Dietary fatty acids and flight-training influence the expression of the eicosanoid hormone prostacyclin in songbirds

Diet shifts can alter tissue fatty acid composition in birds, which is subsequently related to metabolic patterns. Eicosanoids, short-lived fatty acid-derived hormones, have been proposed to mediate these relationships but neither baseline concentrations nor the responses to diet and exercise have been measured in songbirds. We quantified a stable derivative of the vasodilatory eicosanoid prostacyclin in the plasma of male European Starlings (Sturnus vulgaris, N = 25) fed semisynthetic diets with either high (PUFA) or low (MUFA) amounts of n6 fatty acid precursors to prostacyclin. Plasma samples were taken from each bird before, immediately after, and two days following a 15-day flight-training regimen that a subset of birds (N = 17) underwent. We found elevated prostacyclin levels in flight-trained birds fed the PUFA diet compared to those fed the MUFA diet and a positive relationship between prostacyclin and body condition, indexed by fat score. Prostacyclin concentrations also significantly decreased at the final time point. These results are consistent with the proposed influences of precursor availability (i.e., dietary fatty acids) and regulatory feedback associated with exercise (i.e., fuel supply and inflammation), and suggest that prostacyclin may be an important mediator of dietary influence on songbird physiology.

Sarcolipin relates to fattening, but not sarco/endoplasmic reticulum Ca2+-ATPase uncoupling, in captive migratory gray catbirds

In order to complete their energetically demanding journeys, migratory birds undergo a suite of physiological changes to prepare for long-duration endurance flight, including hyperphagia, fat deposition, reliance on fat as a fuel source, and flight muscle hypertrophy. In mammalian muscle, SLN is a small regulatory protein which binds to sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) and uncouples Ca2+ transport from ATP hydrolysis, increasing energy consumption, heat production, and cytosolic Ca2+ transients that signal for mitochondrial biogenesis, fatigue resistance and a shift to fatty acid oxidation. Using a photoperiod manipulation of captive gray catbirds (Dumetella carolinensis), we investigated whether SLN may play a role in coordinating the development of the migratory phenotype. In response to long-day photostimulation, catbirds demonstrated migratory restlessness and significant body fat stores, alongside higher SLN transcription while SERCA2 remained constant. SLN transcription was strongly correlated with h-FABP and PGC1α transcription, as well as fat mass. However, SLN was not significantly correlated with HOAD or CD36 transcripts or measurements of SERCA activity, SR membrane Ca2+ leak, Ca2+ uptake rates, pumping efficiency or mitochondrial biogenesis. Therefore, SLN may be involved in the process of storing fat and shifting to fat as a fuel, but the mechanism of its involvement remains unclear.

Songbirds avoid the oxidative stress costs of high blood glucose levels: a comparative study

Chronically high blood glucose levels (hyperglycaemia) can compromise healthy ageing and lifespan at the individual level. Elevated oxidative stress can play a central role in hyperglycaemia-induced pathologies. Nevertheless, the lifespan of birds shows no species-level association with blood glucose. This suggests that the potential pathologies of high blood glucose levels can be avoided by adaptations in oxidative physiology at the macroevolutionary scale. However, this hypothesis remains unexplored. Here, we examined this hypothesis using comparative analyses controlled for phylogeny, allometry and fecundity based on data from 51 songbird species (681 individuals with blood glucose data and 1021 individuals with oxidative state data). We measured blood glucose at baseline and after stress stimulus and computed glucose stress reactivity as the magnitude of change between the two time points. We also measured three parameters of non-enzymatic antioxidants (uric acid, total antioxidants and glutathione) and a marker of oxidative lipid damage (malondialdehyde). We found no clear evidence for blood glucose concentration being correlated with either antioxidant or lipid damage levels at the macroevolutionary scale, as opposed to the hypothesis postulating that high blood glucose levels entail oxidative costs. The only exception was the moderate evidence for species with a stronger stress-induced increase in blood glucose concentration evolving moderately lower investment into antioxidant defence (uric acid and glutathione). Neither baseline nor stress-induced glucose levels were associated with oxidative physiology. Our findings support the hypothesis that birds evolved adaptations preventing the (glyc)oxidative costs of high blood glucose observed at the within-species level. Such adaptations may explain the decoupled evolution of glycaemia and lifespan in birds and possibly the paradoxical combination of long lifespan and high blood glucose levels relative to mammals.

Flight muscle size reductions and functional changes following long-distance flight under variable humidity conditions in a migratory warbler

Bird flight muscle can lose as much as 20% of its mass during a migratory flight due to protein catabolism, and catabolism can be further exacerbated under dehydrating conditions. However, the functional consequences of exercise and environment induced protein catabolism on muscle has not been examined. We hypothesized that prolonged flight would cause a decline in muscle mass, aerobic capacity, and contractile performance. This decline would be heightened for birds placed under dehydrating environmental conditions, which typically increases lean mass losses. Yellow-rumped warblers (Setophaga coronata) were exposed to dry or humid (12 or 80% relative humidity at 18°C) conditions for up to 6 h while at rest or undergoing flight. The pectoralis muscle was sampled after flight/rest or after 24 h of recovery, and contractile properties and enzymatic capacity for aerobic metabolism was measured. There was no change in lipid catabolism or force generation of the muscle due to flight or humidity, despite reductions in pectoralis dry mass immediately post-flight. However, there was a slowing of myosin-actin crossbridge kinetics under dry compared to humid conditions. Aerobic and contractile function is largely preserved after 6 h of exercise, suggesting that migratory birds preserve energy pathways and function in the muscle.

Transcriptomic effects of Perfluoralkyl acids on the adipose tissue of a songbird species at environmentally relevant concentrations

Perfluoralkyl acids (PFAS) have been regarded as global pollutants for at least twenty years, with potentially negative physiological effects on multiple vertebrate species including humans. Here we analyze the effects of the administration of environmentally-relevant levels of PFAS on caged canaries (Serinus canaria) by using a combination of physiological, immunological, and transcriptomic analyses. This constitutes a completely new approach to understand the toxicity pathway of PFAS in birds. While we observed no effects on physiological and immunological parameters (e.g, body weight, fat index, cell-mediated immunity), the transcriptome of the pectoral fatty tissue showed changes compatible with the known effects of PFAS as obesogens in other vertebrates, particularly in mammals. First, transcripts related to the immunological response were affected (mainly enriched), including several key signaling pathways. Second, we found a repression of genes related to the peroxisome response and fatty acid metabolism. We interpret these results as indicative of the potential hazard of environmental concentrations of PFAS on the fat metabolism and the immunological system of birds, while exemplifying the ability of transcriptomic analyses of detecting early physiological responses to toxicants. As the potentially affected functions are essential for the survival of the animals during, for example, migration, our results underline the need for tight control of the exposure of natural populations of birds to these substances.

Hypothalamic molecular correlates of photoperiod-induced spring migration in intact and castrated male redheaded buntings

This study investigated the molecular changes associated with neural plasticity in photoperiodic induction of spring migration in intact and castrated redheaded bunting, Emberiza bruniceps. We measured the hypothalamic mRNA expression of genes in birds that were photostimulated into winter non-migratory and spring (vernal) migratory phenotypes under short and long photoperiods, respectively. These included genes associated with the appetitive phase of reproduction (spring migration drive, th and ddc genes encoding for tyrosine hydroxylase and dopamine decarboxylase enzymes, respectively), sleep/awake state (pmch gene encoding for pro-melanin concentrating hormone; hcrt and hcrtr2 encoding for the hypocretin/orexin and its receptor, respectively) and neurogenesis (dcx and neuN coding for doublecortin and neuronal nuclear proteins, respectively). Higher th mRNA levels suggested an upregulated dopamine synthesis in the hypothalamus of spring migrants. Similarly, elevated hcrt and hcrtr2 mRNA levels suggested an increased wakefulness, and those of dcx and neuN genes suggested an enhanced neurogenesis during the spring migration state. Further, compared to intact birds, the lower th and pmch, and higher hcrtr2 and neuN mRNA levels in castrates suggested a role of testicular steroids in modulation of the appetitive phase of reproduction, sleep and awake states, and neurogenesis during spring migration period. These results provide insights into molecular changes linked with important hypothalamic molecular pathways and steroidal influence in the photoperiodic induction of spring migration in obligate migratory songbirds.

