecal corticosterone in all seasons. The polar nature of glucocorticoids, however, seems to make a distinction between conjugated and nonconjugated types difficult, and the available avian literature on this topic is discussed.

Noninvasive monitoring of hormones in bird droppings: physiological validation, sampling, extraction, sex differences, and the influence of diet on hormone metabolite levels

During the past several years, the noninvasive measurement of steroid metabolites from mammalian feces and bird droppings has become more and more popular. With an increasing acceptance of the method, investigators may become less aware of the need to validate their assays. It is shown why such validations are essential for each new species investigated and various ways to physiologically validate such noninvasive methods are described. Using the European stonechat (Saxicola torquata rubicola) as a model, it is explained why a validated method to measure androgen metabolites in males does not necessarily work in females. In addition the difficulties that may be neglected owing to the superficial ease of sampling and processing of excreta are investigated. Various issues that may arise during sampling, storage, and extraction of excreta are addressed. Finally, results suggesting that experimental manipulations of the diet may affect hormone metabolite levels in European stonechats are presented. So far, only a few studies have investigated the impact of diet on hormone metabolite levels, and these are the first data to report such an impact in birds. More studies are urgently needed to learn more about differences between the sexes, individuals, and populations and the impact of diet and energy metabolism on hormone metabolites.

Sampling effort/frequency necessary to infer individual acute stress responses from fecal analysis in Greylag geese (Anser anser)

Measuring hormone metabolites from excreta is a powerful method to study hormone-behavior relationships. Currently, fecal corticosterone metabolite concentrations are used to estimate individual short-term stress responses. From the free-roaming, semitame flock of greylag geese (Anser anser), as many fecal samples as possible were collected over 3 h following a challenge (social density stress) or in a control situation. This time span corresponds to the gut passage time of geese. It was asked how many samples were necessary to determine differences in excreted corticosterone immunoreactive metabolites (CORTs) between control and social density stress and which parameters (means, maxima, range) reliably showed this difference. A large variation of CORT was found between consecutive samples. Still, means, maxima, and ranges of the samples in a fecal series consistently showed the response to a stressor both within and between individuals. Three samples sufficed if the maximum value of CORT was used, whereas four or more samples were necessary to work with the mean. It was concluded that by increasing the number of fecal samples collected, the course of CORT could be measured more precisely and an individual's acute stress response inferred more reliably.

Changes in the fecal concentrations of cortisol and androgen metabolites in captive male jaguars (Panthera onca) in response to stress

In the present study we determined the efficacy of the measurement of fecal cortisol and androgen metabolite concentrations to monitor adrenal and testicular activity in the jaguar (Panthera onca). Three captive male jaguars were chemically restrained and electroejaculated once or twice within a period of two months. Fecal samples were collected daily for 5 days before and 5 days after the procedure and stored at -20 degrees C until extraction. Variations in the concentrations of cortisol and androgen metabolites before and after the procedure were determined by solid phase cortisol and testosterone radioimmunoassay and feces dry weight was determined by drying at 37 degrees C for 24 h under vacuum. On four occasions, fecal cortisol metabolite levels were elevated above baseline (307.8 +/- 17.5 ng/g dry feces) in the first fecal sample collected after the procedure (100 to 350% above baseline). On one occasion, we did not detect any variation. Mean (+/- SEM) fecal androgen concentration did not change after chemical restraint and electroejaculation (before: 131.1 +/- 26.7, after: 213.7 +/- 43.6 ng/g dry feces). These data show that determination of fecal cortisol and androgen metabolites can be very useful for a noninvasive assessment of animal well-being and as a complement to behavioral, physiological, and pathological studies. It can also be useful for the study of the relationship between adrenal activity and reproductive performance in the jaguar.

Dietary intake effects on arginine vasotocin and aldosterone in cloacal fluid of whitebellied sunbirds (Nectarinia talatala)

The main objective of this study was to determine whether or not the renal outputs of the osmoregulatory hormones arginine vasotocin (AVT) and aldosterone (ALDO) reflect the osmotic status of whitebellied sunbirds (Nectarinia talatala). The birds were fed a range of sucrose concentrations (from 0.07 to 2.5 mol/l, with osmolalities of 70 to approximately 5,800 mosM/kg), and adjusted their intakes so that they drank large volumes of dilute diets and small volumes of concentrated diets. Renal fluid outputs were appropriately regulated so that large volumes of cloacal fluid (CF) were voided on the dilute diets and small volumes on the concentrated diets. Accordingly, plasma AVT concentrations increased with increasing sugar concentration; however, AVT outputs in CF did not change in a similar manner, rather they decreased as dietary concentration increased. It was not possible to measure plasma ALDO concentrations in the small sunbirds because of insufficient blood samples available; however, ALDO outputs in CF did vary with the sucrose diets and renal function, being highest on the most concentrated diet. In addition ALDO output in CF fell markedly when sodium was added to the 0.5 mol/l sucrose diet. We conclude that in sunbirds fed increasingly concentrated sucrose solutions, changes in CF outputs of ALDO, but not AVT, appear to reflect the water flux and hydration state of these birds.

