Corticosterone exposure is associated with long-term changes in DNA methylation, physiology and breeding decisions in a wild bird.

When facing challenges, vertebrates activate a hormonal stress response that can dramatically alter behaviour and physiology. Although this response can be costly, conceptual models suggest that it can also recalibrate the stress response system, priming more effective responses to future challenges. Little is known about whether this process occurs in wild animals, particularly in adulthood, and if so, how information about prior experience with stressors is encoded. One potential mechanism is hormonally mediated changes in DNA methylation. We simulated the spikes in corticosterone that accompany a stress response using non-invasive dosing in tree swallows (Tachycineta bicolor) and monitored the phenotypic effects 1 year later. In a subset of individuals, we characterized DNA methylation using reduced representation bisulfite sequencing shortly after treatment and a year later. The year after treatment, experimental females had stronger negative feedback and initiated breeding earlier-traits that are associated with stress resilience and reproductive performance in our population-and higher baseline corticosterone. We also found that natural variation in corticosterone predicted patterns of DNA methylation. Finally, corticosterone treatment influenced methylation on short (1-2 weeks) and long (1 year) time scales; however, these changes did not have clear links to functional regulation of the stress response. Taken together, our results are consistent with corticosterone-induced priming of future stress resilience and support DNA methylation as a potential mechanism, but more work is needed to demonstrate functional consequences. Uncovering the mechanisms linking experience with the response to future challenges has implications for understanding the drivers of stress resilience.

Experimentally elevated corticosterone does not affect bacteria killing ability of breeding female tree swallows (Tachycineta bicolor).

The immune system can be modulated when organisms are exposed to acute or chronic stressors. Glucocorticoids (GCs), the primary hormonal mediators of the physiological stress response, are suspected to play a crucial role in immune modulation. However, most evidence of stress-associated immunomodulation does not separate the effects of glucocorticoid-dependent pathways from those of glucocorticoid-independent mechanisms on immune function. In this study, we experimentally elevated circulating corticosterone, the main avian glucocorticoid, in free-living female tree swallows (Tachycineta bicolor) for one to two weeks to test its effects on immune modulation. Natural variation in bacteria killing ability (BKA), a measure of innate constitutive immunity, was predicted by the interaction between timing of breeding and corticosterone levels. However, experimental elevation of corticosterone had no effect on BKA. Therefore, even when BKA is correlated with natural variation in glucocorticoid levels, this relationship may not be causal. Experiments are necessary to uncover the causal mechanisms of immunomodulation and the consequences of acute and chronic stress on disease vulnerability. Findings in other species indicate that acute increases in GCs can suppress BKA; but our results support the hypothesis that this effect does not persist over longer timescales, during chronic elevations in GCs. Direct comparisons of the effects of acute vs. chronic elevation of GCs on BKA will be important for testing this hypothesis.

The effects of humidity on thermoregulatory physiology of a small songbird.

Scholander-Irving curves describe the relationship between ambient temperature and metabolic rate and are fundamental to understanding the energetic demands of homeothermy. However, Scholander-Irving curves are typically measured in dry air, which is not representative of the humidity many organisms experience in nature. Consequently, it is unclear (1) whether Scholander-Irving curves (especially below thermoneutrality) are altered by humidity, given the effects of humidity on thermal properties of air, and (2) whether physiological responses associated with Scholander-Irving curves in the lab reflect organismal performance in humid field conditions. We used laboratory experiments and biophysical models to test the effects of humidity on the thermoregulatory physiology of tree swallows (Tachycineta bicolor). We also tested whether physiological responses measured under lab conditions were correlated with field body temperatures and nestling provisioning rates. We found that humidity reduced rates of evaporative water loss but did not have large effects on body temperature or metabolic rate, suggesting that swallows can decouple evaporative cooling, body temperature and metabolic rate. Although the effect of humidity on metabolic rate in the lab was small, our biophysical models indicated that energetic costs of thermoregulation were ∼8% greater in simulations that used metabolic rates from birds in humid compared with dry conditions. Finally, we found mixed evidence that physiological responses measured in the lab under humid or dry conditions were associated with body temperature and nest provisioning rates in the field. Our results help clarify the effect of humidity on endotherm thermoregulation, which may help forecast organismal responses to environmental change.

