Flexibility underlies differences in mitochondrial respiratory performance between migratory and non-migratory White-crowned Sparrows (Zonotrichia leucophrys)

Migration is one of the most energy-demanding behaviors observed in birds. Mitochondria are the primary source of energy used to support these long-distance movements, yet how mitochondria meet the energetic demands of migration is scarcely studied. We quantified changes in mitochondrial respiratory performance in the White-crowned Sparrow (Zonotrichia leucophrys), which has a migratory and non-migratory subspecies. We hypothesized that the long-distance migratory Gambel's subspecies (Z. l. gambelii) would show higher mitochondrial respiratory performance compared to the non-migratory Nuttall's subspecies (Z. l. nuttalli). We sampled Gambel's individuals during spring pre-migration, active fall migration, and a period with no migration or breeding (winter). We sampled Nuttall's individuals during periods coinciding with fall migration and the winter period of Gambel's annual cycle. Overall, Gambel's individuals had higher citrate synthase, a proxy for mitochondrial volume, than Nuttall's individuals. This was most pronounced prior to and during migration. We found that both OXPHOS capacity (state 3) and basal respiration (state 4) of mitochondria exhibit high seasonal flexibility within Gambel's individuals, with values highest during active migration. These values in Nuttall's individuals were most similar to Gambel's individuals in winter. Our observations indicate that seasonal changes in mitochondrial respiration play a vital role in migration energetics.

Comparison of the Phenotypic Flexibility of Muscle and Body Condition of Migrant and Resident White-Crowned Sparrows

AbstractSeasonally breeding birds express variations of traits (phenotypic flexibility) throughout their life history stages that represent adaptations to environmental conditions. Changes of body condition during migration have been well studied, whereas alterations of skeletal and cardiac muscles, body mass, and fat scores have yet to be characterized throughout the spring or fall migratory stages. Additionally, we examined flexible patterns of muscle, body mass, and fat score in migrant white-crowned sparrows (Zonotrichia leucophrys gambelii) in comparison with those in a resident subspecies (Zonotrichia leucophrys nuttalli) during the stages they share to evaluate the influence of different life histories. Migrants showed hypertrophy of the pectoralis muscle fiber area on the wintering grounds in late prealternate molt, yet increased pectoralis muscle mass was not detected until birds readied for spring departure. While pectoralis profile and fat scores enlarged at predeparture in spring and fall, pectoralis, cardiac, and body masses were greater only in spring stages, suggesting seasonal differences for migratory preparation. Gastrocnemius mass showed little change throughout all stages, whereas gastrocnemius fiber area declined steadily but rebounded in fall on the wintering grounds, where migrants become more sedentary. In general, residents are heavier birds with larger leg structures, while migrants sport longer wings and greater heart mass. Phenotypic flexibility was most prominent among residents with peaks of pectoralis, gastrocnemius, and body masses during the winter stage, when local weather is most severe. Thus, the subspecies express specific patterns of phenotypic flexibility with peaks coinciding with the stages of heightened energy demands: the winter stage for residents and the spring stages for migrants.

Sexual selection for extreme physical performance in a polygynous bird is associated with exceptional sex differences in oxygen carrying capacity

In many animals, males compete for access to fertile females. The resulting sexual selection leads to sex differences in morphology and behaviour, but may also have consequences for physiology. Pectoral sandpipers are an arctic-breeding polygynous shorebird in which males perform elaborate displays around-the-clock and move over long distances to sample potential breeding sites, implying the need for physiological adaptations to cope with extreme endurance. We examined the oxygen carrying capacity of pectoral sandpipers, measured as the volume percentage of red blood cells in blood (haematocrit, Hct). We found a remarkable sex difference in Hct levels, with males having much higher values (58.9 ± 3.8 s.d.) than females (49.8 ± 5.3 s.d.). While Hct values of male pectoral sandpipers are notable for being among the highest recorded in birds, the sex difference we report is unprecedented and more than double that of any previously described. We also show that Hct values declined after arrival to the breeding grounds in females, but not in males, suggesting that males maintain an aerobic capacity during the mating period equivalent to that sustained during trans-hemispheric migration. We conclude that sexual selection for extreme physical performance in male pectoral sandpipers has led to exceptional sex differences in oxygen carrying capacity.

Migratory songbirds exhibit seasonal modulation of the oxygen cascade

Migratory flight requires birds to maintain intensive aerobic exercise for many hours or days. Maintaining O2 supply to flight muscles is therefore important during migration, especially since migratory songbirds have been documented flying at altitudes greater than 5000 m above sea level, where O2 is limited. Whether songbirds exhibit seasonal plasticity of the O2 cascade to maintain O2 uptake and transport during migratory flight is not well understood. We investigated changes in the hypoxic ventilatory response, haematology and pectoralis (flight) muscle phenotype of 6 songbird species from 3 families during migratory and non-migratory conditions. Songbirds were captured during southbound migration in southern Ontario, Canada. Half of the birds were assessed during migration, and the rest were transitioned onto a winter photoperiod to induce a non-migratory phenotype and measured. All species exhibited seasonal plasticity at various stages along the O2 cascade, but not all species exhibited the same responses. Songbirds tended to be more hypoxia tolerant during migration, withstanding 5 kPa O2 and breathed more effectively through slower, deeper breaths. Warblers had a stronger haemoglobin-O2 affinity during autumn migration (decrease of ∼4.7 Torr), while the opposite was observed in thrushes (increase of ∼2.6 Torr). In the flight muscle there was an ∼1.2-fold increase in the abundance of muscle fibres with smaller fibre transverse areas during autumn migration, but no changes in capillary:fibre ratio. These adjustments would enhance O2 uptake and transport to the flight muscle. Our findings demonstrate that in the O2 cascade there is no ideal migratory phenotype for all songbirds.

