Stress and the timing of breeding: glucocorticoid-luteinizing hormones relationships in an arctic seabird

Individual quality overwrites carry-over effects across the annual cycle of a long-distance migrant

In seasonal environments, the fitness of animals depends upon the successful integration of life-history stages throughout their annual cycle. Failing to do so can lead to negative carry-over effects where individuals are transitioning into the next season in different states, consequently affecting their future performance. However, carry-over effects can be masked by individual quality when individuals vary in their efficiency at acquiring resources year after year (i.e. 'quality'), leading to cross-seasonal consistency in individual performance. Here we investigated the relative importance of carry-over effects and individual quality in determining cross-seasonal interactions and consequences for breeding success over the full annual cycle of a migratory seabird (black-legged kittiwake Rissa tridactyla). We monitored the reproduction and annual movement of kittiwakes over 13 years using geolocators to estimate their breeding success, distribution and winter energy expenditure. We combined this with an experimental approach (clutch removal experiment, 2 years) to manipulate the reproductive effort irrespective of individual quality. Piecewise path analyses showed that successful breeders reproduced earlier and were more likely to breed successfully again the following year. This positive interaction among consecutive breeding stages disappeared after controlling for individual quality, suggesting that quality was dominant in determining seasonal interactions. Moreover, controlling experimentally for individual quality revealed underlying carry-over effects that were otherwise masked by quality, with breeding costs paid in higher energy expenditure and delayed onset of reproduction. We highlight the need to combine an experimental approach along with long-term data while assessing apparent carry-over effects in wild animals, and their potential impact on fitness and population demography.

Sex-specific responses to GnRH challenge, but not food supply, in kittiwakes: Evidence for the "sensitivity to information" hypothesis

Seasonal timing of breeding is usually considered to be triggered by endogenous responses linked to predictive cues (e.g., photoperiod) and supplementary cues that vary annually (e.g., food supply), but social cues are also important. Females may be more sensitive to supplementary cues because of their greater role in reproductive timing decisions, while males may only require predictive cues. We tested this hypothesis by food-supplementing female and male colonial seabirds (black-legged kittiwakes, Rissa tridactyla) during the pre-breeding season. We measured colony attendance via GPS devices, quantified pituitary and gonadal responses to gonadotropin releasing hormone (GnRH) challenge, and observed subsequent laying phenology. Food supplementation advanced laying phenology and increased colony attendance. While female pituitary responses to GnRH were consistent across the pre-breeding season, males showed a peak in pituitary sensitivity at approximately the same time that most females were initiating follicle development. The late peak in male pituitary response to GnRH questions a common assumption that males primarily rely on predictive cues (e.g., photoperiod) while females also rely on supplementary cues (e.g., food availability). Instead, male kittiwakes may integrate synchronising cues from their social environment to adjust their reproductive timing to coincide with female timing.

Integument carotenoid-based colouration reflects contamination to perfluoroalkyl substances, but not mercury, in arctic black-legged kittiwakes

Anthropogenic activities are introducing multiple chemical contaminants into ecosystems that act as stressors for wildlife. Perfluoroalkyl substances (PFAS) and mercury (Hg) are two relevant contaminants that may cause detrimental effects on the fitness of many aquatic organisms. However, there is a lack of information on their impact on the expression of secondary sexual signals that animals use for mate choice. We have explored the correlations between integument carotenoid-based colourations, blood levels of carotenoids, and blood levels of seven PFAS and of total Hg (THg) in 50 adult male black-legged kittiwakes (Rissa tridactyla) from the Norwegian Arctic during the pre-laying period, while controlling for other colouration influencing variables such as testosterone and body condition. Kittiwakes with elevated blood concentrations of PFAS (PFOSlin, PFNA, PFDcA, PFUnA, or PFDoA) had less chromatic but brighter bills, and brighter gape and tongue; PFOSlin was the pollutant with the strongest association with bill colourations. Conversely, plasma testosterone was the only significant correlate of hue and chroma of both gape and tongue, and of hue of the bill. Kittiwakes with higher concentrations of any PFAS, but not of THg, tended to have significantly higher plasma concentrations of the carotenoids astaxanthin, zeaxanthin, lutein, and cryptoxanthin. Our work provides the first correlative evidence that PFAS exposure might interfere with the carotenoid metabolism and the expression of integument carotenoid-based colourations in a free-living bird species. This outcome may be a direct effect of PFAS exposure or be indirectly caused by components of diet that also correlate with elevated PFAS concentrations (e.g., proteins). It also suggests that there might be no additive effect of THg co-exposure with PFAS on the expression of colourations. These results call for further work on the possible interference of PFAS with the expression of colourations used in mate choice.

Microbiota composition and diversity of multiple body sites vary according to reproductive performance in a seabird

The microbiota is suggested to be a fundamental contributor to host reproduction and survival, but associations between microbiota and fitness are rare, especially for wild animals. Here, we tested the association between microbiota and two proxies of breeding performance in multiple body sites of the black-legged kittiwake, a seabird species. First we found that, in females, nonbreeders (i.e., birds that did not lay eggs) hosted different microbiota composition to that of breeders in neck and flank feathers, in the choanae, in the outer-bill and in the cloacae, but not in preen feathers and tracheae. These differences in microbiota might reflect variations in age or individual quality between breeders and nonbreeders. Second, we found that better female breeders (i.e., with higher body condition, earlier laying date, heavier eggs, larger clutch, and higher hatching success) had lower abundance of several Corynebacteriaceae in cloaca than poorer female breeders, suggesting that these bacteria might be pathogenic. Third, in females, better breeders had different microbiota composition and lower microbiota diversity in feathers, especially in preen feathers. They had also reduced dispersion in microbiota composition across body sites. These results might suggest that good breeding females are able to control their feather microbiota-potentially through preen secretions-more tightly than poor breeding females. We did not find strong evidence for an association between reproductive outcome and microbiota in males. Our results are consistent with the hypothesis that natural variation in the microbiota is associated with differences in host fitness in wild animals, but the causal relationships remain to be investigated.