Spatial and temporal specificity of neuroestradiol provision in the songbird

Steroid hormones are often synthesized in multiple tissues, affect several different targets, and modulate numerous physiological endpoints. The mechanisms by which this modulation is achieved with temporal and spatial specificity remain unclear. 17β-estradiol for example, is made in several peripheral tissues and in the brain, where it affects a diverse set of behaviors. How is estradiol delivered to the right target, at the right time, and at the right concentration? In the last two decades, we have learned that aromatase (estrogen-synthase) can be induced in astrocytes following damage to the brain and is expressed at central synapses. Both mechanisms of estrogen provision confer spatial and temporal specificity on a lipophilic neurohormone with potential access to all cells and tissues. In this review, I trace the progress in our understanding of astrocytic and synaptic aromatization. I discuss the incidence, regulation, and functions of neuroestradiol provision by aromatization, first in astrocytes and then at synapses. Finally, I focus on a relatively novel hypothesis about the role of neuroestradiol in the orchestration of species-specific behaviors.

Mercury exposure in songbird communities along an elevational gradient on Whiteface Mountain, Adirondack Park (New York, USA)

Mercury (Hg) is a potent neurotoxin that biomagnifies within food webs. Adverse effects have been documented for avian species related to exposure of elevated Hg levels. High elevation, boreal forests generally receive higher atmospheric Hg deposition and regional studies have subsequently identified elevated blood Hg concentrations in songbird species inhabiting these montane habitats. The overall goal of this study was to investigate spatial and seasonal Hg exposure patterns in songbird species along an elevational gradient on Whiteface Mountain in the Adirondack Park of New York State. Songbird blood samples were collected from June-July in 2009 and 2010 along an elevational gradient at 13 study plots (450-1400 m) with a focus on Catharus thrushes, including the hermit thrush, Swainson's thrush, and Bicknell's thrush. The main results of this study documented: (1) an overall linear pattern of increasing blood Hg concentrations with increasing elevation, with additional analysis suggesting a nonlinear elevational pattern of increasing blood Hg concentrations to 1075 m, followed by decreasing concentrations thereafter, for all Catharus thrush species across the elevational gradient; and (2) an overall nonlinear seasonal pattern of increasing, followed by decreasing blood Hg concentrations across target species. Avian exposure patterns appear driven by elevated atmospheric Hg deposition and increased methylmercury bioavailability within high elevation habitats as compared with low elevation forests. Seasonal patterns are likely influenced by a combination of complex and dynamic variables related to dietary selection and annual molting cycles. Considering that few high elevation analyses have been conducted within the context of regional songbird research, this project complements the results from similar studies and highlights the need for further monitoring efforts to investigate environmental Hg contamination within avian communities.

Metabolic reduction after long-duration flight is not related to fat-free mass loss or flight duration in a migratory passerine

Migratory birds catabolize large quantities of protein during long flights, resulting in dramatic reductions in organ and muscle mass. One of the many hypotheses to explain this phenomenon is that decrease in lean mass is associated with reduced resting metabolism, saving energy after flight during refueling. However, the relationship between lean body mass and resting metabolic rate remains unclear. Furthermore, the coupling of lean mass with resting metabolic rate and with peak metabolic rate before and after long-duration flight have not previously been explored. We flew migratory yellow-rumped warblers (Setophaga coronata) in a wind tunnel under one of two humidity regimes to manipulate the rate of lean mass loss in flight, decoupling flight duration from total lean mass loss. Before and after long-duration flights, we measured resting and peak metabolism, and also measured fat mass and lean body mass using quantitative magnetic resonance. Flight duration ranged from 28 min to 600 min, and birds flying under dehydrating conditions lost more fat-free mass than those flying under humid conditions. After flight, there was a 14% reduction in resting metabolism but no change in peak metabolism. Interestingly, the reduction in resting metabolism was unrelated to flight duration or to change in fat-free body mass, indicating that protein metabolism in flight is unlikely to have evolved as an energy-saving measure to aid stopover refueling, but metabolic reduction itself is likely to be beneficial to migratory birds arriving in novel habitats.

Do songbirds in wetlands show higher mercury bioaccumulation relative to conspecifics in non-wetland habitats?

Environmental conditions in wetlands facilitate favorable biogeochemical conditions for the conversion of inorganic mercury into methylmercury. For this reason, wetlands are increasingly classified as mercury hotspots, places where biota exhibit elevated mercury concentrations. While it is clear that wetlands play an important role in methylmercury production, factors such as geographic variation in mercury deposition, wetland type, and trophic dynamics can cause variation in mercury dynamics and bioaccumulation in biota occupying wetlands or connected to wetland trophic systems. Here, we use songbirds as bioindicators in a two-pronged approach aimed at evaluating the state of our understanding of mercury bioaccumulation by songbirds in wetland ecosystems. First, we use a case study in southeast Missouri to compare blood mercury concentrations in tree swallows (Tachycineta bicolor) and eastern bluebirds (Sialia sialis) occupying wetland and non-wetland habitats to test the hypothesis that songbirds in wetlands will have higher mercury bioaccumulation than those in non-wetlands. Adult tree swallows in wetlands had significantly higher blood mercury concentrations than those in non-wetlands; however, no difference between ecosystems was detected in eastern bluebirds. Second, we present a review of the current literature on mercury in songbirds in wetland ecosystems across North America. Mercury concentrations in songbirds varied among wetland types and with geographic location, often in an unpredictable manner. Mercury concentrations in songbird blood varied 3-10 fold at locations separated only by ~10 to several hundred kilometers. This magnitude of difference in blood mercury concentrations among wetlands exceeds documented differences between wetland and non-wetland ecosystems. Therefore, we caution against the automatic assumption that songbirds occupying wetlands will have higher mercury bioaccumulation than conspecifics living in other habitats.

Spatial variation in aquatic invertebrate and riparian songbird mercury exposure across a river-reservoir system with a legacy of mercury contamination

Mercury (Hg) loading and methylation in aquatic systems causes a variety of deleterious effects for fish and wildlife populations. Relatively little research has focused on Hg movement into riparian food webs and how this is modulated by habitat characteristics. This study characterized differences in Hg exposure in aquatic invertebrates and riparian songbirds across a large portion of the Willamette River system in western Oregon, starting at a Hg-contaminated Superfund site in the headwaters (Black Butte Hg Mine) and including a reservoir known to methylate Hg (Cottage Grove Reservoir), all downstream reaches (Coast Fork and Willamette River) and off-channel wetland complexes (Willamette Valley National Wildlife Refuge Complex). After accounting for year, date, and site differences in a mixed effects model, MeHg concentrations in aquatic invertebrates varied spatially among habitat categories and invertebrate orders. Similarly, THg in songbird blood varied by among habitat categories and bird species. The highest Hg concentrations occurred near the Hg mine, but Hg did not decline linearly with distance from the source of contamination. Birds were consistently elevated in Hg in habitats commonly associated with enhanced MeHg production, such as backwater or wetlands. We found a positive but weak correlation between aquatic invertebrate MeHg concentrations and songbird THg concentrations on a site-specific basis. Our findings suggest that Hg risk to riparian songbirds can extend beyond point-source contaminated areas, highlighting the importance of assessing exposure in surrounding habitats where methylmercury production may be elevated, such as reservoirs and wetlands.

Wetland water-management may influence mercury bioaccumulation in songbirds and ducks at a mercury hotspot

Mercury is a persistent, biomagnifying contaminant that can cause negative behavioral, immunological, and reproductive effects in wildlife and human populations. We examined the role of wetland water-management on mercury bioaccumulation in songbirds and ducks at Kellys Slough National Wildlife Refuge Complex, near Grand Forks, North Dakota USA. We assessed mercury concentrations in blood of wetland-foraging songbirds (80 common yellowthroats [Geothlypis trichas] and 14 Nelson's sparrows [Ammospiza nelsoni]) and eggs of upland-nesting ducks (28 gadwall [Mareca strepera], 19 blue-winged teal [Spatula discors], and 13 northern shoveler [S. clypeta]) across four wetland water-management classifications. Nelson's sparrow blood mercury concentrations were elevated (mean: 1.00 µg/g ww; 95% CL: 0.76-1.31) and similar to those reported 6 years previously. Mercury in songbird blood and duck eggs varied among wetland water-management classifications. Songbirds and ducks had 67% and 49% lower mercury concentrations, respectively, when occupying wetlands that were drawn down with water flow compared to individuals occupying isolated-depressional wetlands with no outflow. Additionally, songbirds within impounded and partially drawn-down wetland units with water flow had mercury concentrations that were 26-28% lower, respectively, than individuals within isolated-depressional wetlands with no outflow. Our results confirm that mercury concentrations in songbirds at Kellys Slough continue to be elevated and suggest that water-management could be an important tool for wetland managers to reduce bioaccumulation of mercury in birds.