Physiological impacts of chronic and experimental Plasmodium infection on breeding-condition male songbirds

While Plasmodium parasitism is common in songbirds, its impact on avian reproduction is unclear owing to conflicting reports in the existing literature. Particularly understudied is the impact of phase of infection on variation in host reproductive physiology in wild, breeding-condition birds. However, assessing the full impact of Plasmodium on reproductive success in the wild can be difficult because individuals experiencing severe effects of parasitism may not enter the breeding population and may be less likely to be captured during field studies. To address these factors, we quantified metrics of health and reproductive physiology in wild-caught, breeding-condition male dark-eyed juncos (Junco hyemalis hyemalis) before and after experimental Plasmodium inoculation in a captive setting. Metrics of health and reproductive physiology included activity rate, hematocrit, scaled body mass, testosterone and sperm production. Individuals already infected at capture (i.e., chronically infected) had higher levels of hematocrit than males without chronic infections. Experimentally infected males showed a larger reduction in hematocrit and activity rate as compared to controls. However, chronic infection status did not influence the extent of metric decline. Testosterone production did not vary by treatment and most birds produced sperm following inoculation. Broadly, our results suggest that male juncos exposed to Plasmodium during the breeding season likely experience declines in general health, but Plasmodium infections do not negatively impact reproductive physiology. We conclude that physiological tradeoffs in males may favor maintenance of reproductive function despite infection.

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.

Perspectives on environmental heterogeneity and seasonal modulation of stress response in neotropical birds

Corticosterone (CORT), the main glucocorticoid in birds, regulates physiological and behavioral traits linked to predictable and unpredictable environmental fluctuations (i.e., stressors). Baseline and stress-induced CORT concentrations are known to fluctuate seasonally, linked to life history stages (LHS) such as breeding, molt, and wintering stage. These variations have been relatively well described in North American birds, but poorly addressed in neotropical species. To fill this gap, we explored how baseline and stress-induced CORT variation by LHS was affected by seasonality and environmental heterogeneity (i.e., frequency of unpredictable events such as droughts, flashfloods, etc) within the Neotropics using two approaches. First, we reviewed all currently available data about CORT concentrations for neotropical bird species. Second, we performed an in-depth analysis comparing the CORT responses of the two most common species of the Zonotrichia genus from North and South America (Z. leucophrys and Z. capensis, respectively) and their subspecies to seasonality and environmental heterogeneity. These species have been analyzed with the same methodology, allowing for an in-depth comparison of CORT variations. Despite scant data on neotropical bird species, we observed overlap between molt and breeding, and lower fluctuations of CORT among LHS. These patterns would be considered atypical compared to those described for North temperate species. Further, we found no significant associations between environmental heterogeneity and the stress-responses. In Zonotrichia we observed a positive association between baseline and stress-induced concentrations of CORT and latitude. We also observed differences by LHS. Both baseline and stress-induced CORT concentrations were higher during breeding and lower during molt. In addition, for both species, the overall pattern of seasonal modulation of stress response was heavily influenced by the migration strategy, with long-distance migrants showing significantly higher stress-induced CORT levels. Our results highlight the need for more data collection in the Neotropics. Comparative data would shed further light on the sensitivity of the adrenocortical response to stress under different scenarios of environmental seasonality and unpredictability.

Riding out the storm: depleted fat stores and elevated hematocrit in a small bodied endotherm exposed to severe weather

In the mid-continental grasslands of North America, climate change is increasing the intensity and frequency of extreme weather events. Increasingly severe storms and prolonged periods of elevated temperatures can impose challenges that adversely affect an individual's condition and, ultimately, survival. However, despite mounting evidence that extreme weather events, such as heavy rain storms, can impose short-term physiological challenges, we know little regarding the putative costs of such weather events. To determine the consequences of extreme weather for small endotherms, we tested predictions of the relationships between both severe precipitation events and wet bulb temperatures (an index that combines temperature and humidity) prior to capture with body composition and hematocrit of grasshopper sparrows (Ammodramus savannarum) caught during the breeding season at the Konza Prairie Biological Station, Kansas, USA, between 2014 and 2016. We measured each individual's fat mass, lean mass and total body water using quantitative magnetic resonance in addition to their hematocrit. Individuals exposed to storms in the 24 hours prior to capture had less fat reserves, more lean mass, more water and higher hematocrit than those exposed to moderate weather conditions. Furthermore, individuals stored more fat if they experienced high wet bulb temperatures in the week prior to capture. Overall, the analysis of these data indicate that extreme weather events take a physiological toll on small endotherms, and individuals may be forced to deplete fat stores and increase erythropoiesis to meet the physiological demands associated with surviving a storm. Elucidating the potential strategies used to cope with severe weather may enable us to understand the energetic consequences of increasingly severe weather in a changing world.

Interactive effects of multiscale diversification practices on farmland bird stress

Farmland diversification practices (i.e., methods used to produce food sustainably by enhancing biodiversity in cropping systems) are sometimes considered beneficial to both agriculture and biodiversity, but most studies of these practices rely on species richness, diversity, or abundance as a proxy for habitat quality. Biodiversity assessments may miss early clues that populations are imperiled when species presence does not imply persistence. Physiological stress indicators may help identify low-quality habitats before population declines occur. We explored how avian stress indicators respond to on-farm management practices and surrounding seminatural area (1-km radius) across 21 California strawberry farms. We examined whether commonly used biodiversity metrics correlate with stress responses in wild birds. We used ∼1000 blood and feather samples and body mass and wing chord measurements, mostly from passerines, to test the effects of diversification practices on four physiological stress indicators: heterophil to lymphocyte ratios (H:L), body condition, hematocrit values, and feather growth rates of individual birds. We then tested the relationship between physiological stress indicators and species richness, abundance, occurrence, and diversity derived from 285 bird point count surveys. After accounting for other biological drivers, landscape context mediated the effect of local farm management on H:L and body condition. Local diversification practices were associated with reduced individual stress in intensive agricultural landscapes but increased it in landscapes surrounded by relatively more seminatural area. Feathers grew more slowly in landscapes dominated by strawberry production, suggesting that nutritional condition was lower here than in landscapes with more crop types and seminatural areas. We found scant evidence that species richness, abundance, occurrence, or diversity metrics were correlated with the individual's physiological stress, suggesting that reliance on these metrics may obscure the impacts of management on species persistence. Our findings underscore the importance of considering landscape context when designing local management strategies to promote wildlife conservation.