The effects of food supply on reproductive hormones and timing of reproduction in an income-breeding seabird

Current food supply is a major driver of timing of breeding in income-breeding animals, likely because increased net energy balance directly increases reproductive hormones and advances breeding. In capital breeders, increased net energy balance increases energy reserves, which eventually leads to improved reproductive readiness and earlier breeding. To test the hypothesis that phenology of income-breeding birds is independent of energy reserves, we conducted an experiment on food-supplemented ("fed") and control female black-legged kittiwakes (Rissa tridactyla). We temporarily increased energy costs (via weight handicap) in a 2 × 2 design (fed/unfed; handicapped/unhandicapped) during the pre-laying period and observed movement via GPS-accelerometry. We measured body mass, baseline hormones (corticosterone; luteinising hormone) before and after handicap manipulation, and conducted a gonadotropin-releasing hormone challenge. Females from all treatment groups foraged in similar areas, implying that individuals could adjust time spent foraging, but had low flexibility to adjust foraging distance. Consistent with the idea that income breeders do not accumulate reserves in response to increased food supply, fed birds remained within an energy ceiling by reducing time foraging instead of increasing energy reserves. Moreover, body mass remained constant until the onset of follicle development 20 days prior to laying regardless of feeding or handicap, implying that females were using a 'lean and fit' approach to body mass rather than accumulating lipid reserves for breeding. Increased food supply advanced endocrine and laying phenology and altered interactions between the hypothalamic-pituitary-adrenal axis and the hypothalamic-pituitary-gonadal axis, but higher energy costs (handicap) had little effect. Consistent with our hypothesis, increased food supply (but not net energy balance) advanced endocrine and laying phenology in income-breeding birds without any impact on energy reserves.

Personality-specific carry-over effects on breeding

Carry-over effects describe the phenomenon whereby an animal's previous conditions influence its subsequent performance. Carry-over effects are unlikely to affect individuals uniformly, but the factors modulating their strength are poorly known. Variation in the strength of carry-over effects may reflect individual differences in pace-of-life: slow-paced, shyly behaved individuals are thought to favour an allocation to self-maintenance over current reproduction, compared to their fast-paced, boldly behaved conspecifics (the pace-of-life syndrome hypothesis). Therefore, detectable carry-over effects on breeding should be weaker in bolder individuals, as they should maintain an allocation to reproduction irrespective of previous conditions, while shy individuals should experience stronger carry-over effects. We tested this prediction in black-legged kittiwakes breeding in Svalbard. Using miniature biologging devices, we measured non-breeding activity of kittiwakes and monitored their subsequent breeding performance. We report a number of negative carry-over effects of non-breeding activity on breeding, which were generally stronger in shyer individuals: more active winters were followed by later breeding phenology and poorer breeding performance in shy birds, but these effects were weaker or undetected in bolder individuals. Our study quantifies individual variability in the strength of carry-over effects on breeding and provides a mechanism explaining widespread differences in individual reproductive success.

Monitoring glucocorticoid metabolite concentrations as a proxy of environmental stress across important life-history stages in captive African penguins

Due to considerable global decline in wildlife population numbers and species diversity, because of various anthropogenic activities, conservationists increasingly rely on captive and managed populations as important reservoirs to ensure the survival of endangered and vulnerable species. However, very few of these facilities implement robust, non-invasive monitoring techniques to confirm the effectiveness of their management practices to address animal welfare challenges. This study assessed adrenocortical activity as an indication of environmental stress by investigating the effects of both natural (climate, life-history stages) and anthropogenic (visitor presence) factors on captive-housed African penguins. Seven male-female African penguin breeding pairs were housed in a large, naturalistic outside enclosure at the National Zoological Garden (NZG), South Africa. Weekly urofaecal samples were collected from all individuals over one-year to measure urofaecal glucocorticoid metabolite (ufGCM) concentrations. General linear mixed model analysis determined that visitor presence (for males) and rainfall (for females) were the two factors which best explained the variation in ufGCM concentrations of the study population; however, none of the environmental and anthropogenic factors monitored were found to be significant. A posthoc graphical analysis showed considerable individual variation in terms of ufGCM concentrations within and between sexes when comparing life-history stages. This study confirms that non-invasive steroid monitoring can be an effective tool set for defining and assessing environmental stressors for African penguins and potentially other captive seabirds. However, conservationists and wildlife managers should also consider that individual-, sex-, and population-specific differences in the response to environmental stressors can exist. As such, a generalized management protocol for a specific species may not be sufficient and should be customized according to the specific captive population and/or individual.

Post-natal glucocorticoid elevation affects GnRH-induced luteinizing hormone concentration in female house sparrows

Most non-mammalian studies investigating the long-term effects of early-life stressor exposure on endocrine regulation have focused on the hypothalamic-pituitaryadrenal (HPA) axis. However, the hypothalamic-pituitary-gonadal (HPG) axis may more directly affect fitness by regulating reproduction. Changes in HPG axis regulation could allow vertebrates to adaptively mitigate negative effects of early-life stressor exposure. However, only a few studies have examined long-term effects of early-life stressor experience on the HPG axis, and these have found mixed results. Here, we evaluate long-term effects of post-natal corticosterone exposure on the HPG axis in adult female house sparrows (Passer domesticus). We elevated circulating corticosterone non-invasively in wild nestling house sparrows between 8 and 11 days post-hatching, and then brought birds into captivity at fledging. Early in their first breeding season (ages 285-353d post-hatching), females were given a gonadotropin releasing hormone (GnRH) challenge. We found that early-life corticosterone exposure interacted with current condition such that females exposed to elevated post-natal corticosterone had higher baseline and GnRH-induced luteinizing hormone (LH) concentration than control females, but only if they had a high mass. Our results suggest that female house sparrows may mitigate negative impacts of early-life corticosterone exposure by investing in early reproduction, but only when current energetic condition allows.

Winter corticosterone and body condition predict breeding investment in a nonmigratory bird

Reproduction is an energetically demanding life history stage that requires costly physiological and behavioral changes, yet some individuals will invest more into reproduction and breed more successfully than others. To understand variation in reproductive investment, previous studies have evaluated factors during breeding, but conditions outside of this life history stage may also play a role. Using a free-ranging population of black-capped chickadees (Poecile atricapillus), we assessed the repeatability of plastic traits relating to energetic condition (circulating initial corticosterone concentrations and body condition) during the nonbreeding season and evaluated whether these traits predicted reproductive investment in the subsequent breeding season. We found that initial corticosterone concentrations and an index of body condition, but not fat score, were moderately repeatable over a 1-week period in winter. This trait repeatability supports the interpretation that among-individual variation in these phenotypic traits could reflect an intrinsic strategy to cope with challenging conditions across life history stages. We found that females with larger fat reserves during winter laid eggs sooner and tended to spend more time incubating their eggs and feeding their offspring. In contrast, we found that females with higher residual body mass delayed breeding, after controlling for the relationship between fat score and timing of breeding. Additionally, females with higher initial corticosterone in winter laid lighter eggs. Our findings suggest that conditions experienced outside of the breeding season may be important factors explaining variation in reproductive investment.