Manipulation of a social signal affects DNA methylation of a stress-related gene in a free-living bird.

Social status directly affects the health of humans and other animals. Low status individuals receive more antagonistic encounters, have fewer supportive relationships and have worse health outcomes. However, the physiological and cellular processes that mediate the relationship between the social environment and health are incompletely known. Epigenetic regulation of the hypothalamic-pituitary-adrenal (HPA) axis, the neuroendocrine pathway that activates in response to stressors, may be one process that is sensitive to the social environment. Here, we experimentally manipulated plumage, a key social signal in female tree swallows (Tachycineta bicolor) and quantified methylation of four genes in the HPA axis before and after treatment. We found that dulling the white breast plumage affected methylation in one gene, CRHR1; however, the effect depended on the original brightness of the bird. Methylation in this gene was correlated with baseline corticosterone levels, suggesting that DNA methylation of CRHR1 helps regulate glucocorticoid production in this species. Methylation in two other genes, FKBP5 and GR, changed over the course of the experiment, independent of treatment. These results show that methylation of these genes is labile into adulthood and suggest that epigenetic regulation of the HPA axis could help birds respond to current environmental conditions.

Avian mud nest architecture by self-secreted saliva.

Mud nests built by swallows (Hirundinidae) and phoebes (Sayornis) are stable granular piles attached to cliffs, walls, or ceilings. Although these birds have been observed to mix saliva with incohesive mud granules, how such biopolymer solutions provide the nest with sufficient strength to support the weight of the residents as well as its own remains elusive. Here, we elucidate the mechanism of strong granular cohesion by the viscoelastic paste of bird saliva through a combination of theoretical analysis and experimental measurements in both natural and artificial nests. Our mathematical model considering the mechanics of mud nest construction allows us to explain the biological observation that all mud-nesting bird species should be lightweight.

Experimental competition induces immediate and lasting effects on the neurogenome in free-living female birds.

Periods of social instability can elicit adaptive phenotypic plasticity to promote success in future competition. However, the underlying molecular mechanisms have primarily been studied in captive and laboratory-reared animals, leaving uncertainty as to how natural competition among free-living animals affects gene activity. Here, we experimentally generated social competition among wild, cavity-nesting female birds (tree swallows, Tachycineta bicolor). After territorial settlement, we reduced the availability of key breeding resources (i.e., nest boxes), generating heightened competition; within 24 h we reversed the manipulation, causing aggressive interactions to subside. We sampled females during the peak of competition and 48 h after it ended, along with date-matched controls. We measured transcriptomic and epigenomic responses to competition in two socially relevant brain regions (hypothalamus and ventromedial telencephalon). Gene network analyses suggest that processes related to energy mobilization and aggression (e.g., dopamine synthesis) were up-regulated during competition, the latter of which persisted 2 d after competition had ended. Cellular maintenance processes were also down-regulated after competition. Competition additionally altered methylation patterns, particularly in pathways related to hormonal signaling, suggesting those genes were transcriptionally poised to respond to future competition. Thus, experimental competition among free-living animals shifts gene expression in ways that may facilitate the demands of competition at the expense of self-maintenance. Further, some of these effects persisted after competition ended, demonstrating the potential for epigenetic biological embedding of the social environment in ways that may prime individuals for success in future social instability.

Birds advancing lay dates with warming springs face greater risk of chick mortality.