Time course of photo-induced Egr-1 expression in the hypothalamus of a seasonally breeding songbird

Many seasonally-breeding species use daylength to time reproduction. Light-induced release of progonadal hormones involves a complex cascade of responses both inside and outside the brain. In this study, we used induction of early growth response 1 (Egr-1), the protein product of an immediate early gene, to evaluate the time course of such responses in male white-throated sparrows (Zonotrichia albicollis) exposed to a single long day. Induction of Egr-1 in the pars tuberalis began ∼11 h after dawn. This response was followed ∼6 h later by dramatic induction in the tuberal hypothalamus, including in the ependymal cells lining the third ventricle. At approximately the same time, Egr-1 was induced in dopaminergic and vasoactive intestinal peptide neurons in the tuberal hypothalamus and in dopaminergic neurons of the premammillary nucleus. We noted no induction in gonadotropin-releasing hormone (GnRH) neurons until 2 h after dawn the following morning. Overall, our results indicate that Egr-1 responses in GnRH neurons occur rather late during photostimulation, compared with responses in other cell populations, and that such induction may reflect new synthesis related to GnRH depletion rather than stimulation by light cues.

Behavioural response of the house mosquitoes Culex quinquefasciatus and Culex pipiens molestus to avian odours and its reliance on carbon dioxide

How Culex (Diptera: Culicidae) mosquitoes select and discriminate between potential avian hosts is critical for understanding the epidemiology of West Nile virus. Therefore, the present authors studied the behavioural responses of Culex quinquefasciatus (Say) and Culex pipiens molestus (Forsskål) to headspace volatiles of three avian species [chicken and pigeon (sexes analysed separately), and magpie], presented either alone or in combination with 600 p.p.m. carbon dioxide (CO₂ ). The attraction of Cx. quinquefasciatus to the headspace volatiles of both sexes of chicken, and of female pigeon, in combination with CO₂ was significantly higher than that achieved by the CO₂ and solvent control. Although Cx. p. molestus was attracted to headspace volatiles of chickens and magpies, it was repelled by those of female pigeons when combined with CO₂ . An increased effect between the avian volatiles and CO₂ was observed for Cx. quinquefasciatus, whereas the addition of CO₂ had no effect on the attraction of Cx. p. molestus females. The results of this study demonstrate that Cx. quinquefasciatus and Cx. p. molestus are attracted to the odour of potential avian hosts. Future studies aimed at identifying the bioactive volatile compounds in the headspace of chickens may contribute to the potential development of effective surveillance and control tools against Culex species.

Acetylcholine acts on songbird premotor circuitry to invigorate vocal output

Acetylcholine is well-understood to enhance cortical sensory responses and perceptual sensitivity in aroused or attentive states. Yet little is known about cholinergic influences on motor cortical regions. Here we use the quantifiable nature of birdsong to investigate how acetylcholine modulates the cortical (pallial) premotor nucleus HVC and shapes vocal output. We found that dialyzing the cholinergic agonist carbachol into HVC increased the pitch, amplitude, tempo and stereotypy of song, similar to the natural invigoration of song that occurs when males direct their songs to females. These carbachol-induced effects were associated with increased neural activity in HVC and occurred independently of basal ganglia circuitry. Moreover, we discovered that the normal invigoration of female-directed song was also accompanied by increased HVC activity and was attenuated by blocking muscarinic acetylcholine receptors. These results indicate that, analogous to its influence on sensory systems, acetylcholine can act directly on cortical premotor circuitry to adaptively shape behavior.

Seasonal and elevational variation in glucose and glycogen in two songbird species

Birds naturally maintain high glucose concentrations in the blood and tissues, even when relying on fat to meet the metabolic demands of flight or thermogenesis. One possibility is that high glucose levels might be needed to deal with these metabolic demands. Thus, we hypothesized that birds chronically exposed to colder temperatures and higher elevations have higher circulating glucose and tissue free glucose and glycogen compared to conspecifics living at warmer temperatures and lower elevations. Adult House Sparrows (Passer domesticus) and House Finches (Haemorhous mexicanus) were captured from Phoenix, AZ (340 m elevation), and Albuquerque, NM (1600 m elevation), during the summer and winter months. We measured plasma glucose, as well as free glucose and glycogen from multiple tissues. In general, high elevation and colder temperatures were associated with higher tissue glycogen and higher free glucose concentrations in the brain. These findings indicate that glucose and glycogen are subject to seasonal phenotypic flexibility as well as geographic variations that may relate to local food availability and abundance.

Effect of acute exposure to high ambient temperature on the thermal, metabolic and hygric physiology of a small desert bird

The intensity and frequency of extreme weather events, such as heat waves, are increasing as a consequence of global warming. Acute periods of extreme heat can be more problematic for wildlife than a chronic increase in mean temperature, to which animals can potentially acclimatise. Predicting effects of heat exposure requires a clear understanding of the capacity of individuals to respond to heat waves, so we examined the physiological response of a small desert bird, the zebra finch (Taeniopygia guttata), after acute previous exposure to high ambient temperature, simulating heatwave-like conditions. The standard physiology of the zebra finches was unaffected by prior exposure to heatwave-type conditions, suggesting that periodic exposure to heatwaves is unlikely to impact their longer-term day-to-day energy and water requirements. When finches were thermally challenged, prior experience of heatwave-like conditions did not impact overall body temperature and evaporative water loss, but birds previously experiencing high temperatures did reduce their metabolic heat production, and the variance in water loss and metabolism between individuals was significantly lower. This suggests that some individuals are more likely to become dehydrated if they have not had prior experience of high temperatures, and do not prioritise water conservation over thermoregulation. However, our observations overall suggest that acute periods of heat exposure do little to modify the general physiology of small birds, supporting the hypothesis that periodic extreme heat events may be more problematic for them than chronic warming.

Carotenoid- but not melanin-based plumage coloration is negatively related to metal exposure and proximity to the road in an urban songbird

Rapid urbanization is a global phenomenon that is increasingly exposing organisms to novel stressors. These novel stressors can affect diverse aspects of organismal function, including development of condition-dependent ornaments, which play critical roles in social and sexual selection. We investigated the relationship between metal pollution, proximity to roads, and carotenoid- and melanin-based plumage coloration in a common songbird, the great tit (Parus major). We studied populations located across a well-characterized metal pollution gradient and surrounded by roadway networks. Metal exposure and road-associated pollution could reduce carotenoid-based pigmentation by inducing oxidative stress or affecting habitat quality, but metals could also enhance melanin-based pigmentation, through effects on melanogenesis and testosterone concentrations. Using a large sample size (N > 500), we found that birds residing close to a point source for metals had reduced ultraviolet chroma, a component of carotenoid-based pigmentation. Moreover, birds with high feather metal concentrations had lower carotenoid chroma, hue, and ultraviolet chroma, with effects modified by age class. Birds residing closer to roads also had lower carotenoid chroma and hue. Melanin-based pigmentation showed high between-year repeatability, and no association with anthropogenic pollution. Results suggest that carotenoid-, but not melanin-, based pigmentation is negatively affected by multiple anthropogenic stressors. We are the first to demonstrate a negative association between roads and a plumage-based signaling trait, which could have important implications for sexual signaling dynamics in urban landscapes.

Exploration of tissue-specific gene expression patterns underlying timing of breeding in contrasting temperature environments in a song bird

BACKGROUND

Seasonal timing of breeding is a life history trait with major fitness consequences but the genetic basis of the physiological mechanism underlying it, and how gene expression is affected by date and temperature, is not well known. In order to study this, we measured patterns of gene expression over different time points in three different tissues of the hypothalamic-pituitary-gonadal-liver axis, and investigated specifically how temperature affects this axis during breeding. We studied female great tits (Parus major) from lines artificially selected for early and late timing of breeding that were housed in two contrasting temperature environments in climate-controlled aviaries. We collected hypothalamus, liver and ovary samples at three different time points (before and after onset of egg-laying). For each tissue, we sequenced whole transcriptomes of 12 pools (n = 3 females) to analyse gene expression.

RESULTS

Birds from the selection lines differed in expression especially for one gene with clear reproductive functions, zona pellucida glycoprotein 4 (ZP4), which has also been shown to be under selection in these lines. Genes were differentially expressed at different time points in all tissues and most of the differentially expressed genes between the two temperature treatments were found in the liver. We identified a set of hub genes from all the tissues which showed high association to hormonal functions, suggesting that they have a core function in timing of breeding. We also found ample differentially expressed genes with largely unknown functions in birds.

CONCLUSIONS

We found differentially expressed genes associated with selection line and temperature treatment. Interestingly, the latter mainly in the liver suggesting that temperature effects on egg-laying date may happen down-stream in the physiological pathway. These findings, as well as our datasets, will further the knowledge of the mechanisms of tissue-specific avian seasonality in the future.