Gene expression of sex steroid metabolizing enzymes and receptors in the skeletal muscle of migrant and resident subspecies of white-crowned sparrow (Zonotrichia leucophrys)

Circulating sex steroid concentrations vary dramatically across the year in seasonally breeding animals. The ability of circulating sex steroids to effect muscle function can be modulated by changes in intracellular expression of steroid metabolizing enzymes (e.g., 5α-reductase type 2 and aromatase) and receptors. Together, these combined changes in plasma hormones, metabolizing enzymes and receptors allow for seasonally appropriate changes in skeletal muscle function. We tested the hypothesis that gene expression of sex steroid metabolizing enzymes and receptors would vary seasonally in skeletal muscle and these changes would differ between a migrant and resident life history strategy. We quantified annual changes in plasma testosterone and gene expression in pectoralis and gastrocnemius skeletal muscles using quantitative polymerase chain reaction (qPCR) in free-living migrant (Zonotrichia leucophrys gambelii) and resident (Z. l. nuttalli) subspecies of white-crowned sparrow during breeding, pre-basic molt, and wintering life history stages. Pectoralis muscle profile was largest in migrants during breeding, while residents maintained large muscle profiles year-round. Circulating testosterone peaked during breeding in both subspecies. Pectoralis muscle androgen receptor mRNA expression was lower in females of both subspecies during breeding. Estrogen receptor-α expression was higher in the pectoralis muscle, but not gastrocnemius, of residents throughout the annual cycle when compared to migrants. Pectoralis aromatase expression was higher in resident males compared to migrant males. No differences were observed for 5α-reductase 2. Between these two subspecies, patterns of plasma testosterone and androgen receptors appear to be conserved, however estrogen receptor gene expression appears to have diverged.

Short- and long-distance avian migrants differ in exercise endurance but not aerobic capacity

Background

Migratory birds differ markedly in their migration strategies, particularly those performing short- versus long-distance migrations. In preparation for migration, all birds undergo physiological and morphological modifications including enlargement of fat stores and pectoral muscles to fuel and power their flights, as well as cardiovascular and biochemical adjustments that improve lipid and oxygen delivery and uptake by flight muscles. While the magnitude of these changes varies in relation to migration strategy, the consequence of these variations on aerobic performance is unknown. We tested whether the aerobic performance of four Old-world flycatcher species (Muscicapidae) varied according to migration strategy by comparing minimum resting metabolic rates (RMRmin), exercise-induced maximum metabolic rates (MMR), and exercise endurance times of short-distance and long-distance migratory birds.

Results

As expected, RMRmin did not vary between short-distance and long-distance migrants but differed between the species within a migration strategy and between sexes. Unexpectedly, MMR did not vary with migration strategy, but MMR and blood haemoglobin content were positively related among the birds tested. Exercise endurance times differed substantially between migration strategies with long-distance migrants sustaining exercise for > 60% longer than short-distance migrants. Blood haemoglobin content had a significant positive effect on endurance among all birds examined.

Conclusions

The lack of difference in RMRmin and MMR between long- and short-distance migrants during this stage of migration suggests that the attributes favouring the greater aerobic endurance of long-distance migrants did not come at the expense of increased maintenance costs or require greater aerobic capacity.

Varying aerobic capacity in relation to breeding stage and reproductive success in giant petrels (Macronectes spp.)

Reproduction, and parental care in particular, are among the most energy-demanding activities within the annual cycle of adult birds. Parents that cannot meet the metabolic demands and other physiological costs of raising offspring may opt to abandon chicks in favour of self-maintenance and future reproduction. Recent work examining reproductive trade-offs in birds revealed an important role of oxygen carrying capacity in mediating variation in parental effort. This study explores the aerobic factors underlying the success or failure of parental care in two closely-related petrel species during their breeding season on Bird Island, South Georgia: northern giant petrels (Macronectes halli) and southern giant petrels (M. giganteus). Failed breeders of both sexes and species had significantly lower hematocrit levels (by 5.48 ± 0.64%) than successful breeders, and reticulocyte counts also tended to be lower in failed males, consistent with the hypothesis that parental care and workload depend on aerobic capacity. We discuss these results in relation to differences in the foraging ecology of both species and sexes.

Stress in paradise: effects of elevated corticosterone on immunity and avian malaria resilience in a Hawaiian passerine

Vertebrates confronted with challenging environments often experience an increase in circulating glucocorticoids, which result in morphological, physiological and behavioral changes that promote survival. However, chronically elevated glucocorticoids can suppress immunity, which may increase susceptibility to disease. Since the introduction of avian malaria to Hawaii a century ago, low-elevation populations of Hawaii Amakihi (Chlorodrepanis virens) have undergone strong selection by avian malaria and evolved increased resilience (the ability to recover from infection), while populations at high elevation with few vectors have not undergone selection and remain susceptible. We investigated how experimentally elevated corticosterone affects the ability of high- and low-elevation male Amakihi to cope with avian malaria by measuring innate immunity, hematocrit and malaria parasitemia. Corticosterone implants resulted in a decrease in hematocrit in high- and low-elevation birds but no changes to circulating natural antibodies or leukocytes. Overall, leukocyte count was higher in low- than in high-elevation birds. Malaria infections were detected in a subset of low-elevation birds. Infected individuals with corticosterone implants experienced a significant increase in circulating malaria parasites while untreated infected birds did not. Our results suggest that Amakihi innate immunity measured by natural antibodies and leukocytes is not sensitive to changes in corticosterone, and that high circulating corticosterone may reduce the ability of Amakihi to cope with infection via its effects on hematocrit and malaria parasite load. Understanding how glucocorticoids influence a host's ability to cope with introduced diseases provides new insight into the conservation of animals threatened by novel pathogens.