Egg size is independent of variation in pre-breeding feather corticosterone in Cassin's auklets during favorable oceanographic conditions

The measurement of corticosterone levels in feathers (fCort) is gaining recognition as an effective means for describing links between stages of the annual cycle in birds. Many seabirds are especially good models for exploring these links, or carryover effects, due to their migratory behavior and reproductive investment in a single-egg clutch. Here, we measure fCort in Cassin's auklet (Ptychoramphus aleuticus) breast feathers at two colonies in British Columbia during 2011, a year of favorable oceanographic conditions, and examine its relationship with egg size. These feathers are grown at sea during the late winter period, 1-2 months prior to egg laying. Assuming that fCort provides some measure of nutritional stress and hypothalamic-pituitary-adrenal axis activity during feather growth, we predicted that fCort would be positively correlated with egg size via increased support for foraging activity during the nutritionally demanding molt stage. We also analyzed the δ¹³C and δ¹⁵N stable isotope content of breast feathers, measures commonly used to characterize seabird diet composition. Contrary to prediction, neither fCort nor stable isotope ratios were good predictors of egg size. Our results appear to conflict with two previous studies on alcids in which fCort and stable isotopes showed clear links with egg size; however, both studies were conducted in years when oceanographic and foraging conditions were poor. Under these conditions, upregulation of corticosterone may be needed to mediate the energetic tradeoffs between self-maintenance and reproduction, supporting increased foraging effort and thus increasing both the likelihood of reproduction and large egg size. However, when foraging conditions are favorable, we suggest that such tradeoffs and associated physiological constraints are minimized and regulation of egg size may be effectively independent of circulating corticosterone levels and diet type.

Dose-response effects of light at night on the reproductive physiology of great tits (Parus major): Integrating morphological analyses with candidate gene expression

Artificial light at night (ALAN) is increasingly recognized as a potential threat to wildlife and ecosystem health. Among the ecological effects of ALAN, changes in reproductive timing are frequently reported, but the mechanisms underlying this relationship are still poorly understood. Here, we experimentally investigated these mechanisms by assessing dose‐dependent photoperiodic responses to ALAN in the great tit (Parus major). We individually exposed photosensitive male birds to one of three nocturnal light levels (0.5, 1.5, and 5 lux), or to a dark control. Subsequent histological and molecular analyses on their testes indicated a dose‐dependent reproductive response to ALAN. Specifically, different stages of gonadal growth were activated after exposure to different levels of light at night. mRNA transcript levels of genes linked to the development of germ cells (stra8 and spo11) were increased under 0.5 lux compared to the dark control. The 0.5 and 1.5 lux groups showed slight increases in testis size and transcript levels associated with steroid synthesis (lhr and hsd3b1) and spermatogenesis (fshr, wt1, sox9, and cldn11), although spermatogenesis was not detected in histological analysis. In contrast, all birds under 5 lux had 10 to 30 times larger testes than birds in all other groups, with a parallel strong increase in mRNA transcript levels and clear signs of spermatogenesis. Across treatments, the volume of the testes was generally a good predictor of testicular transcript levels. Overall, our findings indicate that even small changes in nocturnal light intensity can increase, or decrease, effects on the reproductive physiology of wild organisms.

Linking pre-laying energy allocation and timing of breeding in a migratory arctic raptor

For migratory species, acquisition and allocation of energy after arrival on the breeding grounds largely determine reproductive decisions. Few studies have investigated underlying physiological mechanisms driving variation in breeding phenology so far. We linked physiological state to individual timing of breeding in pre-laying arctic-nesting female peregrine falcons (Falco peregrinus tundrius). We captured females from two populations 2-20 days before egg-laying to assess plasma concentration of β-hydroxybutyric acid (BUTY) and triglyceride (TRIG), two metabolites known to reflect short-term changes in fasting and fattening rate, respectively. We also assessed baseline corticosterone (CORTb), a hormone that mediates energy allocation, and the scaled mass index (SMI) as an indicator of somatic body reserves. Plasma BUTY was slightly higher during the pre-recruiting period compared to the period of rapid follicular growth, indicating a reduction in catabolism of lipid reserves before investment in follicle development. Conversely, TRIG levels increased in pre-recruiting females, and best-predicted individual variation in pre-laying interval and lay date. A marked increase in CORTb occurred concomitantly with the onset of rapid follicle growth. SMI was highly variable possibly reflecting variation in food availability or individuals at different stages. Results suggest that (1) lower rates of pre-laying fattening and/or lower mobilization rate of lipoproteins to ovarian follicles delayed laying, and (2) an elevation in pre-laying CORTb may result from, or be required to compensate for, the energetic costs of egg production. Results of this study illustrate how variation in the allocation of energy before laying can influence individual fitness-related reproductive decisions.

The seasonal glucocorticoid response of male Rufous-winged Sparrows to acute stress correlates with changes in plasma uric acid, but neither glucose nor testosterone

We sought to clarify functional relationships between baseline and acute stress-induced changes in plasma levels of the stress hormone corticosterone (CORT) and the reproductive hormone testosterone (T), and those of two main metabolites, uric acid (UA) and glucose (GLU). Acute stress in vertebrates generally stimulates the secretion of glucocorticoids, which in birds is primarily CORT. This stimulation is thought to promote behavioral and metabolic changes, including increased glycemia. However, limited information in free-ranging birds supports the view that acutely elevated plasma CORT stimulates glycemia. Acute stress also often decreases the secretion of reproductive hormones (e.g., T in males), but the role of CORT in this decrease and the contribution of T to the regulation of plasma GLU remain poorly understood. We measured initial (pre-stress) and acute stress-induced plasma CORT and T as well as GLU in adult male Rufous-winged Sparrows, Peucaea carpalis, sampled during the pre-breeding, breeding, post-breeding molt, and non-breeding stages. Stress increased plasma CORT and the magnitude of this increase did not differ across life history stages. The stress-induced elevation of plasma CORT was consistently associated with decreased plasma UA, suggesting a role for CORT in the regulation of plasma UA during stress. During stress plasma GLU either increased (pre-breeding), did not change (breeding), or decreased (molt and non-breeding), and plasma T either decreased (pre-breeding and breeding) or did not change (molt and non-breeding). These data provide only partial support to the hypothesis that CORT secretion during acute stress exerts a hyperglycemic action or is responsible for the observed decrease in plasma T taking place at certain life history stages. They also do not support the hypothesis that rapid changes in plasma T influence glycemia.