PPAR expression, muscle size and metabolic rates across the gray catbird's annual cycle are greatest in preparation for fall migration

Phenotypic flexibility across the annual cycle allows birds to adjust to fluctuating ecological demands. Varying energetic demands associated with time of year have been demonstrated to drive metabolic and muscle plasticity in birds, but it remains unclear what molecular mechanisms control this flexibility. We sampled gray catbirds at five stages across their annual cycle: tropical overwintering (January), northward spring (late) migration (early May), breeding (mid June), the fall pre-migratory period (early August) and southward fall (early) migration (end September). Across the catbird's annual cycle, cold-induced metabolic rate (V̇O2summit) was highest during migration and lowest during tropical wintering. Flight muscles exhibited significant hypertrophy and/or hyperplasia during fall migratory periods compared with breeding and the fall pre-migratory period. Changes in heart mass were driven by the tropical wintering stage, when heart mass was lowest. Mitochondrial content of the heart and pectoralis remained constant across the annual cycle as quantified by aerobic enzyme activities (CS, CCO), as did lipid catabolic capacity (HOAD). In the pectoralis, transcription factors PPARα, PPARδ and ERRβ, coactivators PGC-1α and β, and genes encoding proteins associated with fat uptake (FABPpm, Plin3) were unexpectedly upregulated in the tropical wintering stage, whereas those involved in fatty acid oxidation (ATGL, LPL, MCAD) were downregulated, suggesting a preference for fat storage over utilization. Transcription factors and coactivators were synchronously upregulated during pre-migration and fall migration periods in the pectoralis but not the heart, suggesting that these pathways are important in preparation for and during early migration to initiate changes to phenotypes that facilitate long-distance migration.

New insights into the chemical forms of extremely high methylmercury in songbird feathers from a contaminated site

The chemical forms of mercury (Hg), particularly methylmercury (MeHg), in songbird feathers from an abandoned mining region were analyzed via X-ray absorption near-edge structure analysis (XANES). In feathers, proportions of MeHg as total mercury (75.6-100%) quantified by the XANES were directly comparable to the chemical extraction values (74.1-95.9%). Most of MeHg were bound with cysteine (Cys) and reduced glutathione (GSH), whereas inorganic mercury (IHg) was mainly bound with GSH. These results were consistent with those found in fish muscles and human hairs of both fish consumers and occupational Hg exposure populations. Our study suggested that chemical forms and speciation of Hg were highly dependent on the exposure sources and food consumption, respectively. Bird feathers were able to selectively accumulate MeHg due to their special binding ways. However, detailed mechanisms of Hg accumulation in bird feathers remain to be further elucidated.

Do concentrations of perfluoroalkylated acids (PFAAs) in isopods reflect concentrations in soil and songbirds? A study using a distance gradient from a fluorochemical plant

Perfluoroalkylated acids (PFAAs) are persistent chemicals that have been detected globally in the environment and in wildlife. Although it is known that PFAAs sorb to solid matrices, little is known on PFAA concentrations in soils. PFAA pollution has often been studied in aquatic invertebrates. However, this has rarely been done on terrestrial species. In the present study, we examined whether the concentrations of 15 PFAAs in isopods (Oniscidae), collected at a fluorochemical plant and in four other areas, representing a gradient in distance from the pollution source (1 km to 11 km), were related to those in the soil and in eggs of a songbird, the great tit (Parus major), collected in the same areas. Additionally, we examined the effect of physicochemical properties such as total organic carbon (TOC) and clay content on the relationship between the concentrations in soil and isopods. Finally, we examined the composition profile in the soil and isopods. Mean PFOS and PFOA concentrations of 1700 ng/g dw and 24 ng/g dw were detected in the soil at the plant. PFOS and PFPeA were the dominant PFAAs in isopods and were detected at mean concentrations of 253 and 108 ng/g ww, respectively. The great tit eggs showed elevated mean PFOS concentrations of 55,970 ng/g ww. In most cases, PFAA concentrations decreased with increasing distance from the plant. As PFAA concentrations in isopods were correlated with concentrations in the soils, isopods could serve as a bioindicator for PFAA concentrations in soils. Additionally, there were indications that isopods could also serve as a bioindicator for PFAA concentrations in eggs of great tits. However, these indications were only the case at two locations, showing the need to further monitor the possibilities of using isopods as a bioindicator for PFAA concentrations in song bird eggs. CAPSULE: Elevated PFAA concentrations in isopods reflected concentrations in songbird eggs and in soil, indicating that trophic transfer of PFAAs from soil, via isopods, to songbirds might play a role in the PFAA exposure of terrestrial songbirds.

Seasonal Variation in Genome-Wide DNA Methylation Patterns and the Onset of Seasonal Timing of Reproduction in Great Tits

In seasonal environments, timing of reproduction is a trait with important fitness consequences, but we know little about the molecular mechanisms that underlie the variation in this trait. Recently, several studies put forward DNA methylation as a mechanism regulating seasonal timing of reproduction in both plants and animals. To understand the involvement of DNA methylation in seasonal timing of reproduction, it is necessary to examine within-individual temporal changes in DNA methylation, but such studies are very rare. Here, we use a temporal sampling approach to examine changes in DNA methylation throughout the breeding season in female great tits (Parus major) that were artificially selected for early timing of breeding. These females were housed in climate-controlled aviaries and subjected to two contrasting temperature treatments. Reduced representation bisulfite sequencing on red blood cell derived DNA showed genome-wide temporal changes in more than 40,000 out of the 522,643 CpG sites examined. Although most of these changes were relatively small (mean within-individual change of 6%), the sites that showed a temporal and treatment-specific response in DNA methylation are candidate sites of interest for future studies trying to understand the link between DNA methylation patterns and timing of reproduction.

Variation in PFAA concentrations and egg parameters throughout the egg-laying sequence in a free-living songbird (the great tit, Parus major): Implications for biomonitoring studies

Over the past decades, there has been growing scientific attention and public concern towards perfluoroalkyl acids (PFAAs), due to their widespread presence in the environment and associations with adverse effects on various organisms. Bird eggs have often been used as less-invasive biomonitoring tools for toxicological risk assessments of persistent organic pollutants, including some PFAAs. Hereby, it is typically assumed that one random egg is representative for the PFAA concentrations of the whole clutch. However, variation of PFAA concentrations within clutches due to laying sequence influences can have important implications for the egg collection strategy and may impede interpretations of the quantified concentrations. Therefore, the main objective of this paper was to study variation patterns and possible laying sequence associations with PFAA concentrations in eggs of the great tit (Parus major). Eight whole clutches (4-8 eggs) were collected at a location in the Antwerp region, situated about 11 km from a known PFAA point source. The ∑ PFAA concentrations ranged from 8.9 to 75.1 ng g^-1 ww. PFOS concentrations ranged from 6.7 to 55.1 ng g^-1 ww and this compound was the dominant contributor to the total PFAA profile (74%), followed by PFDoA (7%), PFOA (7%), PFDA (5%), PFTrA (4%) and PFNA (3%). The within-clutch variation (70.7%) of the ∑ PFAA concentrations was much larger than the among-clutch variation (29.3%) and concentrations decreased significantly for some PFAA compounds throughout the laying sequence. Nevertheless, PFAA concentrations were positively and significantly correlated between some egg pairs within the same clutch, especially between egg 1 and egg 3. For future PFAA biomonitoring studies, we recommend to consistently collect the same egg along the laying sequence, preferably the first or third egg if maximizing egg exposure metrics is the main objective.

Glucocorticoid response to both predictable and unpredictable challenges detected as corticosterone metabolites in collared flycatcher droppings

In most vertebrate animals, glucocorticoid hormones are the chief mediators of homeostasis in response to ecological conditions and as they progress through their lifecycle. In addition, glucocorticoids are a major part of the stress response and stress induced elevations of the hormone can make it difficult to assess glucocorticoid secretion in response to changes in life-stage and current environmental conditions in wild animals. Particularly when quantifying circulating levels of glucocorticoids in the blood which fluctuate rapidly in response to stress. An alternative method of quantifying glucocorticoids is as hormone metabolites in faeces or urine giving a historical sample related to the gut passage time and urinary tract that is less sensitive to stressful events which cause spikes in the circulating hormone level. Although the concentration of glucocorticoid metabolites are influenced by faecal mass thereby potentially affecting any differences in hormone metabolites detected amongst samples. In the present study, we aimed to detect changes in levels of corticosterone, the primary bird glucocorticoid, in relation to the phase of reproduction, in a breeding population of collared flycatchers by sampling corticosterone metabolites in droppings. We also tested how corticosterone metabolite concentrations were affected by ambient temperature and related to body condition in adult birds. Our results indicate that the upregulation of corticosterone between incubation and nestling feeding in female birds is crucial for successful reproduction in this species. Also, females appear to downregulate corticosterone during incubation in response to lower ambient temperature and poorer body condition. Our results did not indicate a relationship between dropping mass and corticosterone metabolite concentrations, which suggests that our findings were linked to the regulation of corticosterone in response to predictable and unpredictable challenges.