In response to a warming planet with earlier springs, migratory animals are adjusting the timing of essential life stages. Although these adjustments may be essential for keeping pace with resource phenology, they may prove insufficient, as evidenced by population declines in many species. However, even when species can match the tempo of climate change, other consequences may emerge when exposed to novel conditions earlier in the year. Here, using three long-term datasets on bird reproduction, daily insect availability, and weather, we investigated the complex mechanisms affecting reproductive success in an aerial insectivore, the tree swallow (Tachycineta bicolor). By examining breeding records over nearly half a century, we discovered that tree swallows have continuously advanced their egg laying by ∼3 d per decade. However, earlier-hatching offspring are now exposed to inclement weather events twice as often as they were in the 1970s. Our long-term daily insect biomass dataset shows no long-term trends over 25 y but precipitous drops in flying insect numbers on days with low ambient temperatures. Insect availability has a considerable impact on chick survival: Even a single inclement weather event can reduce offspring survival by >50%. Our results highlight the multifaceted threats that climate change poses on migrating species. The decoupling between cold snap occurrence and generally warming spring temperatures can affect reproductive success and threaten long-term persistence of populations. Understanding the exact mechanisms that endanger aerial insectivores is especially timely because this guild is experiencing the steepest and most widespread declines across North America and Europe.

Interacting effects of cold snaps, rain, and agriculture on the fledging success of a declining aerial insectivore.

Climate change predicts the increased frequency, duration, and intensity of inclement weather periods such as unseasonably low temperatures (i.e., cold snaps) and prolonged precipitation. Many migratory species have advanced the phenology of important life history stages and, as a result, are likely to be exposed to these periods of inclement spring weather more often, therefore risking reduced fitness and population growth. For declining avian species, including aerial insectivores, anthropogenic landscape changes such as agricultural intensification are another driver of population declines. These landscape changes may affect the foraging ability of food provisioning parents and reduce the survival of nestlings exposed to inclement weather through, for example, pesticide exposure impairing thermoregulation and punctual anorexia. Breeding in agro-intensive landscapes may therefore exacerbate the negative effects of inclement weather under climate change. We observed that a significant reduction in the availability of insect prey occurred when daily maximum temperatures fell below 18.3°C, and thereby defined any day when the maximum temperature fell below this value as a day witnessing a cold snap. We then combined daily information on the occurrence of cold snaps and measures of precipitation to assess their impact on the fledging success of Tree Swallows (Tachycineta bicolor) occupying a nest box system placed across a gradient of agricultural intensification. Estimated fledging success of this declining aerial insectivore was 36.2% lower for broods experiencing 4 cold-snap days during the 12 days post-hatching period versus broods experiencing none, and this relationship was worsened when facing more precipitation. We further found that the overall negative effects of a brood experiencing periods of inclement weather was exacerbated in more agro-intensive landscapes. Our results indicate that two of the primary hypothesized drivers of many avian population declines may interact to further increase the rate of declines in certain landscape contexts.

Chronic noise exposure has context-dependent effects on stress physiology in nestling Tree Swallows (Tachycineta bicolor).

Anthropogenic noise is increasing in intensity and scope, resulting in changes to acoustic landscapes and largely negative effects on a range of species. In birds, noise can mask acoustic signals used in a variety of communication systems, including parent-offspring communication. As a result, nestling birds raised in noise may have challenges soliciting food from parents and avoiding detection by predators. Given that passerine nestlings are confined to a nest and therefore cannot escape these challenges, noise may also act as a chronic stressor during their development. Here, we raised Tree Swallow (Tachycineta bicolor) nestlings with or without continuous, white noise to test whether noise exposure affected baseline and stress-induced plasma, integrated feather corticosterone levels, and immune function. Stress physiology and immune function may also vary with the competitive environment during development, so we also examined whether noise effects varied with brood size and nestling mass. We found that overall, exposure to noise did not alter nestling stress physiology or immune function. However, light nestlings raised in noise exhibited lower baseline plasma and integrated feather corticosterone than heavy nestlings, suggesting alternative physiological responses to anthropogenic stimuli. Furthermore, light nestlings in larger broods had reduced PHA-induced immune responses compared to heavy nestlings, and PHA-induced immune responses were associated with higher levels of baseline plasma and feather CORT. Overall, our findings suggest that noise can alter the stress physiology of developing birds; however, these effects may depend on developmental conditions and the presence of other environmental stressors, such as competition for resources. Our findings may help to explain why populations are not uniformly affected by noise.