Summary: Amakihi innate immunity, as measured by natural antibodies and leukocytes, is not sensitive to changes in corticosterone, but high circulating corticosterone may reduce the ability of Amakihi to cope with avian malaria infection via its effects on hematocrit and malaria parasite load.

Snow Buntings Maintain Winter-Level Cold Endurance While Migrating to the High Arctic

Arctic breeding songbirds migrate early in the spring and can face winter environments requiring cold endurance throughout their journey. One such species, the snow bunting (Plectrophenax nivalis), is known for its significant thermogenic capacity. Empirical studies suggest that buntings can indeed maintain winter cold acclimatization into the migratory and breeding phenotypes when kept captive on their wintering grounds. This capacity could be advantageous not only for migrating in a cold environment, but also for facing unpredictable Arctic weather on arrival and during preparation for breeding. However, migration also typically leads to declines in the sizes of several body components linked to metabolic performance. As such, buntings could also experience some loss of cold endurance as they migrate. Here, we aimed to determine whether free-living snow buntings maintain a cold acclimatized phenotype during spring migration. Using a multi-year dataset, we compared body composition (body mass, fat stores, and pectoralis muscle thickness), oxygen carrying capacity (hematocrit) and metabolic performance (thermogenic capacity – Msum and maintenance energy expenditure – BMR) of birds captured on their wintering grounds (January–February, Rimouski, QC, 48°N) and during pre-breeding (April–May) in the Arctic (Alert, NU, 82°). Our results show that body mass, fat stores and Msum were similar between the two stages, while hematocrit and pectoralis muscle thickness were lower in pre-breeding birds than in wintering individuals. These results suggest that although tissue degradation during migration may affect flight muscle size, buntings are able to maintain cold endurance (i.e., Msum) up to their Arctic breeding grounds. However, BMR was higher during pre-breeding than during winter, suggesting higher maintenance costs in the Arctic.

Seasonal differences in hypothalamic thyroid-stimulating hormone β, gonadotropin-releasing hormone-I and deiodinase expression between migrant and resident subspecies of white-crowned sparrow (Zonotrichia leucophrys)

Across taxa, the seasonal transition between non-breeding and breeding states is controlled by localised thyroid hormone signalling in the deep brain via reciprocal switching of deiodinase enzyme expression from type 3 (DIO3) to type 2 (DIO2). This reciprocal switch is considered to be mediated by increasing thyroid-stimulating hormone β (TSHβ) release from the pars tuberalis, which occurs in response to a change in photoperiod. Although well characterised in a handful of model organisms in controlled laboratory settings, this pathway remains largely unexplored in free-living animals under natural environmental conditions. In this comparative gene expression study, we investigated hypothalamic thyroid hormone signalling in two seasonally breeding subspecies of white-crowned sparrow (Zonotrichia leucophrys), across the entirety of their annual cycles. The migratory Gambel's (Z. l. gambelii) and resident Nuttall's (Z. l. nuttalii) subspecies differ with respect to timing of reproduction, as well as life history stage and migratory strategies. Although DIO3 mRNA expression was elevated and DIO2 mRNA expression was reduced in the wintering period in both subspecies, DIO2 peaked in both subspecies prior to the onset of reproduction. However, there was differential timing between subspecies in peak DIO2 expression. Intriguingly, seasonal modulation of TSHβ mRNA was only observed in migrants, where expression was elevated at the start of breeding, consistent with observations from other highly photoperiodic species. There was no correlation between TSHβ, DIO2 and gonadotropin-releasing hormone-I mRNA or reproductive metrics in residents. Based on these observed differences, we discuss potential implications for our understanding of how changes in medial basal hypothalamic gene expression mediates initiation of seasonal reproduction.

Flight muscle and heart phenotypes in the high-flying ruddy shelduck

Ruddy shelduck migrate from wintering grounds in lowland India and Myanmar to breeding grounds in central China and Mongolia, sustaining flight over the Himalayas, where oxygen availability is greatly reduced. We compared phenotypes of the pectoralis muscle and the ventricle of the heart from ruddy shelduck and common shelduck (a closely related low-altitude congener) that were raised in common conditions at sea level, predicting that oxidative capacity would be greater in ruddy shelduck to support high-altitude migration. Fibre-type composition of the pectoralis and the maximal activity of eight enzymes involved in mitochondrial energy metabolism in the pectoralis and heart, were compared between species. Few differences distinguished ruddy shelduck from common shelduck in the flight muscle, with the exception that ruddy shelduck had higher activities of complex II and higher ratios of complex IV (cytochrome c oxidase) and complex II when expressed relative to citrate synthase activity. There were no species differences in fibre-type composition, so these changes in enzyme activity may reflect an evolved modification in the functional properties of muscle mitochondria, potentially influencing mitochondrial respiratory capacity and/or oxygen affinity. Ruddy shelduck also had higher lactate dehydrogenase activity concurrent with lower pyruvate kinase and hexokinase activity in the left ventricle, which likely reflects an increased capacity for lactate oxidation by the heart. We conclude that changes in pathways of mitochondrial energy metabolism in the muscle and heart may contribute to the ability of ruddy shelduck to fly at high altitude.