Exposure to oxychlordane is associated with shorter telomeres in arctic breeding kittiwakes

Telomeres are DNA-protein complexes located at the end of chromosomes, which play an important role in maintaining the genomic integrity. Telomeres shorten at each cell division and previous studies have shown that telomere length is related to health and lifespan and can be affected by a wide range of environmental factors. Among them, some persistent organic pollutants (POPs) have the potential to damage DNA. However, the effect of POPs on telomeres is poorly known for wildlife. Here, we investigated the relationships between some legacy POPs (organochlorine pesticides and polychlorobiphenyls) and telomere length in breeding adult black-legged kittiwakes (Rissa tridactyla), an arctic seabird species. Our results show that among legacy POPs, only blood concentration of oxychlordane, the major metabolite of chlordane mixture, is associated with shorter telomere length in females but not in males. This suggests that female kittiwakes could be more sensitive to oxychlordane, potentially explaining the previously reported lower survival rate in most oxychlordane-contaminated kittiwakes from the same population. This study is the first to report a significant and negative relationship between POPs and telomere length in a free-living bird and highlights sex-related susceptibility to banned pesticides.

Antioxidant Responses in Relation to Persistent Organic Pollutants and Metals in a Low- and a High-Exposure Population of Seabirds

Oxidative stress occurs when there is an imbalance between the production of reactive oxygen species (ROS) and antioxidant defense. Exposure to pollutants may increase ROS and affect antioxidant levels, and the resulting oxidative stress may negatively affect both reproduction and survival. We measured concentrations of 18 persistent organic pollutants (POPs) and 9 toxic elements in blood, as well as total antioxidant capacity (TAC), total glutathione (tGSH), and carotenoids in plasma of Baltic and Arctic female common eiders (Somateria mollissima) (N = 54) at the end of their incubation-related fasting. The more polluted Baltic population had higher TAC and tGSH concentrations compared to the Arctic population. Carotenoid levels did not differ between populations. The effect of mixtures of pollutants on the antioxidants was assessed, and the summed molar blood concentrations of 14 POPs were positively related to TAC. There was no significant relationship between the analyzed pollutants and tGSH concentrations. The adaptive improvement of the antioxidant defense system in the Baltic population may be a consequence of increased oxidative stress. However, both increased oxidative stress and energy allocation toward antioxidant defense may have adverse consequences for Baltic eiders at the incubation stage, when energy resources reach an annual minimum due to incubation-related fasting.

Does corticosterone regulate the onset of breeding in free-living birds?: The CORT-Flexibility Hypothesis and six potential mechanisms for priming corticosteroid function

For many avian species, the decision to initiate breeding is based on information from a variety of environmental cues, including photoperiod, temperature, food availability, and social interactions. There is evidence that the hormone corticosterone may be involved in delaying the onset of breeding in cases where supplemental cues, such as low food availability and inclement weather, indicate that the environment is not suitable. However, not all studies have found the expected relationships between breeding delays and corticosterone titers. In this review, we present the hypothesis that corticosterone physiology mediates flexibility in breeding initiation (the "CORT-Flexibility Hypothesis"), and propose six possible corticosterone-driven mechanisms in pre-breeding birds that may delay breeding initiation: altering hormone titers, negative feedback regulation, plasma binding globulin concentrations, intracellular receptor concentrations, enzyme activity and interacting hormone systems. Based on the length of the breeding season and species-specific natural history, we also predict variation in corticosterone-regulated pre-breeding flexibility. Although few studies thus far have examined mechanisms beyond plasma hormone titers, the CORT-Flexibility Hypothesis is grounded on a solid foundation of research showing seasonal variation in the physiological stress response and knowledge of physiological mechanisms modulating corticosteroid effects. We propose six possible mechanisms as testable and falsifiable predictions to help clarify the extent of HPA axis regulation of the initiation of breeding.

Feather and faecal corticosterone concentrations predict future reproductive decisions in harlequin ducks (Histrionicus histrionicus)

Understanding sources of reproductive variation can inform management and conservation decisions, population ecology and life-history theory. Annual reproductive variation can drive population growth rate and can be influenced by factors from across the annual cycle (known as carry-over effects). The majority of studies, however, focus solely on the role of current environmental events. Past events often influence future reproductive decisions and success but can be logistically difficult to collect and quantify, especially in migratory species. Recent work indicates that glucocorticoids may prove good indicators to evaluate carry-over effects across life-history transitions. Here, we evaluated three different measures of glucocorticoid physiology (feathers, faeces and plasma) to evaluate the predictability of future breeding decision in the harlequin duck (Histrionicus histrionicus). We collected tail and back feathers, plasma and faeces for glucocorticoid analysis, and fitted female harlequin ducks with very high-frequency transmitters to track their breeding decisions. Both back feathers (moulted immediately before the current season) and faecal glucocorticoid metabolites were identified as important predictive factors of reproductive decisions; high concentrations of glucocorticoid metabolites in back feathers and faeces predicted a higher likelihood of reproductive deferral for the year. Although back and tail feather corticosterone concentrations were correlated, tail feathers (moulted at the end of the previous breeding season) did not predict breeding decisions. Plasma corticosterone concentrations were collected over too broad a time range after capture to be useful in this study. This study demonstrates the utility of non-invasive corticosterone metrics in predicting breeding decisions and supports the use of feathers to measure carry-over effects in migratory birds. With this technique, we identified the prenuptial moult as an important life-history phase that contributes to reproductive decisions. Identification of critical life-history phases is paramount to efficient management of species.