Differences in perceived predation risk associated with variation in relative size of extra-pair and within-pair offspring.

Extra-pair paternity (EPP) is a widespread phenomenon in birds. Researchers have long hypothesized that EPP must confer a fitness advantage to extra-pair offspring (EPO), but empirical support for this hypothesis is definitively mixed. This could be because genetic benefits of EPP only exist in a subset of environmental contexts to which a population is exposed. From 2013 to 2015, we manipulated perceived predator density in a population of tree swallows (Tachycineta bicolor) breeding in New York to see whether fitness outcomes of extra-pair and within-pair offspring (WPO) varied with predation risk. In nests that had been exposed to predators, EPO were larger, longer-winged and heavier than WPO. In nonpredator nests, WPO tended to be larger, longer-winged and heavier than EPO, though the effect was nonsignificant. We found no differences in age, morphology or stress physiology between extra-pair and within-pair sires from the same nest, suggesting that additive genetic benefits cannot fully explain the differences in nestling size that we observed. The lack of an effect of predator exposure on survival or glucocorticoid stress physiology of EPO and WPO further suggests that observed size differences do not reflect more general variation in intrinsic genetic quality. Instead, we suggest that size differences may have arisen through differential investment into EPO and WPO by females, perhaps because EPO and WPO represent different reproductive strategies, with each type of nestling conferring a fitness advantage in specific ecological contexts.

How research on female vertebrates contributes to an expanded challenge hypothesis.

The bi-directional links between hormones and behavior have been a rich area of research for decades. Theory on the evolution of testosterone (T) was greatly advanced by the challenge hypothesis, which presented a framework for understanding interspecific, seasonal, and social variation in T levels in males, and how they are shaped by the competing demands of parental care and male-male competition. Female competition is also widespread in nature, although it is less clear whether or how the challenge hypothesis applies to females. Here, we evaluate this issue in four parts: (1) We summarize and update prior analyses of seasonal plasticity and interspecific variation in T in females. (2) We evaluate experimental links between T and female aggression on shorter timescales, asking how T manipulations affect aggression and conversely, how social manipulations affect T levels in female mammals, birds, lizards, and fishes. (3) We examine alternative mechanisms that may link aggression to the social environment independently of T levels in circulation. (4) We present a case study, including new data analyses, in an aggressive female bird (the tree swallow, Tachycineta bicolor) to explore how variation in tissue-level processing of T may bridge the gap between circulating T and variation in behavior that is visible to natural selection. We close by connecting these multivariate levels of sex steroid signaling systems alongside different temporal scales (social, seasonal, and evolutionary) to generate broadly applicable insights into how animals respond to their social environment, regardless of whether they are male or female.

A range-wide domino effect and resetting of the annual cycle in a migratory songbird.

Latitudinal differences in timing of breeding are well documented but how such differences carry over to influence timing of events in the annual cycle of migratory birds is not well understood. We examined geographical variation in timing of events throughout the year using light-level geolocator tracking data from 133 migratory tree swallows ( Tachycineta bicolor) originating from 12 North American breeding populations. A swallow's breeding latitude influenced timing of breeding, which then carried over to affect breeding ground departure. This resulted in subsequent effects on the arrival and departure schedules at autumn stopover locations and timing of arrival at non-breeding locations. This 'domino effect' between timing events was no longer apparent by the time individuals departed for spring migration. Our range-wide analysis demonstrates the lasting impact breeding latitude can have on migration schedules but also highlights how such timing relationships can reset when individuals reside at non-breeding sites for extended periods of time.