Interspecific variation in the spatially-explicit risks of trace metals to songbirds

Many wild animals can be adversely affected by trace metals around point sources but little is known about the risks to birds across their ranges. Trace metals in the soil are ubiquitously, if heterogeneously distributed, across the world due to natural and anthropogenic sources. Here, we built, parameterized and applied a spatially explicit modelling framework to determine the risks of soil-associated metals to 30 invertebrate-consuming passerine species across their spatial distribution in England and Wales. The model uses a risk characterization approach to assess the risks of soil-associated metals. Various monitoring datasets were used as input parameters: soil metal concentrations in England and Wales, bird spatial distribution; bird diet, bioaccumulation and toxicity data were extracted from the literature. Our model highlights significant differences in toxicity risks from Cd, Cu, Pb and Zn across the UK distributions of different species; Pb and Zn posed risks to all species across most of species' distributions, with more localised risks to some species of conservation concern from Cd and Cu. No single taxa of invertebrate prey drove avian exposure to metal toxicity. Adults were found to be at higher risk from Pb and Zn toxicity across their distributions than nestlings. This risk was partially driven by diet, with age differences in diets identified. Our spatially explicit model allowed us to identify areas of each species' national distribution in which the population was at risk. Overall, we determined that for all species studied an average of 32.7 ± 0.2%, 8.0 ± 0.1%, 86.1 ± 0.1% and 93.2 ± 0.1% of the songbird spatial distributions in the UK were characterized at risk of Cd, Cu, Pb and Zn, respectively. Despite some limitations, our spatially explicit model helps in understanding the risks of metals to wildlife and provides an efficient method of prioritising areas, contaminants and species for environmental risk assessments. The model could be further evaluated using a targeted monitoring dataset of metal concentration in bird tissues. Our model can assess and communicate to stakeholders the potential risks of environmental contaminants to wildlife species at a national and potentially international scale.

Neuroestrogens rapidly shape auditory circuits to support communication learning and perception: Evidence from songbirds

Contribution to Special Issue on Fast effects of steroids. Steroid hormones, such as estrogens, were once thought to be exclusively synthesized in the ovaries and enact transcriptional changes over the course of hours to days. However, estrogens are also locally synthesized within neural circuits, wherein they rapidly (within minutes) modulate a range of behaviors, including spatial cognition and communication. Here, we review the role of brain-derived estrogens (neuroestrogens) as modulators within sensory circuits in songbirds. We first present songbirds as an attractive model to explore how neuroestrogens in auditory cortex modulate vocal communication processing and learning. Further, we examine how estrogens may enhance vocal learning and auditory memory consolidation in sensory cortex via mechanisms similar to those found in the hippocampus of rodents and birds. Finally, we propose future directions for investigation, including: 1) the extent of developmental and hemispheric shifts in aromatase and membrane estrogen receptor expression in auditory circuits; 2) how neuroestrogens may impact inhibitory interneurons to regulate audition and critical period plasticity; and, 3) dendritic spine plasticity as a candidate mechanism mediating estrogen-dependent effects on vocal learning. Together, this perspective of estrogens as neuromodulators in the vertebrate brain has opened new avenues in understanding sensory plasticity, including how hormones can act on communication circuits to influence behaviors in other vocal learning species, such as in language acquisition and speech processing in humans.

Spatial extent of mercury contamination in birds and their prey on the floodplain of a contaminated river

Mercury (Hg) exposure has been extensively studied in aquatic and piscivorous wildlife, but, historically, less attention has been directed towards terrestrial species. However, it has become apparent that aquatic Hg crosses ecosystem boundaries along with beneficial subsidies, thereby entering the terrestrial food chain. It is still not known how far from contaminated waterways Hg exposure remains a risk. We examined the spatial extent of exposure in terrestrial songbirds breeding in the floodplain along a 40-km stretch of Hg-contaminated river in Virginia, USA. Four songbird species were chosen that readily occupied artificial nest cavities placed at known distances from the river; Carolina chickadees (Poecile carolinensis), Carolina wrens (Thryothorus ludovicianus), eastern bluebirds (Sialia sialis), and house wrens (Troglodytes aedon). We examined Hg exposure at varying distances from the edge of the river for three endpoints: 1) adult bird blood, 2) nestling bird feather and 3) spider whole body (collected in pitfall traps and directly from the mouths of nestling birds). We used mixed effects models to understand 1) how total Hg (i.e., inorganic and organic, THg) concentrations differed between contaminated and reference sites and 2) how THg concentrations declined with distance away from the river on contaminated sites. Adult bird blood THg indicated that a species-by-distance interaction was significant, after accounting for site and year. Importantly, despite the decline with distance, we found that THg levels in some songbird species and their invertebrate prey remained elevated above reference levels for up to 400m away from the river. Our results predict a spatial extent of contamination that is an order of magnitude greater than similar studies investigating the lateral impact of other aquatically derived contaminants. To our knowledge, this study is the first to indicate that exposure to legacy aquatic Hg is possible for terrestrial wildlife across the entire floodplain, rather than being confined to narrow riparian corridors.

Carotenoid metabolism strengthens the link between feather coloration and individual quality

Thirty years of research has made carotenoid coloration a textbook example of an honest signal of individual quality, but tests of this idea are surprisingly inconsistent. Here, to investigate sources of this heterogeneity, we perform meta-analyses of published studies on the relationship between carotenoid-based feather coloration and measures of individual quality. To create color displays, animals use either carotenoids unchanged from dietary components or carotenoids that they biochemically convert before deposition. We hypothesize that converted carotenoids better reflect individual quality because of the physiological links between cellular function and carotenoid metabolism. We show that feather coloration is an honest signal of some, but not all, measures of quality. Where these relationships exist, we show that converted, but not dietary, carotenoid coloration drives the relationship. Our results have broad implications for understanding the evolutionary role of carotenoid coloration and the physiological mechanisms that maintain signal honesty of animal ornamental traits.

Understanding sources of methylmercury in songbirds with stable mercury isotopes: Challenges and future directions

Mercury (Hg) stable isotope analysis is an emerging technique that has contributed to a better understanding of many aspects of the biogeochemical cycling of Hg in the environment. However, no study has yet evaluated its usefulness in elucidating the sources of methylmercury (MeHg) in songbird species, a common organism for biomonitoring of Hg in forested ecosystems. In the present pilot study, we examined stable mercury isotope ratios in blood of 4 species of songbirds and the invertebrates they are likely foraging on in multiple habitats in a small watershed of mixed forest and wetlands in Acadia National Park in Maine (USA). We found distinct isotopic signatures of MeHg in invertebrates (both mass-dependent fractionation [as δ²⁰² Hg] and mass-independent fractionation [as Δ¹⁹⁹ Hg]) among 3 interconnected aquatic habitats. It appears that the Hg isotopic compositions in bird blood cannot be fully accounted for by the isotopic compositions of MeHg in lower trophic levels in each of the habitats examined. Furthermore, the bird blood isotope results cannot be simply explained by an isotopic offset as a result of metabolic fractionation of δ²⁰² Hg (e.g., internal demethylation). Our results suggest that many of the birds sampled obtain MeHg from sources outside the habitat they were captured in. Our findings also indicate that mass-independent fractionation is a more reliable and conservative tracer than mass-dependent fractionation for identifying sources of MeHg in bird blood. The results demonstrate the feasibility of Hg isotope studies of songbirds but suggest that larger numbers of samples and an expanded geographic area of study may be required for conclusive interpretation. Environ Toxicol Chem 2018;37:166-174. © 2017 SETAC.