Annual regulation of adrenocortical function in migrant and resident subspecies of white-crowned sparrow

Corticosterone affects physiology and behavior both during normal daily processes but also in response to environmental challenges and is known to mediate life history trade-offs. Many studies have investigated patterns of corticosterone production at targeted times of year, while ignoring underlying annual profiles. We aimed to understand the annual regulation of hypothalamic-pituitary-adrenal (HPA) axis function of both migrant (Zonotrichia leucophrys gambelii; n = 926) and resident (Z. l. nutalli; n = 688) subspecies of white-crowned sparrow and how it is influenced by environmental conditions - wind, precipitation, and temperature. We predicted that more dramatic seasonal changes in baseline and stress-induced corticosterone would occur in migrants to precisely time the onset of breeding and cope with environmental extremes on their arctic breeding grounds, while changes in residents would be muted as they experience a more forgiving breeding schedule and comparatively benign environmental conditions in coastal California. During the course of a year, the harshest conditions were experienced the summer breeding grounds for migrants, at which point they had higher corticosterone levels compared to residents. For residents, the winter months coincided with harshest conditions at which point they had higher corticosterone levels than migrants. For both subspecies, corticosterone tended to rise as environmental conditions became colder and windier. We found that the annual maxima in stress-induced corticosterone occurred prior to egg lay for all birds except resident females. Migrants had much higher baseline and acute stress-induced corticosterone during breeding compared to residents; where in a harsher environment the timing of the onset of reproduction is more critical because the breeding season is shorter. Interestingly, molt was the only stage within the annual cycle in which subspecies differences were absent suggesting that a requisite reduction in corticosterone may have to be met for feather growth. These data suggest that modulation of the HPA axis is largely driven by environmental factors, social cues, and their potential interactions with a genetic program.

Evaluating endoplasmic reticulum stress and unfolded protein response through the lens of ecology and evolution

Considerable progress has been made in understanding the physiological basis for variation in the life-history patterns of animals, particularly with regard to the roles of oxidative stress and hormonal regulation. However, an underappreciated and understudied area that could play a role in mediating inter- and intraspecific variation of life history is endoplasmic reticulum (ER) stress, and the resulting unfolded protein response (UPR^ER ). ER stress response and the UPR^ER maintain proteostasis in cells by reducing the intracellular load of secretory proteins and enhancing protein folding capacity or initiating apoptosis in cells that cannot recover. Proper modulation of the ER stress response and execution of the UPR^ER allow animals to respond to intracellular and extracellular stressors and adapt to constantly changing environments. ER stress responses are heritable and there is considerable individual variation in UPR^ER phenotype in animals, suggesting that ER stress and UPR^ER phenotype can be subjected to natural selection. The variation in UPR^ER phenotype presumably reflects the way animals respond to ER stress and environmental challenges. Most of what we know about ER stress and the UPR^ER in animals has either come from biomedical studies using cell culture or from experiments involving conventional laboratory or agriculturally important models that exhibit limited genetic diversity. Furthermore, these studies involve the assessment of experimentally induced qualitative changes in gene expression as opposed to the quantitative variations that occur in naturally existing populations. Almost all of these studies were conducted in controlled settings that are often quite different from the conditions animals experience in nature. Herein, we review studies that investigated ER stress and the UPR^ER in relation to key life-history traits including growth and development, reproduction, bioenergetics and physical performance, and ageing and senescence. We then ask if these studies can inform us about the role of ER stress and the UPR^ER in mediating the aforementioned life-history traits in free-living animals. We propose that there is a need to conduct experiments pertaining to ER stress and the UPR^ER in ecologically relevant settings, to characterize variation in ER stress and the UPR^ER in free-living animals, and to relate the observed variation to key life-history traits. We urge others to integrate multiple physiological systems and investigate how interactions between ER stress and oxidative stress shape life-history trade-offs in free-living animals.

Recovery of hematocrit and fat deposits varies by cage size in food-restricted captive red crossbills (Loxia curvirostra)

Hematocrit-or the percent volume of red blood cells in whole blood-is thought to fluctuate adaptively in response to changing oxygen demands that occur during different life activities and in different environments. Because red blood cells are made from materials that can be limiting, however, it is thought that hematocrit may also reflect general body condition and access to resources. We tested the effect of hydration state, resource restriction (i.e., time available to forage), and activity (i.e., different cage sizes) on hematocrit in captive red crossbills (Loxia curvirostra). We found no evidence that a mild dehydration protocol impacts hematocrit and only weak support that mild food restriction impacts hematocrit. Food restriction did, however, reduce fat deposits and fat loss was more significant in birds that were also sampled for hematocrit. Furthermore, food-restricted birds housed in flight aviaries recovered hematocrit but not fat stores following repeated blood sampling, whereas birds housed in small cages lost additional hematocrit but mitigated fat loss following successive bleeds. Together these results suggest that different flight demands may determine response to blood loss during food restriction, potentially revealing a trade-off between fat storage and red blood cell development. Our results also demonstrate the need for scientists to carefully record hematocrit data and the time course across which multiple tubes of blood are collected to avoid confounding real patterns with variation generated by sampling protocol.

The threat of global mercury pollution to bird migration: potential mechanisms and current evidence

Mercury is a global pollutant that has been widely shown to adversely affect reproduction and other endpoints related to fitness and health in birds, but almost nothing is known about its effects on migration relative to other life cycle processes. Here I consider the physiological and histological effects that mercury is known to have on non-migrating birds and non-avian vertebrates to identify potential mechanisms by which mercury might hinder migration performance. I posit that the broad ability of mercury to inactivate enzymes and compromise the function of other proteins is a single mechanism by which mercury has strong potential to disrupt many of the physiological processes that make long-distance migration possible. In just this way alone, there is reason to expect mercury to interfere with navigation, flight endurance, oxidative balance, and stopover refueling. Navigation and flight could be further affected by neurotoxic effects of mercury on the brain regions that process geomagnetic information from the visual system and control biomechanics, respectively. Interference with photochemical reactions in the retina and decreases in scotopic vision sensitivity caused by mercury also have the potential to disrupt visual-based magnetic navigation. Finally, migration performance and possibly survival might be limited by the immunosuppressive effects of mercury on birds at a time when exposure to novel pathogens and parasites is great. I conclude that mercury pollution is likely to be further challenging what is already often the most difficult and perilous phase of a migratory bird's annual cycle, potentially contributing to global declines in migratory bird populations.