Seasonal and urban effects on the endocrinology of a wild passerine

In order to maximize their fitness, organisms in seasonal environments rely on external cues to optimally time their life-history stages. One of the most important zeitgeber to time reproduction is the photoperiod, but further environmental cues are assessed to fine-tune reproduction due to year-to-year variation in environmental conditions. However, in urbanized environments, the pervasive artificial light at night has altered the natural signal of light and darkness. Accordingly, artificial light at night was repeatedly shown to affect avian reproductive physiology and to advance seasonal reproduction in birds. However, these experiments were mainly conducted in the absence of further environmental cues to facilitate the investigation of the mechanisms which are still poorly understood. Here, we investigate whether the endocrine system of free-ranging European blackbirds (Turdus merula) correlates with the amount of artificial light at night along a rural to urban gradient while the birds still encounter complementary environmental cues including seasonal variation in day length and temperature. Testosterone and estrone were assessed as metabolites in fecal samples and corticosterone in blood from mist-netted blackbirds. We demonstrate that seasonal fluctuations in abiotic factors, individual conditions, but also light at night affect the reproductive and stress physiology of wild European blackbirds. Elevated artificial night light intensities were significantly positively correlated with corticosterone and negatively with female estrone levels. No effects of artificial light were found for testosterone levels. Our results suggest that female blackbirds in particular perceive even low levels of artificial light at night as a weak but chronic stressor that interacts with the hypothalamic-pituitary-gonadal axis and leads to a reduced secretion of reproductive hormones. These findings point out that the impacts of light pollution are diverse and we only slowly disentangle its multiple effects on physiology, ecology, and biodiversity.

Age-related mercury contamination and relationship with luteinizing hormone in a long-lived Antarctic bird

Seabirds, as long-lived top predators, accumulate contaminants such as mercury (Hg), an established endocrine disruptor. In long lived species hormonal secretion varies with age; therefore, Hg-induced endocrine disruption may be exacerbated in some age classes. Here we investigated relationships between blood total Hg and luteinizing hormone (LH, a key pituitary hormone for the onset of breeding), in pre-laying known-age (11-45 years old) snow petrels (Pagodroma nivea) from Adélie Land, Antarctica. We predicted that 1) blood Hg would increase with advancing age as a consequence of bio-accumulation; and that 2) increasing blood Hg would be related to decreased concentrations of LH in the most Hg-contaminated individuals. Hg concentrations were higher in females than in males (p<0.001), and contrary to our prediction, decreased with advancing age in males (p = 0.009) and tended to do so in females (p = 0.06). The analysis of stable isotopes (δ13C and δ15N) suggested that this unexpected pattern could originate from age and sex-related variations in trophic niche, and hence Hg exposure. Regarding LH, our prediction was only supported in young birds (≤23 years) where baseline LH was inversely correlated with Hg concentrations (p = 0.04). Hg burden did not predict baseline LH or GnRH-induced LH in birds that were more than 23 years old. These results show that age and contaminants may interfere with major endocrine mechanisms and, together with other recent studies, support the view that Hg could be connected to LH secretion and could then impair the fitness of long-lived birds.

Regulation of plasma testosterone, corticosterone, and metabolites in response to stress, reproductive stage, and social challenges in a desert male songbird

In many male vertebrates, the secretion of reproductive (gonadal androgens) and adrenocortical (glucocorticoids) hormones varies seasonally and in response to environmental stimuli, and these hormones exert numerous behavioral and metabolic effects. We performed two field studies on adult male Rufous-winged Sparrows, Peucaea carpalis, a Sonoran Desert rain-dependent sedentary species, to (a) determine seasonal changes in initial (baseline) and acute stress-induced plasma testosterone (T), corticosterone (CORT), and two metabolites (uric acid and glucose) and (b) compare the effects of two types of social challenge (song playback or simulated territorial intrusion consisting of song playback plus exposure to a live decoy bird) on plasma T, CORT, these metabolites, and territorial behavior. Initial plasma T was higher during the summer breeding period than during post-breeding molt. Acute stress resulting from capture and restraint for 30 min decreased plasma T in breeding condition birds but not in the fall, revealing that this decrease is seasonally regulated. Initial plasma CORT did not change seasonally, but plasma CORT increased in response to acute stress. This increase was likewise seasonally regulated, being relatively smaller during autumnal molt than in the summer. We found no evidence that acute stress levels of CORT are functionally related to stress-depressed plasma T and, therefore, that plasma T decreases during stress as a result of elevated plasma CORT. Thirty minutes of exposure to simulated territorial intrusion resulted in different behavior than 30 min of exposure to song playback, with increased time spent near the decoy and decreased number of overhead flights. Neither type of social challenge influenced plasma T, thus offering no support for the hypothesis that plasma T either responds to or mediates the behavioral effects of social challenge. Exposure to both social challenges elevated plasma CORT, but simulated territorial intrusion was more effective in this respect than song playback. Plasma uric acid and glucose decreased during acute stress, but only plasma uric acid decreased during social challenge. Thus, an elevation in plasma CORT was consistently associated with a decrease in plasma uric acid, but not with a change in glycemia. These results enhance our understanding of the short-term relationships between T, CORT, and avian territorial behavior. They provide novel information on the endocrine effects of acute stress, in particular on plasma T, in free-ranging birds, and are among the first in these birds to link these effects to metabolic changes.

At the crossroads of physiology and ecology: food supply and the timing of avian reproduction

This article is part of a Special Issue “Energy Balance”. The decision of when to breed is crucial to the reproductive success and fitness of seasonally breeding birds. The availability of food for adults prior to breeding has long been thought to play a critical role in timing the initiation of seasonal reproductive events, in particular laying. However, unequivocal evidence for such a role remains limited and the physiological mechanisms by which an increase in food availability results in seasonal activation of the reproductive system are largely speculative. This lack of mechanistic information partly reflects a lack of integration of ecological and physiological approaches to study seasonal reproduction. Indeed, most work pertaining to the role of food availability for adults on the timing of avian reproduction has been ecological and has focused almost exclusively on female traits associated with reproductive timing (e.g., lay date and clutch size). By contrast, most work on the physiological bases of the relationship between food availability and the timing of reproduction has investigated male traits associated with reproductive development (e.g., reproductive hormones and gonadal development). To advance our understanding of these topics, we review the role of proximate factors including food availability, social factors, and ambient temperature in the control of breeding decisions, and discuss the role of three potential candidates (leptin, glucocorticoids, and GnIH-neuropeptide Y) that may mediate the effects of food availability on these decisions. We emphasize that future progress in this area is heavily contingent upon the use of physiology-based approaches and their integration into current ecological frameworks.