Rainy springs linked to poor nestling growth in a declining avian aerial insectivore ( Tachycineta bicolor).

As species shift their ranges and phenology to cope with climate change, many are left without a ready supply of their preferred food source during critical life stages. Food shortages are often assumed to be driven by reduced total food abundance, but here we propose that climate change may cause short-term food shortages for foraging specialists without affecting overall food availability. We frame this hypothesis around the special case of birds that forage on flying insects for whom effects mediated by their shared food resource have been proposed to cause avian aerial insectivores' decline worldwide. Flying insects are inactive during cold, wet or windy conditions, effectively reducing food availability to zero even if insect abundance remains otherwise unchanged. Using long-term monitoring data from a declining population of tree swallows ( Tachycineta bicolor), we show that nestlings' body mass declined substantially from 1977 to 2017. In 2017, nestlings had lower body mass if it rained during the preceding 3 days, though females increased provisioning rates, potentially in an attempt to compensate. Adult body mass, particularly that of the males, has also declined over the long-term study. Mean rainfall during the nestling period has increased by 9.3 ± 0.3 mm decade-1, potentially explaining declining nestling body mass and population declines. Therefore, we suggest that reduced food availability, distinct from food abundance, may be an important and previously overlooked consequence of climate change, which could be affecting populations of species that specialize on foraging on flying insects.

Genome-wide variation in DNA methylation is associated with stress resilience and plumage brightness in a wild bird.

Individuals often differ in their ability to cope with challenging environmental and social conditions. Evidence from model systems suggests that patterns of DNA methylation are associated with variation in coping ability. These associations could arise directly if methylation plays a role in controlling the physiological response to stressors by, among other things, regulating the release of glucocorticoids in response to challenges. Alternatively, the association could arise indirectly if methylation and resilience have a common cause, such as early-life conditions. In either case, methylation might act as a biomarker for coping ability. At present, however, relatively little is known about whether variation in methylation is associated with organismal performance and resilience under natural conditions. We studied genome-wide patterns of DNA methylation in free-living female tree swallows (Tachycineta bicolor) using methylated DNA immunoprecipitation (MeDIP) and a tree swallow genome that was assembled for this study. We identified areas of the genome that were differentially methylated with respect to social signal expression (breast brightness) and physiological traits (ability to terminate the glucocorticoid stress response through negative feedback). We also asked whether methylation predicted resilience to a subsequent experimentally imposed challenge. Individuals with brighter breast plumage and higher stress resilience had lower methylation at differentially methylated regions across the genome. Thus, widespread differences in methylation predicted both social signal expression and the response to future challenges under natural conditions. These results have implications for predicting individual differences in resilience, and for understanding the mechanistic basis of resilience and its environmental and social mediators.

Changes in thyroid function of nestling tree swallows (Tachycineta bicolor) in relation to polycyclic aromatic compounds and other environmental stressors in the Athabasca Oil Sands Region.

In the Canadian Athabasca Oil Sands Region (AOSR), nestling tree swallows (Tachycineta bicolor) raised near mining-related activities accumulated greater concentrations of polycyclic aromatic compounds (PACs) that contributed to their poorer condition, growth, and reproductive success. Here, we report changes in thyroid function of the same 14 day old (do) nestlings (N ≤ 68) at these mining-related sites (OS1, OS2) compared to reference nestlings (REF1), and in relation to multiple environmental stressors that influence avian thyroid function. Thyroid function was compromised for OS1 nestlings but generally comparable between OS2 and REF1 chicks. In 2012, circulating total triiodothyronine (TT3) and thyroxine (TT4) were similar among all nestlings. The OS1 chicks had more active thyroid glands based on histological endpoints. Hepatic T4 outer-ring deiodinase (T4-ORD) activity was suppressed in OS1 and OS2 chicks. Despite inter-annual differences, OS1 chicks continued experiencing compromised thyroid function with significantly higher circulating TT4 and more active thyroid glands in 2013. The OS2 chicks had less active thyroid glands, which conceivably contributed to their suppressed growth (previously reported) relative to the heavier OS1 nestlings with more active thyroid glands. Thyroid gland activity was more influenced by the chicks' accumulation of (muscle), than exposure (feces) to naphthalene, C2-naphthalenes, and C1-fluorenes. Of four major volatile organic contaminants, sulfur dioxide (SO2) primarily influenced thyroid gland activity and structure, supporting previous findings with captive birds. When collectively considering environmental-thyroidal stressors, chicks had a greater thyroidal response when they experienced colder temperatures, accumulated more C2-naphthalenes, and consumed aquatic-emerging insects with higher PAC burdens than terrestrial insects (carbon (δ13C)). We hypothesize that the more active thyroid glands and higher circulating TT4 of the OS1 chicks supported their growth and survival despite having the highest PAC burdens, whereas the lack of thyroid response in the OS2 chicks combined with high PAC burdens, contributed to their smaller size, poorer condition and poorer survival.