Season and sex have different effects on hematology and cytokines in striped hamsters (Cricetulus barabensis)

Animals in the temperate zones face seasonal variations in environments and hence their immune responses change seasonally. In the current study, seasonal changes in hematological parameters and cytokines in striped hamsters (Cricetulus barabensis) were examined to test the winter immunoenhancement hypothesis, which states that immune function tends to increase in fall and winter compared with other seasons. Male and female hamsters were captured from the wild in the fall and winter of 2014 and in the spring and summer of 2015. Maximum body mass in both sexes and relative fatness in female hamsters occurred in the summer, indicating that body condition was the best during this season. All hematological parameters were not different between male and female hamsters, and were also not affected by the interaction of season and sex except neutrophil granulocytes (GRAN). Red blood cells (RBC) and haematocrit (PCV) were higher in the fall and winter, and hemoglobin concentration (HGB) was the highest in winter in hamsters compared with the spring and summer, implying that their oxygen-carrying capacity and oxygen affinity of the blood increased during these seasons. Compared with other seasons, the number of white blood cells (WBC) was higher in winter than in summer, intermediate granulocytes (MID), the percent of MID (MID%), GRAN and the percent of GRAN (GRAN%) were the highest in winter, which all supported the winter immunoenhancement hypothesis. However, the count of lymphocytes (LYMF) was the highest in spring, being inconsistent with this hypothesis. IL-2 levels, but not TNF-α, were influenced by seasons, sex and their interaction in hamsters. Regardless of sex, IL-4 titres were higher in spring and summer than in fall and winter in hamsters. INF-γ titres in male hamsters did not differ between the spring and summer, while its titres in female hamsters was lower in spring in contrast with winter and summer. Higher IL-2 and IL-4 levels during the breeding seasons might be crucial in controlling the increased possibilities of infections in these seasons. In summary, season and sex had disparate effects on different hematological profiles and the levels of cytokines in hamsters.

Corticosterone negative feedback is weaker during spring vs. autumn migration in a songbird (Junco hyemalis)

Birds face many challenges during seasonal migrations and must make important decisions about whether to accelerate, maintain, or delay travel to their final destinations. Spring migration is likely more challenging than autumn migration as spring journeys are completed more quickly and weather conditions are harsher during this time. These differential challenges may be reflected in the endocrine stress response, as the hypothalamic-pituitary-adrenal (HPA) axis is important for both daily energetic needs and coping with stressors. Indeed, most avian studies have found that both baseline and stress-induced corticosterone (CORT) levels tend to be higher in spring migrants than in autumn migrants. We hypothesized that CORT negative feedback efficacy also differs across the season, and is likely weaker during times of year when birds must be most sensitive to stressors. We therefore predicted that CORT negative feedback efficacy would be weaker during spring vs. autumn migration as spring migrants are more likely to encounter situations where they must decide whether to turn back or delay their travel. We examined male dark-eyed juncos (Junco hyemalis) during their spring and autumn stopovers in Fargo, ND, USA. Our prediction was met as we did find that negative feedback efficacy was weaker during spring vs. autumn, although we notably did not find any seasonal differences in baseline and stress-induced CORT. We also found that spring migrants were heavier, had greater subcutaneous fat stores, and had slightly higher hematocrit compared to autumn migrants. These findings suggest that CORT negative feedback sensitivity may help migrating birds effectively cope with the differential challenges of autumn and spring migration.

Haematological traits co-vary with migratory status, altitude and energy expenditure: a phylogenetic, comparative analysis

Aerobic capacity is assumed to be a main predictor of workload ability and haematocrit (Hct) and haemoglobin (Hb) have been suggested as key determinants of aerobic performance. Intraspecific studies have reported increases in Hct and Hb in response to increased workload. Furthermore, Hct and Hb vary markedly among individuals and throughout the annual cycle in free-living birds and it has been suggested that this variation reflects adaptive modulation of these traits to meet seasonal changes in energy demands. We used a comparative dataset of haematological traits, measures of metabolic rate (57 species), and life-history traits (160 species) to test several hypotheses for adaptive variation in haematology in relation to migration and altitude. We then extended these general ideas to test relationships between Hct and basal metabolic rate, daily energy expenditure and activity energy expenditure, using the 57 species that we have metabolic rate information for. We found that at the interspecific level, full migrants have higher Hct and Hb than partial migrants and non-migrants, and that altitude is positively correlated with Hb but not Hct. Hct is positively associated with activity energy expenditure (energy spent specifically on costly activities), suggesting that haematological traits could be adaptively modulated based on life-history traits and that Hct is a potential physiological mediator of energetic constraint.

Blood biochemistry and haematology of migrating loggerhead turtles (Caretta caretta) in the Northwest Atlantic: reference intervals and intra-population comparisons

We documented blood biochemistry and haematology of healthy loggerhead turtles (Caretta caretta) in the Northwest (NW) Atlantic in order to establish clinical reference intervals (RIs) for this threatened population. Blood samples were analysed from migratory loggerheads captured off the Mid-Atlantic coast of the USA in 2011, 2012, 2013 and 2016 as part of a long-term research program. Blood variables were determined using a point-of-care analyser, and a veterinary diagnostic laboratory service. We calculated 95% RIs with associated 90% confidence intervals (CIs) for each blood variable. We compared results obtained from our study of migratory loggerheads with published data for similarly sized loggerheads resident at a seasonal temperate latitude foraging area. Significant differences in several blood variables between migratory and resident turtles provided insight on energetic and health status during different behavioural states. Temperature was significantly correlated with several blood variables: lactate, pCO₂, sodium, haemoglobin and lactate dehydrogenase. Our assessment of blood chemistry in healthy loggerhead turtles in the NW Atlantic provides a baseline for clinical comparisons with turtles impacted by anthropogenic and environmental threats, and highlights the importance of identifying unique aspects of biochemical and haematological profiles for sea turtles at the intra-population level.