Elemental composition in feathers of a migratory passerine for differentiation of sex, age, and molting areas

The bulk analysis of single feathers of 263 feathers belonging to 238 individuals of a migratory passerine (collared flycatcher, Ficedula albicollis, originating from a breeding population in the Pilis-Visegrád Mountains in Hungary) by inductively coupled plasma sector field mass spectrometry (ICP-SF-MS) for determination of elements after proper dissolution allowed the quantitative determination of 38 elements. Calcium, Mg, Mn, Fe, and Zn were found to have a quantitative determination frequency larger than 80 % and a concentration greater than 100 μg/g. Among ecotoxicologically relevant elements, Ni, Cd, Hg, and Pb could be determined in more than 55 % of the tail feather samples. The concentration of Hg with a quantification limit of 0.006 μg/g and Pb with that of 0.015 μg/g was higher than 1 and 10 μg/g, respectively, in more than 80 % of the investigated samples, but generally lower than levels that could cause adverse behavioral effects. The principal component analyses of elemental concentration data followed by the application of general linear models revealed that, for male collared flycatchers, the concentration of Sn, Pb, Ni, Sr, Mg, Zn, Ba, and Sc differed significantly in the wing and tail feathers collected from the same individuals. With females, only the Ca and Sc concentration showed a significant difference between wing and tail feathers. Moreover, the concentration of rare earth elements, V, Fe, Sr, Mg, Mn, Zn, Pb, and Ba in tail feathers allowed differentiation between sexes while the concentration of Se, Bi, and Sc between yearling and adult male individuals. At the same time, Sc differentiated age categories in females. Distribution of major elements along the rachis of feathers could be monitored by laser ablation ICP-SF-MS after normalization of the intensities to either ¹³C or ³⁴S signals.

Androgen and estrogen sensitivity of bird song: a comparative view on gene regulatory levels

Singing of songbirds is sensitive to testosterone and its androgenic and estrogenic metabolites in a species-specific way. The hormonal effects on song pattern are likely mediated by androgen receptors (AR) and estrogen receptor alpha (ERα), ligand activated transcription factors that are expressed in neurons of various areas of the songbirds' vocal control circuit. The distribution of AR in this circuit is rather similar between species while that of ERα is species variant and concerns a key vocal control area, the HVC (proper name). We discuss the regulation of the expression of the cognate AR and ERα and putative splice variants. In particular, we suggest that transcription factor binding sites in the promoter of these receptors differ between bird species. Further, we suggest that AR- and ERα-dependent gene regulation in vocal areas differs between species due to species-specific DNA binding sites of putative target genes that are required for the transcriptional activity of the receptors. We suggest that species differences in the distribution of AR and ERα in vocal areas and in the genomic sensitivity to these receptors contribute to species-specific hormonal regulation of the song.

Maternal transfer of mercury to songbird eggs

We evaluated the maternal transfer of mercury to eggs in songbirds, determined whether this relationship differed between songbird species, and developed equations for predicting mercury concentrations in eggs from maternal blood. We sampled blood and feathers from 44 house wren (Troglodytes aedon) and 34 tree swallow (Tachycineta bicolor) mothers and collected their full clutches (n = 476 eggs) within 3 days of clutch completion. Additionally, we sampled blood and feathers from 53 tree swallow mothers and randomly collected one egg from their clutches (n = 53 eggs) during mid to late incubation (6-10 days incubated) to evaluate whether the relationship varied with the timing of sampling the mother's blood. Mercury concentrations in eggs were positively correlated with mercury concentrations in maternal blood sampled at (1) the time of clutch completion for both house wrens (R² = 0.97) and tree swallows (R² = 0.97) and (2) during mid to late incubation for tree swallows (R² = 0.71). The relationship between mercury concentrations in eggs and maternal blood did not differ with the stage of incubation when maternal blood was sampled. Importantly, the proportion of mercury transferred from mothers to their eggs decreased substantially with increasing blood mercury concentrations in tree swallows, but increased slightly with increasing blood mercury concentrations in house wrens. Additionally, the proportion of mercury transferred to eggs at the same maternal blood mercury concentration differed between species. Specifically, tree swallow mothers transferred 17%-107% more mercury to their eggs than house wren mothers over the observed mercury concentrations in maternal blood (0.15-1.92 μg/g ww). In contrast, mercury concentrations in eggs were not correlated with those in maternal feathers and, likewise, mercury concentrations in maternal blood were not correlated with those in feathers (all R² < 0.01). We provide equations to translate mercury concentrations from maternal blood to eggs (and vice versa), which should facilitate comparisons among studies and help integrate toxicity benchmarks into a common tissue.

Songbird chemical signals reflect uropygial gland androgen sensitivity and predict aggression: implications for the role of the periphery in chemosignaling

Chemical signals can provide useful information to potential mates and rivals. The production mechanisms of these signals are poorly understood in birds, despite emerging evidence that volatile compounds from preen oil may serve as chemosignals. Steroid hormones, including testosterone (T), may influence the production of these signals, yet variation in circulating T only partly accounts for this variation. We hypothesized that odor is a T-mediated signal of an individual's phenotype, regulated in part by androgen sensitivity in the uropygial gland. We quantified natural variation in chemosignals, T, uropygial gland androgen sensitivity, and aggressive behavior in dark-eyed juncos (Junco hyemalis). The interaction between circulating T and androgen receptor transcript abundance significantly correlated with volatile concentrations in male, but not female, preen oil. In both sexes, odorant variables correlated with aggressive response to an intruder. Our results suggest that preen oil volatiles could function as signals of aggressive intent, and, at least in males, may be regulated by local androgen receptor signaling in the uropygial gland. Because these behavioral and chemical traits have been linked with reproductive success, local regulation of androgen sensitivity in the periphery has the potential to be a target of selection in the evolution of avian olfactory signaling.

Heavy metal bioaccumulation in two passerines with differing migration strategies

Various anthropogenic activities have resulted in concentration of heavy metals and contamination of surrounding environments. Historically, heavy metal contamination at the Savannah River Site (SRS) in South Carolina has resulted from accidental releases of stored waste generated from nuclear weapon production in the early 1950s. Songbirds inhabiting and using resources from these areas have the potential to bioaccumulate metals but there is limited information on metal concentration levels in areas suspected of contamination as well as uncontaminated sites. Nonlethal tissues samples from avian blood and feathers provide a reliable approach for determining the bioavailability of these pollutants (As, Cd, Cr, Cu, Hg, Ni, Pb, Se, and Zn). The objectives of this study were to survey terrestrial heavy metal contamination at the SRS on potentially bioavailable contaminated (PBC) sites through blood and feather samples from resident Northern Cardinals (Cardinalis cardinalis) and migratory Great Crested Flycatchers (Myiarchus crinitus) and quantify sex-specific concentrations within species. Samples were collected in April to June of 2016. Cardinals had lower blood concentrations of Hg (β=-0.17, 85% CL=-0.26, -0.09) and Se (β=-0.33, 85% CL=-0.50, -0.16) than flycatchers. Cr feather concentrations were less in cardinals (β=-1.46, 85% CL=-2.44, -0.49) and all feathers of both species from reference locations had significantly less Zn (β=-67.92, 85% CL=-128.71, -7.14). Results indicate flycatchers were exposed to differing heavy metal levels during feather formation on their wintering grounds as compared to their recent exposure (through bloods samples) on their breeding grounds. Sex of individuals did not have a significant impact on bioaccumulation in either species. Overall, metal concentration levels in both species indicate minimal risk for acute toxicity; however, there is limited research on wild passerine populations with similar concentration levels. Therefore, further research on reproductive success of these birds should be explored.

Pre-GnRH and GnRH-induced testosterone levels do not vary across behavioral contexts: A role for individual variation

Hormones can facilitate the expression of behavior, but relatively few studies have considered individual variation and repeatability in hormone-behavior relationships. Repeated measures of hormones are valuable because repeatability in hormone levels might be a mechanism that drives repeatability in behavior ("personality"). Testosterone is predicted to promote territorial aggression and suppress parental behaviors. In our population of eastern bluebirds (Sialia sialis), parental care and nest defense aggression toward a heterospecific are repeatable. We tested the hypothesis that repeatability of testosterone levels within individuals underlies repeatable behaviors observed in our population. We measured nestling provisioning and aggressive nest defense against a heterospecific. After behavioral observations we captured either the male or female bluebird, and determined initial testosterone levels and maximum capacity of the gonads to secrete testosterone by injecting gonadotropin-releasing hormone (GnRH). We found among-individual variation in initial testosterone levels for males and females. Individual males were repeatable in both initial and GnRH-induced testosterone levels across behavioral contexts, while individual females were repeatable in GnRH-induced testosterone levels. However, testosterone levels were not significantly related to parental or nest defense behaviors, suggesting that repeatable testosterone levels may not drive repeatable parental and heterospecific nest defense behaviors in this population. The absence of a relationship between testosterone and parental and heterospecific nest defense behaviors might be due to among-individual variation in testosterone levels. Considering the sources of variation in testosterone levels may reveal why some populations exhibit high individual variation in hormone levels.