Traffic noise exposure alters nestling physiology and telomere attrition through direct, but not maternal, effects in a free-living bird.

Anthropogenic impacts, such as noise pollution from transportation networks, can serve as stressors to some wildlife species. For example, increased exposure to traffic noise has been found to alter baseline and stress-induced corticosterone levels, reduce body condition and reproductive success, and increase telomere attrition in free-living birds. However, it remains unknown if alterations in nestling phenotype are due to direct or indirect effects of noise exposure. For example, indirect (maternal) effects of noise may occur if altered baseline and stress-induced corticosterone in mothers results in differential deposition of yolk steroids or other components in eggs. Noise exposure may also alter nestling corticosterone levels directly, given that nestlings cannot escape the nest during development. Here, we examined maternal versus direct effects of traffic noise exposure on baseline and stress-induced corticosterone levels, and body condition (as measured by size-corrected mass) in nestling tree swallows (Tachycineta bicolor). We used a two-way factorial design and partially cross-fostered eggs between nests exposed to differing levels (i.e. amplitudes) of traffic noise. For nestlings that were not cross-fostered, we also investigated the effects of traffic noise on telomere dynamics. Our results show a positive relationship between nestling baseline and stress-induced corticosterone and nestling noise exposure, but not maternal noise exposure. While we did not find a relationship between noise and body condition in nestlings, nestling baseline corticosterone was negatively associated with body condition. We also found greater telomere attrition for nestlings from nests with greater traffic noise amplitudes. These results suggest that direct, rather than maternal, effects result in potentially long-lasting consequences of noise exposure. Reduced nestling body condition and increased telomere attrition have been shown to reduce post-fledging survival in this species. Given that human transportation networks continue to expand, strategies to mitigate noise exposure on wildlife during critical periods (i.e. breeding) may be needed to maintain local population health in free-living passerines, such as tree swallows.

Fluctuating survival selection explains variation in avian group size.

Most animal groups vary extensively in size. Because individuals in certain sizes of groups often have higher apparent fitness than those in other groups, why wide group size variation persists in most populations remains unexplained. We used a 30-y mark-recapture study of colonially breeding cliff swallows (Petrochelidon pyrrhonota) to show that the survival advantages of different colony sizes fluctuated among years. Colony size was under both stabilizing and directional selection in different years, and reversals in the sign of directional selection regularly occurred. Directional selection was predicted in part by drought conditions: birds in larger colonies tended to be favored in cooler and wetter years, and birds in smaller colonies in hotter and drier years. Oscillating selection on colony size likely reflected annual differences in food availability and the consequent importance of information transfer, and/or the level of ectoparasitism, with the net benefit of sociality varying under these different conditions. Averaged across years, there was no net directional change in selection on colony size. The wide range in cliff swallow group size is probably maintained by fluctuating survival selection and represents the first case, to our knowledge, in which fitness advantages of different group sizes regularly oscillate over time in a natural vertebrate population.