The stress of being contaminated? Adrenocortical function and reproduction in relation to persistent organic pollutants in female black legged kittiwakes

High levels of environmental pollutants such as persistent organic pollutants (POPs) including PCB and DDT have been found in the Arctic and many of those pollutants may impair reproduction through endocrine disruption. Nevertheless, their effects on stress hormones remain poorly understood, especially in free-ranging birds. Corticosterone, the principal glucocorticoid in birds, can indirectly impair reproduction. The aim of the present study was to examine the relationships between POPs and reproduction through their potential consequences on different reproductive traits (breeding decision, egg-laying date, breeding success) and corticosterone secretion (baseline and stress-induced levels). We addressed those questions in an Arctic population of female black-legged kittiwakes during the pre-breeding stage and measured several legacy POPs (PCBs and pesticides: HCB, p,p'-DDE, CHL) in whole blood. POP levels were not related to breeding decision neither to breeding success, whereas females with high levels of pesticides laid their eggs earlier in the season. We found a negative relationship between POP levels and body condition index in non-breeding females. Black-legged kittiwakes with higher levels of PCB showed stronger adrenocortical response when subjected to a capture-handling stress protocol. We suggest that PCBs may disrupt corticosterone secretion whereas the positive relationship between pesticides and egg-laying date could either originate from a direct effect of pesticides or may be related to other confounding factors such as age or individual's quality. Although no direct negative reproduction output of POPs was found in this study, it is possible that the most contaminated individuals would be more sensitive to environmental stress and would be less able to maintain parental investment than less polluted individuals.

Longitudinal monitoring of sex steroid hormones in excrement of spectacled eiders (Somateria fischeri)

From the 1970s to the 1990s, the breeding population of spectacled eiders (Somateria fischeri) in western Alaska declined by 96%, which led to the listing of this species as threatened under the Endangered Species Act in 1993. Since then, the population has stabilized, but has not recovered to pre-decline numbers. While little is known about reproductive endocrinology in spectacled eiders, in other avian species, estrogen and testosterone are known to initiate and modulate various reproductive processes including yolk protein synthesis, reproductive behaviors and secondary sex characteristics. Measurement of the metabolites of estrogen and testosterone (EM and TM, respectively) in excrement reflect circulating hormone concentrations and provide a non-invasive method to monitor reproductive physiology. We measured concentrations of excreted EM in captive females and TM in males to (1) determine the efficacy of commercially available radioimmunoassay kits to detect EM and TM, (2) describe annual profiles of EM and TM concentrations, and (3) define the reproductive season of captive spectacled eiders using endocrine status. Excrement samples were collected from captive female and male spectacled eiders three times per week throughout 1 year. Female EM and male TM levels were quantified using radioimmunoassay. Mean female EM profile exhibited values exceeding the threshold for "peak" values (EM>193.3 ng/g) from mid-February to early July, and again in September. Additionally, the highest average concentrations of EM were seen in March, May and September. Elevated TM concentrations occurred in mid March, mid May and late June. These data suggest that levels of excreted sex steroids reflect patterns predicted by breeding landmarks in the annual cycle and will assist in field monitoring and captive breeding programs for spectacled eiders.

The effect of keel fractures on egg production, feed and water consumption in individual laying hens

The impact of keel bone fractures on egg production, egg weight and feed and water consumption in individual laying hens. A total of 165 Lohmann brown laying hens were obtained from a commercial farm that consisted of 105 with keel fractures and 60 without keel fractures. 2. After a 4-d period of acclimatisation, hens were individually housed and provided with ad libitum food and water for a 24-h period. The number of eggs laid, egg weight, feed and water consumption during this period were recorded. Keel bone strength was also assessed. 3. Hens free from keel fractures laid more eggs (91.7% vs. 84.9%) of significantly heavier weight (61.9 g vs. 60.2 g), ate less feed (139 g vs. 151 g) and drank less water (212 ml vs. 237 ml) than hens with fractures. 4. There was a significant positive association between keel fracture severity and water consumption, and a significant negative association between keel fracture severity and egg weight and keel bone strength. 5. This small-scale study on individual birds shows that keel bone fractures may have an impact on the economics of egg production.

Postmigratory body condition and ovarian steroid production predict breeding decisions by female gray-headed albatrosses

Carryover effects have been documented in many migratory bird species, but we know little about the physiological mechanisms that mediate those effects. Here we show that the energetic, endocrine, and aerobic characteristics of postmigratory female gray-headed albatrosses (Thalassarche chrysostoma) can affect their decision to breed. All females in this study, whether breeding or not, were secreting ovarian steroids when they arrived at the breeding colony at Bird Island, South Georgia, which suggests that all were responding to seasonal cues. However, deferring, nonbreeding birds were characterized by a steroid profile of high progesterone (P4) and low testosterone (T), whereas breeding birds showed the opposite pattern. Deferring birds also had low body mass, hematocrit, and hemoglobin. These results suggest that postmigratory condition can influence patterns of ovarian steroidogenesis and that the maintenance of high P4 without subsequent conversion to T favors breeding deferral. Whereas breeding females normally convert P4 to T, which is a key deterministic step toward 17β-estradiol synthesis, vitellogenesis, and follicle development, deferring females did not make this conversion and instead maintained high levels of P4, perhaps due to inhibition of the hydroxylase-lyase enzyme complex, thus rendering them infertile for the current season. Results are discussed within the context of the biennial breeding system of this species, and comparisons with other biennially and annually breeding albatrosses are made.