Early life exposure to artificial light at night affects the physiological condition: An experimental study on the ecophysiology of free-living nestling songbirds

Light pollution or artificial light at night (ALAN) is increasingly recognised to be an important anthropogenic environmental pressure on wildlife, affecting animal behaviour and physiology. Early life experiences are extremely important for the development, physiological status and health of organisms, and as such, early exposure to artificial light may have detrimental consequences for organism fitness. We experimentally manipulated the light environment of free-living great tit nestlings (Parus major), an important model species in evolutionary and environmental research. Haptoglobin (Hp) and nitric oxide (NOx), as important indicators of immunity, health, and physiological condition, were quantified in nestlings at baseline (13 days after hatching) and after a two night exposure to ALAN. We found that ALAN increased Hp and decreased NOx. ALAN may increase stress and oxidative stress and reduce melatonin which could subsequently lead to increased Hp and decreased NOx. Haptoglobin is part of the immune response and mounting an immune response is costly in energy and resources and, trade-offs are likely to occur with other energetically demanding tasks, such as survival or reproduction. Acute inhibition of NOx may have a cascading effect as it also affects other physiological aspects and may negatively affect immunocompetence. The consequences of the observed effects on Hp and NOx remain to be examined. Our study provides experimental field evidence that ALAN affects nestlings' physiology during development and early life exposure to ALAN could therefore have long lasting effects throughout adulthood.

Corticosterone levels in relation to trace element contamination along an urbanization gradient in the common blackbird (Turdus merula)

In a rapidly urbanizing world, trace element pollution may represent a threat to human health and wildlife, and it is therefore crucial to assess both exposition levels and associated effects of trace element contamination on urban vertebrates. In this study, we investigated the impact of urbanization on trace element contamination and stress physiology in a wild bird species, the common blackbird (Turdus merula), along an urbanization gradient (from rural to moderately urbanized areas). Specifically, we described the contamination levels of blackbirds by 4 non-essential (Ag, Cd, Hg, Pb) and 9 essential trace elements (As, Co, Cr, Cu, Fe, Mn, Ni, Se, Zn), and explored the putative disrupting effects of the non-essential element contamination on corticosterone levels (a hormonal proxy for environmental challenges). We found that non-essential trace element burden (Cd and Pb specifically) increased with increasing urbanization, indicating a significant trace element contamination even in medium sized cities and suburban areas. Interestingly, the increased feather non-essential trace element concentrations were also associated with elevated feather corticosterone levels, suggesting that urbanization probably constrains birds and that this effect may be mediated by trace element contamination. Future experimental studies are now required to disentangle the influence of multiple urban-related constraints on corticosterone levels and to specifically test the influence of each of these trace elements on corticosterone secretion.

Hybrid Dysfunction Expressed as Elevated Metabolic Rate in Male Ficedula Flycatchers

Studies of ecological speciation are often biased towards extrinsic sources of selection against hybrids, resulting from intermediate hybrid morphology, but the knowledge of how genetic incompatibilities accumulate over time under natural conditions is limited. Here we focus on a physiological trait, metabolic rate, which is central to life history strategies and thermoregulation but is also likely to be sensitive to mismatched mitonuclear interactions. We measured the resting metabolic rate of male collared, and pied flycatchers as well as of naturally occurring F1 hybrid males, in a recent hybrid zone. We found that hybrid males had a higher rather than intermediate metabolic rate, which is indicative of hybrid physiological dysfunction. Fitness costs associated with elevated metabolic rate are typically environmentally dependent and exaggerated under harsh conditions. By focusing on male hybrid dysfunction in an eco-physiological trait, our results contribute to the general understanding of how combined extrinsic and intrinsic sources of hybrid dysfunction build up under natural conditions.

Elevated corticosterone during egg production elicits increased maternal investment and promotes nestling growth in a wild songbird

Glucocorticoids circulating in breeding birds during egg production accumulate within eggs, and may provide a potent form of maternal effect on offspring phenotype. However, whether these steroids affect offspring development remains unclear. Here, we employed a non-invasive technique that experimentally elevated the maternal transfer of corticosterone to eggs in a wild population of house wrens. Feeding corticosterone-injected mealworms to free-living females prior to and during egg production increased the number of eggs that females produced and increased corticosterone concentrations in egg yolks. This treatment also resulted in an increase in the amount of yolk allocated to eggs. Offspring hatching from these eggs begged for food at a higher rate than control offspring and eventually attained increased prefledging body condition, a trait predictive of their probability of recruitment as breeding adults in the study population. Our results indicate that an increase in maternal glucocorticoids within the physiological range can enhance maternal investment and offspring development.

Organophosphate pesticide method development and presence of chlorpyrifos in the feet of nearctic-neotropical migratory songbirds from Canada that over-winter in Central America agricultural areas

Recent modeling analysis suggests that numerous birds may be at risk of acute poisoning in insecticide-treated fields. Although the majority of avian field studies on pesticides have focused on treated seed, granule, insect or vegetation (oral exposure) ingestion, dermal exposure is an important exposure route when birds come into contact with deposited pesticides on foliage and other surfaces. Some nearctic-neotropical migratory songbirds are likely exposed to pesticides on their non-breeding habitats and include treated crops, plantations or farmlands. In the present study, we developed a method for four environmentally-relevant organophosphate (OP) pesticides (fenthion, fenamiphos, chlorpyrifos and diazinon) in the feet of migratory songbirds (i.e. Common yellowthroat, Gray catbird, Indigo bunting, America redstart, Northern waterthrush, Northern parula, and an additional 12 species of warblers). A total of 190 specimens of the 18 species of songbirds were sampled from available window-killed birds (spring of 2007 and 2011) in downtown Toronto, Canada. The species that were available most likely over-wintered in Mexican/Central American crops such as citrus, coffee and cacao. The feet of the dead birds were sampled and where OP foot exposure likely occurred during over-wintering foraging on pesticide-treated crops. Chlorpyrifos was the only measurable OP (pg mg feet weight(-1)) and in the 2011-collected feet of Black throated blue warbler (0.5), Tennessee warbler (1.0), Northern parula (1.2), Northern waterthrush (0.6), Common yellowthroat (1.0) and the Blue winged warbler (0.9). Dermal contact with OP pesticides during over-wintering in agricultural areas resulted in low levels of chlorpyrifos and long time retention on the feet of a subset of songbirds.

Dawn-song onset coincides with increased HVC androgen receptor expression but is decoupled from high circulating testosterone in an equatorial songbird

The song of songbirds is a testosterone-sensitive behavior that is controlled by brain regions expressing androgen receptors. At higher latitudes, seasonal singing is stimulated by increasing day-length and elevated circulating testosterone. However, a large number of songbird species inhabit equatorial regions under a nearly constant photoperiod, and the neuroendocrine mechanisms of seasonal song in these species have rarely been investigated. We studied males from an equatorial population of the silver-beaked tanager (Ramphocelus carbo), an Amazonian songbird. We found seasonality in dawn-song behavior, which was displayed continuously for more than half a year throughout an extended breeding territoriality stage. The seasonal activation of dawn-song was correlated with an increased area of androgen receptor expression in HVC, a major brain area of song control. However, testosterone levels remained low for several weeks after activation of dawn-song. Circulating levels of testosterone were elevated only later in the breeding season, coinciding with a higher dawn-song output and with the mating period. Our results suggest that the seasonal activation of dawn-song and territoriality involves an increase of androgen target cells in HVC. This mechanism could potentially function to circumvent adverse effects of high testosterone levels in a species with an extended breeding season.

Mercury Concentrations in Birds from Two Atmospherically Contaminated Sites in North Texas, USA

Mercury (Hg) is a ubiquitous and highly toxic contaminant that can have negative effects on wildlife. Only a few studies have measured Hg concentrations in birds from the south central United States, and the potential threat of Hg contamination to birds in this region is largely unknown. In the present study, we assess Hg concentrations in blood and feathers from five bird species [eastern bluebird (Sialis sialis), Carolina wren (Thryothorus ludovicianus), wood duck (Aix sponsa), great egret (Ardea alba), and great blue heron (Ardea herodias)] that occupy different trophic levels at Caddo Lake and Lewisville Lake, located in northeast and north central Texas, respectively. Both sites are contaminated with Hg from the atmosphere. Adult passerines had higher Hg concentrations in their blood than conspecific nestlings. Mercury concentrations in feathers differed between species by more than an order of magnitude with large piscivorous species having higher concentrations than smaller insectivorous species. Mercury concentrations in eastern bluebirds were higher at Caddo Lake than Lewisville Lake. The present study represents one of the first studies of Hg concentrations in multiple bird species in north Texas and suggests that Hg concentrations in birds from atmospherically polluted sites in this region may be high enough to compromise fitness in those species.