Fecal corticosteroids in a territorial bird selected for different personalities: daily rhythm and the response to social stress

In this study we tested the hypothesis that in a passerine bird (great tit, Parus major) individuals differing for coping strategies differ in the magnitude of the adrenocortical response to social stress as well. Furthermore, we aimed at characterizing daily rhythms in corticosteroid release before and after social stress. We used 16 males from either of two lines bidirectionally selected for different coping strategies (fast and slow explorers). Social stress was induced by confrontation with an aggressive resident male. Corticosteroid metabolites were analyzed in feces collected at 90-min intervals from 900 to 1630 h on a baseline day, on the day of the social conflict, and on the following day. In both days and in both lines levels varied with time of day in a robust rhythm with a peak in the first sample of the morning and a trough at the end of the light phase. This rhythm correlates with activity (perch hopping). An overall increase in levels relative to baseline day was observed between 30 and 140 min after the challenge. Birds of the less aggressive and more cautious line (slow explorers) showed a trend for a higher response compared to birds of the more aggressive and bolder line (fast explorers), which showed almost no response. On the day after the challenge the birds of the slow line exhibited significantly reduced corticosteroid secretion, probably due to an increased negative feedback. The results provide evidence for a physiological basis of different coping strategies in birds, emerging in response to social stress and with a pattern similar to that in other vertebrates.

The concept of allostasis in biology and biomedicine

Living organisms have regular patterns and routines that involve obtaining food and carrying out life history stages such as breeding, migrating, molting, and hibernating. The acquisition, utilization, and storage of energy reserves (and other resources) are critical to lifetime reproductive success. There are also responses to predictable changes, e.g., seasonal, and unpredictable challenges, i.e., storms and natural disasters. Social organization in many populations provides advantages through cooperation in providing basic necessities and beneficial social support. But there are disadvantages owing to conflict in social hierarchies and competition for resources. Here we discuss the concept of allostasis, maintaining stability through change, as a fundamental process through which organisms actively adjust to both predictable and unpredictable events. Allostatic load refers to the cumulative cost to the body of allostasis, with allostatic overload being a state in which serious pathophysiology can occur. Using the balance between energy input and expenditure as the basis for applying the concept of allostasis, we propose two types of allostatic overload. Type 1 allostatic overload occurs when energy demand exceeds supply, resulting in activation of the emergency life history stage. This serves to direct the animal away from normal life history stages into a survival mode that decreases allostatic load and regains positive energy balance. The normal life cycle can be resumed when the perturbation passes. Type 2 allostatic overload begins when there is sufficient or even excess energy consumption accompanied by social conflict and other types of social dysfunction. The latter is the case in human society and certain situations affecting animals in captivity. In all cases, secretion of glucocorticosteroids and activity of other mediators of allostasis such as the autonomic nervous system, CNS neurotransmitters, and inflammatory cytokines wax and wane with allostatic load. If allostatic load is chronically high, then pathologies develop. Type 2 allostatic overload does not trigger an escape response, and can only be counteracted through learning and changes in the social structure.

Non-invasive methods to measure androgen metabolites in excrements of European stonechats, Saxicola torquata rubicola

Traditionally androgen concentrations are measured invasively in blood plasma. However, non-invasive methods to detect androgens are desirable, as this reduces interference with the natural behavior of the study species and multiple samples can be obtained relatively easy. The aim of this study was to validate a method to measure androgens non-invasively in excrements of male European stonechats (Saxicola torquata rubicola). Extracts of excrements of a male stonechat injected with [3H]testosterone ([3H]T) were chromatographically separated using high performance liquid chromatography (HPLC). The resulting HPLC fractions were then analyzed with a radioimmunoassay against testosterone (T-RIA). The results showed that the assay picked up major metabolites of [3H]T. The physiological relevance of excreted androgen metabolites was further validated by showing that injection of exogenous GnRH to seven males led to a significant increase in excreted androgen metabolites. In contrast, androgen metabolite levels of six saline-injected control males did not increase. Furthermore, excrements from nine males were collected from January until April to see whether the typical seasonal increase in testosterone levels can also be traced when measuring excreted androgen metabolites. As expected, there was a significant seasonal increase in androgen metabolite concentrations. Thus, the T-RIA measures androgen metabolites in droppings of male European stonechats and to our knowledge this study represents the first validation of a non-invasive androgen assay in a passerine bird.

Corticosterone and nocturnal torpor in the rufous hummingbird (Selasphorus rufus)

Three experiments were designed to investigate whether corticosterone (CORT), known to have a role in restoration of energy homeostasis, regulates nocturnal torpor, an energy conservation state used by some small mammals and birds to offset environmental challenges to energy balance. In two experiments, one during autumn migration and one during early spring molt, captive rufous hummingbirds (Selasphorus rufus) were fed control and dilute (85% strength) nectar on alternate days. In migratory birds, torpor occurred more frequently over all, and nectar dilution resulted in increased torpor duration and increased concentration of CORT in evening but not midday cloacal fluid (CF) samples. In molting birds, torpor occurred infrequently on both control and food dilution days, but, although there was a significant increase in evening CF CORT on food dilution days, torpor duration did not increase significantly in response and there was no correlation between torpor duration and CF CORT at either time of day. Daily CF CORT patterns showed an increase from midday to evening during migration, but the reverse pattern during the molt. In a third experiment, CORT administered in the nectar elevated the use of torpor and depressed food intake. The results of these three experiments support the hypothesis that CORT is involved in the regulation of torpor, but suggest that some feature of the CORT signal other than concentration per se may be required to fully explain seasonal changes in the relations among energy challenge, CORT, and nocturnal torpor in hummingbirds.