Differential reproductive responses to stress reveal the role of life-history strategies within a species

Life-history strategies describe that 'slow'- in contrast to 'fast'-living species allocate resources cautiously towards reproduction to enhance survival. Recent evidence suggests that variation in strategies exists not only among species but also among populations of the same species. Here, we examined the effect of experimentally induced stress on resource allocation of breeding seabirds in two populations with contrasting life-history strategies: slow-living Pacific and fast-living Atlantic black-legged kittiwakes. We tested the hypothesis that reproductive responses in kittiwakes under stress reflect their life-history strategies. We predicted that in response to stress, Pacific kittiwakes reduce investment in reproduction compared with Atlantic kittiwakes. We exposed chick-rearing kittiwakes to a short-term (3-day) period of increased exogenous corticosterone (CORT), a hormone that is released during food shortages. We examined changes in baseline CORT levels, parental care and effects on offspring. We found that kittiwakes from the two populations invested differently in offspring when facing stress. In response to elevated CORT, Pacific kittiwakes reduced nest attendance and deserted offspring more readily than Atlantic kittiwakes. We observed lower chick growth, a higher stress response in offspring and lower reproductive success in response to CORT implantation in Pacific kittiwakes, whereas the opposite occurred in the Atlantic. Our findings support the hypothesis that life-history strategies predict short-term responses of individuals to stress within a species. We conclude that behaviour and physiology under stress are consistent with trade-off priorities as predicted by life-history theory. We encourage future studies to consider the pivotal role of life-history strategies when interpreting inter-population differences of animal responses to stressful environmental events.

Fecal cortisol levels predict breeding but not survival of females in the short-lived rodent, Octodon degus

The cort-adaptation hypothesis indicates that an association between glucocorticoid (cort) levels and fitness may vary with the extent to which reproduction or breeding effort is a major determinant of cort levels. Support for a context dependent association between cort and fitness comes mostly from relatively long-lived, bird species. We tested the hypothesis that there are gender and context (life-history) specific cort-fitness relationships in degus, a short-lived and generally semelparous social rodent. In particular, we used demographical records on a natural population to estimate adult survival through seasons and years and linked that to records of baseline cort (based on fecal cortisol metabolites). We found no evidence for a direct relationship between baseline cort and adult survival across seasons, and this lack of association was recorded irrespective of sex and life history stage. Yet, cort levels during early lactation predicted the probability that females produce a second litter during the same breeding season, supporting a connection between baseline cort levels and breeding effort. Overall, the differential effects of cort on survival and breeding supported that the extent of cort-fitness relationships depends on the fitness component examined.

To breed or not to breed: endocrine response to mercury contamination by an Arctic seabird

Mercury, a ubiquitous toxic element, is known to alter expression of sex steroids and to impair reproduction across vertebrates but the mechanisms underlying these effects are not clearly identified. We examined whether contamination by mercury predicts the probability to skip reproduction in black-legged kittiwakes (Rissa tridactyla) from Svalbard. We also manipulated the endocrine system to investigate the mechanism underlying this relationship. During the pre-laying period, we injected exogenous GnRH (gonadotropin-releasing hormone) to test the ability of the pituitary to release luteinizing hormone (LH, a key hormone for the release of sex steroids and hence breeding) in relation to mercury burden. Birds that skipped reproduction had significantly higher mercury concentration in blood than breeders. Endocrine profiles of these birds also varied based on breeding status (breeders versus non-breeders), mercury contamination and sex. Specifically, in skippers (birds that did not breed), baseline LH decreased with increasing mercury concentration in males, whereas it increased in females. GnRH-induced LH levels increased with increasing mercury concentration in both sexes. These results suggest that mercury contamination may disrupt GnRH input to the pituitary. Thus, high mercury concentration could affect the ability of long-lived birds to modulate their reproductive effort (skipping or breeding) according to ongoing environmental changes in the Arctic, thereby impacting population dynamics.

Corticosterone regulation of ovarian follicular development is dependent on the energy status of laying hens

Glucocorticoids participate in the arousal of stress responses and trigger physiological adjustments that shift energy away from reproduction toward survival. Ovarian follicular development in avians is accompanied by the supply of yolk precursors, which are mainly synthesized in the liver. Therefore, we hypothesized energy status and hepatic lipogenesis are involved in the induction of reproductive disorders by glucocorticoids in laying hens. The results show that corticosterone decreased the laying performance by suppressing follicular development in energy-deficit state, rather than in energy-sufficient state. In corticosterone-treated hens, the suppressed follicular development was associated with the reduced availability of yolk precursor, indicated by the plasma concentration of VLDL and vitellogenin and the decreased proportion of yolk-targeted VLDL (VLDLy). Corticosterone decreased the expression of apolipoprotein B and apolipoprotein VLDL-II in the liver. A drop in VLDL receptor content and an increase in the expression of tight junction proteins occludin and claudin1 were also observed in hierarchical follicles. The results suggest corticosterone-suppressed follicular development is energy dependent. The decreased apolipoprotein synthesis and VLDLy secretion by liver are responsible for the decreased availability of circulating yolk precursor, and the upregulation of occludin and claudin expression further prevents yolk deposition into oocytes.

Corticosterone metabolism by chicken follicle cells does not affect ovarian reproductive hormone synthesis in vitro

Glucocorticoids affect reproductive hormone production in many species. In chickens, elevated plasma corticosterone down-regulates testosterone and progesterone concentrations in plasma, but also in egg yolk. This suppression could be mediated via the hypothalamic-pituitary system but also via local inhibition of gonadal activity by glucocorticoids. As the latter has not been tested in birds yet, we tested if corticosterone directly inhibits ovarian steroid synthesis under in vitro conditions. We hypothesized that degradation of corticosterone by follicular cells impairs their ability to synthesize reproductive hormones due to either inhibition of enzymes or competition for common co-factors. Therefore, we first established whether follicles degrade corticosterone. Follicular tissue was harvested from freshly euthanized laying hens and incubated with radiolabelled corticosterone. Radioactive metabolites were visualized and quantified by autoradiography. Follicles converted corticosterone in a time-dependent manner into metabolites with a higher polarity than corticosterone. The predominant metabolite co-eluted with 20β-dihydrocorticosterone. Other chicken tissues mostly formed the same metabolite when incubated with corticosterone. In a second experiment, follicles were incubated with either progesterone or dehydroepiandrosterone. Corticosterone was added in increasing dosages up to 1000 ng per ml medium. Corticosterone did not inhibit the conversion of progesterone and dehydroepiandrosterone into a number of different metabolites, including 17α-hydroxyprogesterone, androstenedione and testosterone. In conclusion, avian tissues degrade corticosterone mostly to 20β-dihydrocorticosterone and even high corticosterone dosages do not affect follicular hormone production under in vitro conditions.