The importance of neural aromatization in the acquisition, recall, and integration of song and spatial memories in passerines

This article is part of a Special Issue "Estradiol and cognition". In addition to their well-studied and crucial effects on brain development and aging, an increasing number of investigations across vertebrate species indicate that estrogens like 17β-estradiol (E2) have pronounced and rapid effects on cognitive function. The incidence and regulation of the E2-synthesizing enzyme aromatase at the synapse in regions of the brain responsible for learning, memory, social communication and other complex cognitive processes suggest that local E2 production and action affect the acute and chronic activity of individual neurons and circuits. Songbirds in particular are excellent models for the study of this "synaptocrine" hormone provision given that aromatase is abundantly expressed in neuronal soma, dendrites, and at the synapse across many brain regions in both sexes. Additionally, songbirds readily acquire and recall memories in laboratory settings, and their stereotyped behaviors may be manipulated and measured with relative ease. This leads to a rather unparalleled advantage in the use of these animals in studies of the role of neural aromatization in cognition. In this review we describe the results of a number of experiments in songbird species with a focus on the influence of synaptic E2 provision on two cognitive processes: auditory discrimination reliant on the caudomedial nidopallium (NCM), a telencephalic region likely homologous to the auditory cortex in mammals, and spatial memory dependent on the hippocampus. Data from these studies are providing evidence that the local and acute provision of E2 modulates the hormonal, electrical, and cognitive outputs of the vertebrate brain and aids in memory acquisition, retention, and perhaps the confluence of memory systems.

Condition and Health of Rufous Bush Robin (Cercotrichas galactotes) Nestlings in a Polluted Oasis Habitat in Southern Tunisia

We investigated whether the proximity to the Gabès-Ghannouche factory complex of phosphate treatment, in south-eastern Tunisia, was associated with notable changes in the condition and health of Rufous bush robin (Cercotrichas galactotes) nestlings hatched in the neighbouring oasis habitat. Results demonstrated that excrements of nestlings hatched in one oasis close to the factory complex contained higher concentrations of lead and zinc than the excrements of nestlings from one oasis situated 20 km away. Furthermore, when effects of age and nestling number in the nest were controlled, nestlings hatched near the factory complex showed reduced tarsus length, rectrix length, haematocrit level and haemoglobin concentration, but higher levels of fluctuating asymmetry compared to those from the more distant oasis. Overall, results suggest that the proximity to the factory complex was associated with increased exposure to metals and deterioration in nestling condition and development.

Oxidative stress in songbirds exposed to dietary methylmercury

Long-term, sublethal methylmercury exposure can cause reproductive depression, immune suppression, endocrine disruption and other problems in birds. We used two biomarkers to detect oxidative stress in livers of zebra finches (Taeniopygia guttata) developmentally exposed to sublethal levels of dietary methylmercury (0.0, 0.3, 0.6, 1.2, or 2.4 μg/g wet weight in diet). Our findings indicate that young adult finches exposed to environmentally relevant concentrations of mercury in ovo and through their diets, exhibited oxidative stress in their livers. We measured the ratio of the antioxidant glutathione in its reduced form (GSH) versus its oxidized form (GSSG) and the activity of the superoxide dismutase (SOD) enzyme suite. Blood total mercury served as a proxy for liver mercury concentration, and was on average 8.4 times the dietary dose (e.g., birds consuming 0.6 μg/g had blood mercury levels of ~5 μg/g on a wet weight basis). Consistent with what is known from large, aquatic bird species, there was a significant, negative relationship between GSH/GSSG ratios and tissue mercury concentrations, which is indicative of oxidative stress. This relationship was driven by a significant increase in the oxidized glutathione in the livers of birds with higher blood mercury levels. SOD activity was also found to have a significant, negative relationship with blood mercury.

PHA-stimulated immune-responsiveness in mercury-dosed zebra finches does not match results from environmentally exposed songbirds

Dietary mercury exposure is associated with suppressed immune responsiveness in birds. This study examined the immune-responsiveness of domestic zebra finches (Taeniopygia guttata) experimentally exposed to mercury through their diet. We used the phytohemagglutinin (PHA) skin-swelling test to assay the effect of two modes of mercury exposure. Some finches received exposure to mercury only after reaching sexual maturity, while others were maintained on a mercury-dosed diet throughout life, including development. Each bird received one of five dietary concentrations of methylmercury cysteine (0.0, 0.3, 0.6, 1.2 or 2.4 ppm). In contrast to a study on wild songbirds at a mercury-contaminated site, we detected no relationship between mercury level and immunological response to PHA, regardless of mode of exposure. This result represents the first major difference found by our laboratory between wild birds exposed to environmental mercury and captive birds experimentally exposed to mercury.

Nesting behavior is associated with VIP expression and VIP-Fos colocalization in a network-wide manner

Many species, including humans, engage in a series of behaviors that are preparatory to the arrival of offspring. Such "nesting behaviors" are of obvious importance, but relevant neuroendocrine mechanisms remain little studied. We here focus on the potential roles of vasoactive intestinal polypeptide (VIP) in the performance of appetitive and consummatory nesting behaviors in male and female zebra finches (Taeniopygia guttata). Using combined immunocytochemistry for Fos and in situ hybridization for VIP, we now show that many VIP cell groups show increased transcriptional activity in response to nest building in male and female zebra finches. Particularly strong data come from the preoptic area (medial preoptic area and medial preoptic nucleus), where VIP-Fos co-expression correlates positively with three different measures of nesting behavior, as does the number of VIP-expressing cells. Remarkably, we find that VIP mRNA and/or VIP-Fos co-expression is correlated with nesting behavior in virtually every brain area that we examined, including the medial amygdala (anterior and posterior), medial bed nucleus of the stria terminalis, medial preoptic area, medial preoptic nucleus, anterior hypothalamus, ventromedial hypothalamus, periaqueductal gray complex (central gray and nucleus intercollicularis), and ventral tegmental area. Near-significant effects are also obtained in the tuberoinfundibular hypothalamus. Although most correlations are positive, negative correlations are observed for the VIP cell group of the anterior hypothalamus, a population that selectively promotes aggression, and also the periaqueductal gray complex. These data demonstrate a network-wide relationship between peptide production and social behavior that is, to our knowledge, unparalleled by other peptidergic modulators.

Hg-contaminated terrestrial spiders pose a potential risk to songbirds at Caddo Lake (Texas/Louisiana, USA)

Methylmercury (MeHg) is an environmental contaminant that can have adverse effects on wildlife. Because MeHg is produced by bacteria in aquatic ecosystems, studies of MeHg contamination of food webs historically have focused on aquatic organisms. However, recent studies have shown that terrestrial organisms such as songbirds can be contaminated with MeHg by feeding on MeHg-contaminated spiders. In the present study, the authors examined the risk that MeHg-contaminated terrestrial long-jawed orb weaver spiders (Tetragnatha sp.) pose to songbirds at Caddo Lake (Texas/Louisiana, USA). Methylmercury concentrations in spiders were significantly different in river, wetland, and open-water habitats. The authors calculated spider-based wildlife values (the minimum spider MeHg concentrations causing physiologically significant doses in consumers) to assess exposure risks for arachnivorous birds. Methylmercury concentrations in spiders exceeded wildlife values for Carolina chickadee (Poecile carolinensis) nestlings, with the highest risk in the river habitat. The present study indicates that MeHg concentrations in terrestrial spiders vary with habitat and can pose a threat to small-bodied nestling birds that consume large amounts of spiders at Caddo Lake. This MeHg threat to songbirds may not be unique to Caddo Lake and may extend throughout the southeastern United States.

Hypothalamic gene switches control transitions between seasonal life history states in a night-migratory photoperiodic songbird

This study investigated photoperiodic plasticity in hypothalamic expression of genes implicated in the photoperiodic light perception (rhodopsin, melanopsin, neuropsin and peropsin), transduction (pax6, bmal1, clock, per2 and casr), induction (eya3, tshβ, dio2 and dio3, gnrh and gnih) and metabolism (NPY, sirtuin1, foxO1, hmgcr, citrate synthase and dehydrogenases) in photosensitive and photorefractory redheaded buntings. There was a significant increase in eya3, tsh β, dio2, pax6 and rhodopsin and decrease in dio3 mRNA expression at hour 15 and/or 19 on the day photosensitive buntings were subjected to a 13- or 16 h, but not to 8- and 11 h light exposure. Downstream reproductive and metabolic gene expression was not altered, except for an increase in those genes coding for succinate and malate dehydrogenase enzymes involved in lipogenesis. Photorefractory buntings had high dio3 mRNA expression which significantly declined after 1 short day exposure, suggesting possible involvement of dio3 in the maintenance of photorefractoriness. Positive correlation of rhodopsin on eya 3 and tshβ indicates its role in photoperiodic timing, perhaps involving the peropsin and pax6 genes. These results suggest that rapid switching of hypothalamic gene expression underlies photoperiod-induced seasonal plasticity and regulates transitions from photosensitive to photostimulated and from photorefractory to photosensitive states in migratory songbirds.