Low intensity blood parasite infections do not reduce the aerobic performance of migratory birds

Blood parasites (Haemosporidia) are thought to impair the flight performance of infected animals, and therefore, infected birds are expected to differ from their non-infected counterparts in migratory capacity. Since haemosporidians invade host erythrocytes, it is commonly assumed that infected individuals will have compromised aerobic capacity, but this has not been examined in free-living birds. We tested if haemosporidian infections affect aerobic performance by examining metabolic rates and exercise endurance in migratory great reed warblers (Acrocephalus arundinaceus) experimentally treated with Plasmodium relictum pGRW04 and in naturally infected wild birds over consecutive life-history stages. We found no effect of acute or chronic infections on resting metabolic rate, maximum metabolic rate or exercise endurance in either experimentally treated or free-living birds. Oxygen consumption rates during rest and while undergoing maximum exercise as well as exercise endurance increased from breeding to migration stages in both infected and non-infected birds. Importantly, phenotypic changes associated with preparation for migration were similarly unaffected by parasitaemia. Consequently, migratory birds experiencing parasitaemia levels typical of chronic infection do not differ in migratory capacity from their uninfected counterparts. Thus, if infected hosts differ from uninfected conspecifics in migration phenology, other mechanisms besides aerobic capacity should be considered.

Urban residency and leukocyte profiles in a traditionally migratory songbird

Many animals are shifting migrations in response to human activities. In particular, human-induced changes to climate and habitat (e.g., urbanization) likely facilitate animals becoming year-round residents. Because migration can be energetically expensive, shifts to sedentary behavior could minimize energetic demands incurred and any immunosuppressive effects. Residency in urban habitats could also provide abundant resources and allow sedentary animals to invest more in immunity. However, urban habitats could also expose sedentary animals to novel stressors that counter such benefits. To examine how recent shifts to residency affects physiology in ways that may shape infectious disease dynamics, we analyzed leukocyte profiles of two dark-eyed junco (Junco hyemalis) populations from southern California: the Laguna Mountain population, in which birds breed in high-elevation forests and migrate altitudinally, and the urban University of California San Diego population, which was likely established by overwintering migrants in the 1980s and has since become non-migratory. Over a two-year study of each population’s breeding season, we found no difference in the ratios of heterophils to lymphocytes between populations. However, urban residents had more leukocytes than birds from the altitudinal migrant population. A multivariate analysis suggested urban residents had fewer monocytes, but effect sizes were small. These results suggest no differences in energy demands or stressors between urban resident and altitudinal migrant populations during their breeding season. However, urban residency may confer immunological benefits through anthropogenic resources, which could have important consequences for disease dynamics..

Effects of experimental manipulation of hematocrit on avian flight performance in high- and low-altitude conditions

Despite widely held assumptions that hematocrit (Hct) is a key determinant of aerobic capacity and exercise performance, this relationship has not often been tested rigorously in birds and results to date are mixed. Migration in birds involves high-intensity exercise for long durations at various altitudes. Therefore, it provides a good model system to examine the effect of Hct on flight performance and physiological responses of exercise at high altitude. We treated yellow-rumped warblers (Setophaga coronata) with avian erythropoietin (EPO) and anti-EPO to experimentally manipulate Hct and assessed flight performance at low and high altitudes using a hypobaric wind tunnel. We showed that anti-EPO-treated birds had lower Hct than vehicle- and EPO--treated birds post-treatment. Anti-EPO-treated birds also had marginally lower exercise performance at low altitude, committing a higher number of strikes (mistakes) in the first 30 min of flight. However, anti-EPO-treated birds performed significantly better at high altitude, attaining a higher altitude in a ramped altitude challenge to 3000 m equivalent altitude, and with a longer duration of flight at high altitude. Birds exercising at high altitude showed decreased Hct, increased glucose mobilization and decreased antioxidant capacity, regardless of treatment. In summary, we provide experimental evidence that the relationship between Hct and exercise performance is dependent on altitude. Future studies should investigate whether free-living birds adaptively modulate their Hct, potentially through a combination of erythropoiesis and plasma volume regulation (i.e. hemodilution), based on the altitude they fly at during migratory flight.

Increasing photoperiod stimulates the initiation of spring migratory behaviour and physiology in a facultative migrant, the pine siskin

The transition to a migratory state involves coordinated changes in physiology and behaviour. In species with regular, predictable (obligate) migrations, increasing day length triggers the expression of a spring migratory state and androgens play an important role in stimulating its development. By contrast, we know little about the environmental cues and endocrine mechanisms that regulate migration in species with less predictable (facultative) migrations. Here, we tested whether photoperiod stimulates a migratory state in a facultative nomadic migrant, the pine siskin (Spinus pinus). We exposed wintering birds to either a naturally increasing or short-day photoperiod and measured physiological and behavioural changes indicative of a migratory state. We also examined changes in circulating hormones that may play a role in the migratory transition. Natural-day, but not short-day, birds displayed physiological preparations for migration, including increases in fat deposition, and showed increased levels of migratory restlessness. We found no evidence for a role of corticosterone in the migratory transition, but testosterone may be important. This study is the first experimental test of the role of photoperiod in regulating facultative migration and demonstrates that the predictive cue used by many obligate migrants to time spring migration is also important in a facultative migrant.