Rapid stress-induced inhibition of plasma testosterone in free-ranging male rufous-winged sparrows, Peucaea carpalis: characterization, time course, and recovery

Chronic stress generally inhibits the activity of the reproductive system. Acute stress also is often inhibitory, but the mechanism involved and its persistence of action once animals are no longer exposed to the stressor are poorly understood. We investigated the effect of capture and restraint stress on plasma testosterone (T), luteinizing hormone (LH), and corticosterone (CORT) in free-ranging male rufous-winged sparrows, Peucaea carpalis. Stress decreased plasma T between 10 and 30 min after capture and restraint but did not influence plasma LH, the main hormone that controls T secretion, suggesting that stress did not decrease plasma T by inhibiting LH secretion. The stress-induced decrease in plasma T was associated with elevated plasma CORT, but there was no evidence that these effects were functionally related. Plasma stress-induced T was positively related to plasma initial T measured within 2 min of capture. This relationship was, however, complex as plasma T decreased proportionally more in response to stress in sparrows with high than low plasma initial T. The relative sensitivity to the same stressor was, therefore, individually variable and this variation was related to initial plasma T. Birds caught and restrained for 30 min, and then released on their breeding territory before recapture up to 6 h later, maintained depressed plasma T, indicating that the effect of acute stress on this hormone persists after the stressor removal. These studies provide new information on the effects of acute stress on plasma T in free-ranging birds. In particular, they are among the first to characterize the time course and to describe the persistence of these effects. The findings also contribute to identifying factors that are associated with individual differences in plasma hormone levels.

Relationships between POPs and baseline corticosterone levels in black-legged kittiwakes (Rissa tridactyla) across their breeding cycle

Chronic exposure to persistent organic pollutants (POPs) in wildlife might alter the response to environmental changes through interference with the regulation of stress hormones. Here, we examined the relationship between blood concentrations of several POPs and baseline plasma corticosterone levels in the black-legged kittiwake (Rissa tridactyla) during three distinct periods in the breeding season. The concentrations of POPs and corticosterone increased, whereas body mass decreased progressively from the pre-laying period to the incubation and the chick rearing period. ∑PCB (polychlorinated biphenyls) correlated positively with the baseline corticosterone levels during the pre-laying period, which might suggest that PCBs affect the regulation of corticosterone. However, this relationship was not found during the incubation or the chick rearing period. Possible explanations are discussed with emphasis on how total stress/allostatic load is handled during different periods and conditions.

Migratory carryover effects and endocrinological correlates of reproductive decisions and reproductive success in female albatrosses

Physiological mechanisms mediating carryover effects, wherein events or activities occurring in one season, habitat, or life-history stage affect important processes in subsequent life-history stages, are largely unknown. The mechanism most commonly invoked to explain carryover effects from migration centres on the acquisition and utilization of resources (e.g. body mass, or individual 'condition'). However, other mechanisms are plausible, e.g. trade-offs reflecting conflict or incompatibility between physiological regulatory systems required for different activities or life-history stages (migration vs. reproduction). Here we show that in female black-browed albatrosses (Thalassarche melanophris) the decision to reproduce or to defer reproduction, made prior to their arrival at breeding colonies after long-distance migration, is associated with condition-related (body mass, hematocrit, hemoglobin concentrations) and hormonal (progesterone, testosterone, estrogen-dependent yolk precursors) traits. In contrast, reproductive success showed little association with condition but showed significant associations with the steroidogenic processes underlying follicle development. Specifically, success was determined by reproductive readiness via differences in steroid hormones and hormone-dependent traits. Successful albatrosses were characterized by high progesterone and high estradiol-dependent yolk precursor levels, whereas failed albatrosses had high testosterone and low yolk precursor levels. Results are discussed with reference to migratory carryover effects and how these can differentially affect the physiologies influencing reproductive decisions and reproductive success.

Elevated plasma corticosterone decreases yolk testosterone and progesterone in chickens: linking maternal stress and hormone-mediated maternal effects

Despite considerable research on hormone-mediated maternal effects in birds, the underlying physiology remains poorly understood. This study investigated a potential regulation mechanism for differential accumulation of gonadal hormones in bird eggs. Across vertebrates, glucocorticoids can suppress reproduction by downregulating gonadal hormones. Using the chicken as a model species, we therefore tested whether elevated levels of plasma corticosterone in female birds influence the production of gonadal steroids by the ovarian follicles and thus the amount of reproductive hormones in the egg yolk. Adult laying hens of two different strains (ISA brown and white Leghorn) were implanted subcutaneously with corticosterone pellets that elevated plasma corticosterone concentrations over a period of nine days. Steroid hormones were subsequently quantified in plasma and yolk. Corticosterone-implanted hens of both strains had lower plasma progesterone and testosterone levels and their yolks contained less progesterone and testosterone. The treatment also reduced egg and yolk mass. Plasma estrogen concentrations decreased in white Leghorns only whereas in both strains yolk estrogens were unaffected. Our results demonstrate for the first time that maternal plasma corticosterone levels influence reproductive hormone concentrations in the yolk. Maternal corticosterone could therefore mediate environmentally induced changes in yolk gonadal hormone concentrations. In addition, stressful situations experienced by the bird mother might affect the offspring via reduced amounts of reproductive hormones present in the egg as well as available nutrients for the embryo.

Experimentally reduced corticosterone release promotes early breeding in black-legged kittiwakes

Breeding at the right time is important for successful reproduction. In birds, stressful environmental conditions are known to delay the timing of breeding but the underlying mechanisms are poorly understood. The stress hormone corticosterone appears to be a good candidate for mediating egg-laying date according to early environmental conditions and physiological state. By experimentally reducing the release of corticosterone in black-legged kittiwakes during the pre-laying period, we tested whether egg-laying date was mechanistically linked to corticosterone levels. Male and female kittiwakes were implanted with a low dose of exogenous corticosterone to inhibit endogenous corticosterone production. According to our predictions, the experimental reduction of corticosterone release was paralleled by a significant advancement of egg laying in females (around 4 days earlier). In addition, females with experimentally reduced corticosterone release gained mass during the pre-laying period compared with controls. Ultimately, the advancement of egg laying in females with experimentally reduced corticosterone levels was associated with an enhanced breeding success. This effect was strongly sex specific. In corticosterone-treated male kittiwakes, egg-laying date and reproductive success were not affected, but breeding probability was lower than in controls. This corticosterone treatment did not influence immediate clutch size, or return rate and breeding decision the following year. Our results support the hypothesis that corticosterone secretion during the pre-laying period mediates the timing of breeding in this long-lived seabird, possibly through the dynamics of energy reserves.