Omega-3 long-chain polyunsaturated fatty acids support aerial insectivore performance more than food quantity.

Once-abundant aerial insectivores, such as the Tree Swallow (Tachycineta bicolor), have declined steadily in the past several decades, making it imperative to understand all aspects of their ecology. Aerial insectivores forage on a mixture of aquatic and terrestrial insects that differ in fatty acid composition, specifically long-chain omega-3 polyunsaturated fatty acid (LCPUFA) content. Aquatic insects contain high levels of both LCPUFA and their precursor omega-3 PUFA, alpha-linolenic acid (ALA), whereas terrestrial insects contain much lower levels of both. We manipulated both the quantity and quality of food for Tree Swallow chicks in a full factorial design. Diets were either high-LCPUFA or low in LCPUFA but high in ALA, allowing us to separate the effects of direct LCPUFA in diet from the ability of Tree Swallows to convert their precursor, ALA, into LCPUFA. We found that fatty acid composition was more important for Tree Swallow chick performance than food quantity. On high-LCPUFA diets, chicks grew faster, were in better condition, and had greater immunocompetence and lower basal metabolic rates compared with chicks on both low LCPUFA diets. Increasing the quantity of high-LCPUFA diets resulted in improvements to all metrics of performance while increasing the quantity of low-LCPUFA diets only resulted in greater immunocompetence and lower metabolic rates. Chicks preferentially retained LCPUFA in brain and muscle when both food quantity and LCPUFA were limited. Our work suggests that fatty acid composition is an important dimension of aerial insectivore nutritional ecology and reinforces the importance of high-quality aquatic habitat for these declining birds.

Perfluoroalkyl Contaminant Exposure and Effects in Tree Swallows Nesting at Clarks Marsh, Oscoda, Michigan, USA.

A site in northeastern Michigan, Oscoda Township, has some of the highest recorded exposure in birds to perfluorinated substances (PFASs) in the United States. Some egg and plasma concentrations at that location exceeded the lowest reproductive effect threshold established for two avian laboratory species. The objectives of this study were to determine whether there were reproductive effects or physiological responses in a model bird species, the tree swallow (Tachycineta bicolor), associated with this extremely high exposure to PFASs. The lack of exposure above background to other contaminants at this site allowed for an assessment of PFAS effects without the complication that responses may be caused by other contaminants. A secondary objective was to determine the distribution of PFASs in multiple tissue types to better understand and interpret residues in different tissues. This can best be done at highly exposed locations where tissue concentrations would be expected to be above detectable levels if they are present in that tissue. There were no demonstrable effects of PFAS exposure on reproduction nor on most physiological responses.

The lingering impact of stress: brief acute glucocorticoid exposure has sustained, dose-dependent effects on reproduction.

Acutely stressful experiences can have profound and persistent effects on phenotype. Across taxa, individuals differ remarkably in their susceptibility to stress. However, the mechanistic causes of enduring stress effects, and of individual differences in stress susceptibility, are poorly understood. Here, we tested whether brief, acute increases in glucocorticoid hormones have persistent effects on phenotype, and whether effects differ according to the magnitude or duration of elevation. We used a novel method to non-invasively manipulate hormone levels on short time scales: the application of corticosterone gel to a model egg secured in the nest. Free-living female tree swallows (Tachycineta bicolor) exposed to several brief corticosterone increases during incubation showed dose-dependent differences in behaviour throughout the reproductive period. Birds receiving treatments that simulated higher or longer acute stress responses later provisioned larger broods at lower rates; the resulting offspring were smaller in size. Treatment did not influence female body condition, oxidative stress, reproductive success or inter-annual survival, but exposed females maintained higher baseline corticosterone after treatments ceased. Overall, these results indicate that brief, acute elevations in glucocorticoids in adulthood can have long-term consequences. Furthermore, individuals that mount a greater or longer acute stress response may be more likely to experience lingering effects of stress.