Life-history dependent relationships between body condition and immunity, between immunity indices in male Eurasian tree sparrows

In free-living animals, recent evidence indicates that innate, and acquired, immunity varies with annual variation in the demand for, and availability of, food resources. However, little is known about how animals adjust the relationships between immunity and body condition, and between innate and acquired immunity to optimize survival over winter and reproductive success during the breeding stage. Here, we measured indices of body condition (size-corrected mass [SCM], and hematocrit [Hct]), constitutive innate immunity (plasma total complement hemolysis activity [CH₅₀]) and acquired immunity (plasma immunoglobulin A [IgA]), plus heterophil/lymphocyte (H/L) ratios, in male Eurasian tree sparrows (Passer montanus) during the wintering and the breeding stages. We found that birds during the wintering stage had higher IgA levels than those from the breeding stage. Two indices of body condition were both negatively correlated with plasma CH₅₀ activities, and positively with IgA levels in wintering birds, but this was not the case in the breeding birds. However, there was no correlation between CH₅₀ activities and IgA levels in both stages. These results suggest that the relationships between body condition and immunity can vary across life-history stage, and there are no correlations between innate and acquired immunity independent of life-history stage, in male Eurasian tree sparrows. Therefore, body condition indices predict immunological state, especially during the non-breeding stage, which can be useful indicators of individual immunocompetences for understanding the variations in innate and acquired immunity in free-living animals.

Behavioral and physiological traits of migrant and resident White-crowned Sparrows: a common garden approach

To accommodate a migratory life history, migrants express a greater number of physiological and behavioral stages per annum than residents and are thus considered to have higher finite state diversity (FSD). To investigate the physiological mechanisms and constraints associated with migration, direct comparison of two subspecies of White-crowned Sparrow - migrant, Zonotrichia leucophrys gambelii and resident, Z.l.nuttalli - were made under common garden conditions of photoperiod and housing, as birds progressed from winter through the vernal life history stages. We tested the hypothesis that migrants (higher FSD) respond differently than residents (lower FSD) to the initial predictive cue, photoperiod, to initiate and integrate the progression of vernal stages of prenuptial molt, migration and development of breeding. If differences in vernal phenology were noted then the basis for the distinctions was considered genetic. Results: 1. residents had a lower threshold to vernal photoperiod with elevations of plasma androgen, growth and development of reproductive structures preceeding migrants, 2. only migrants displayed prenuptial molt, preparations for migration and migratory restlessness, 3. neither baseline nor stress-induced plasma corticosterone differed across subspecies suggesting energetic demands of the common garden were insufficient to induce a differential adrenocortical response in either subspecies highlighting the impact of environmental conditions on corticosterone secretion. Thus, in a common garden, Z.l.gambelii responds differently to the initial predictive cue, photoperiod, to initiate and execute the vernal stages of molt, migration and development of breeding in comparisons to the shared stage of breeding with Z.l.nuttalli confirming a genetic basis for the subspecies differences.

Altered expression of pectoral myosin heavy chain isoforms corresponds to migration status in the white-crowned sparrow (Zonotrichia leucophrys gambelii)

Birds undergo numerous changes as they progress through life-history stages, yet relatively few studies have examined how birds adapt to both the dynamic energetic and mechanical demands associated with such transitions. Myosin heavy chain (MyHC) expression, often linked with muscle fibre type, is strongly correlated with a muscle's mechanical power-generating capability, thus we examined several morphological properties, including MyHC expression of the pectoralis, in a long-distance migrant, the white-crowned sparrow (Zonotrichia leucophrys gambelii) throughout the progression from winter, spring departure and arrival on breeding grounds. White-crowned sparrows demonstrated significant phenotypic flexibility throughout the seasonal transition, including changes in prealternate moult status, lipid fuelling, body condition and flight muscle morphology. Pectoral MyHC expression also varied significantly over the course of the study. Wintering birds expressed a single, newly classified adult fast 2 isoform. At spring departure, pectoral isoform expression included two MyHC isoforms: the adult fast 2 isoform along with a smaller proportion of a newly present adult fast 1 isoform. By spring arrival, both adult fast isoforms present at departure remained, yet expression had shifted to a greater relative proportion of the adult fast 1 isoform. Altering pectoral MyHC isoform expression in preparation for and during spring migration may represent an adaptation to modulate muscle mechanical output to support long-distance flight.

The effect of extreme spring weather on body condition and stress physiology in Lapland longspurs and white-crowned sparrows breeding in the Arctic

Climate change is causing rapid shifts in temperature while also increasing the frequency, duration, and intensity of extreme weather. In the northern hemisphere, the spring of 2013 was characterized as extreme due to record high snow cover and low temperatures. Studies that describe the effects of extreme weather on phenology across taxa are limited while morphological and physiological responses remain poorly understood. Stress physiology, as measured through baseline and stress-induced concentrations of cortisol or corticosterone, has often been studied to understand how organisms respond to environmental stressors. We compared body condition and stress physiology of two long-distance migrants breeding in low arctic Alaska - the white-crowned sparrow (Zonotrichia leucophrys) and Lapland longspur (Calcarius lapponicus) - in 2013, an extreme weather year, with three more typical years (2011, 2012, and 2014). The extended snow cover in spring 2013 caused measureable changes in phenology, body condition and physiology. Arrival timing for both species was delayed 4-5days compared to the other three years. Lapland longspurs had reduced fat stores, pectoralis muscle profiles, body mass, and hematocrit levels, while stress-induced concentrations of corticosterone were increased. Similarly, white-crowned sparrows had reduced pectoralis muscle profiles and hematocrit levels, but in contrast to Lapland longspurs, had elevated fat stores and no difference in mass or stress physiology relative to other study years. An understanding of physiological mechanisms that regulate coping strategies is of critical importance for predicting how species will respond to the occurrence of extreme events in the future due to global climate change.