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Highlights: [•] Individual baseline GCs were followed across two reproductive attempts. [•] Current reproductive investment was experimentally reduced. [•] Individuals responded by increasing future reproductive investment. [•] Inter-individual flexbility in baseline GCs predicted the increase in investment. [•] Flexibility in baseline GCs prepare individuals for predictable changes. [Copyright &y& Elsevier]

Abstract: The proximate mechanisms that underlie the evolution of within-sex variation in mating behavior, sexual characters and reproductive investment patterns are still poorly understood. Species exhibiting alternative reproductive tactics (ARTs) are ideal model systems to examine these mechanisms. Chinook salmon (Oncorhynchus tshawytscha) exhibits two distinct ARTs: hooknoses, which are large males that establish spawning dominance hierarchies via intense male–male competition and jacks, which are smaller precocious sneaking males that steal fertilizations via sperm competition. In this study, we examine plasma testosterone (T), 11-ketotestosterone (11-KT) and maturation-inducing steroid (MIS; 17α,20β-dihydroxy-4-pregnen-3-one) profiles of spawning hooknoses and jacks. Furthermore, we examine relationships between androgens and primary (gonad mass, gonadosomatic index and sperm traits) and secondary (total mass, body size, hump depth and kype length) sexual characters. Relationships between MIS and sperm traits are also examined. We found that hooknoses and jacks did not significantly differ in terms of plasma T, 11-KT or MIS concentrations. Moreover, we found significant positive relationships between levels of both androgens within each ART. There were no significant relationships between androgens, MIS and sperm traits. T and 11-KT concentrations co-varied positively with gonad investment and kype length in jacks. In hooknoses, 11-KT concentration was positively related to total mass, hump depth and condition factor. Overall, these findings suggest that there are differential androgen effects for each of the ARTs in Chinook salmon. [Copyright &y& Elsevier]

Abstract: Optimal functioning of the hypothalamo–pituitary–adrenal (HPA) axis is paramount to maximizing fitness in vertebrates. Research in laboratory mammals has suggested that maternally-induced stress can cause significant variation in the responsiveness of an offspring''s HPA axis involving both pre- and post-natal developmental mechanisms. However, very little is known regarding effects of maternal stress on the variability of offspring adrenocortical functioning in free-living vertebrates. Here we use an experimental approach that independently lowers the quality of both the pre- and post-natal developmental environment to examine programming and plasticity in the responsiveness of the HPA axis in fledglings of a free-living passerine, the European starling (Sturnus vulgaris). We found that mimicking a hormonal signal of poor maternal condition via an experimental pre-natal increase in yolk corticosterone decreased the subsequent responsiveness of the HPA axis in fledglings. Conversely, decreasing the quality of the post-natal developmental environment (by decreasing maternal provisioning capability via a maternal feather-clipping manipulation) increased subsequent responsiveness of the HPA axis in fledglings, apparently through direct effects on nestling body condition. The plasticity of these responses was sex-specific with smaller female offspring showing the largest increase in HPA reactivity. We suggest that pre-natal, corticosterone-induced, plasticity in the HPA axis may be a ‘predictive adaptive response’ (PAR): a form of adaptive developmental plasticity where the advantage of the induced phenotype is manifested in a future life-history stage. Further, we introduce a new term to define the condition-driven post-natal plasticity of the HPA axis to an unpredictable post-natal environment, namely a ‘reactive adaptive response’ (RAR). This study confirms that the quality of both the pre- and post-natal developmental environment can be a significant source of variation in the responsiveness of the HPA axis, and provides a frame-work for examining ecologically-relevant sources of stress-induced programming and plasticity in this endocrine system in a free-living vertebrate, respectively. [Copyright &y& Elsevier]

Current research in birds suggests that a conflict should exist during reproduction for the role of the glucocorticoid corticosterone (CORT). While elevated levels have been correlated with the increased energetic demand of raising offspring, elevated CORT levels have traditionally been implicated in reproductive abandonment. We examined the relationship between CORT and nest desertion in breeding wild female European starlings (Sturnus vulgaris) incorporating analyses of both total circulating levels and ‘free’, unbound CORT through analysis of corticosteroid-binding globulin (CBG). Free baseline CORT levels of nest-abandoning birds were significantly higher than nonabandoning birds within each stage, with chick-rearing birds exhibiting the highest free baseline CORT levels, while concurrently remaining the most resistant stage to nest desertion. Elevated free baseline CORT levels in chick-rearing birds were not due to increased total CORT secretion, but rather to a decrease in CBG levels. Overall, our results suggest that CORT and CBG interact to play a role in mediating the increased energetic demand of offspring, while minimizing the chances of nest desertion, thereby alleviating any potential behavioral conflict for CORT during reproduction. Furthermore, these results demonstrate that the traditional view of the role of CORT during reproduction is much more complex than previously appreciated. Together with mounting evidence, we suggest that elevated corticosteroid levels are an inherent and necessary part of reproduction in nonmammalian tetrapods. [Copyright &y& Elsevier]

Although it is well documented that hatching asynchrony in birds can lead to competitive and developmental hierarchies, potentially greatly affecting growth and survival of nestlings, hatching asynchrony may also precipitate modulations in neuroendocrine development or function. Here we examine sibling variation in adrenocortical function in postnatally developing, asynchronously hatching American kestrels (Falco sparverius) by measurements of baseline and stress-induced levels of corticosterone at ages 10, 16, 22, and 28 days posthatching. There was a significant effect of hatching order on both baseline and stress-induced corticosterone levels during development and these effects grew stronger through development. First-hatched chicks exhibited higher baseline levels than later-hatched chicks throughout development and higher stress-induced levels during the latter half of development. Furthermore, there was significant hatching span (difference in days between first- and last-hatched chicks) × hatching order interaction on both baseline and stress-induced corticosterone levels during development. Hatching span was also positively correlated with both measures of corticosterone and body mass in first-hatched chicks, but was negatively correlated with these factors through most of the development in last-hatched chicks. It is known that hatching asynchrony creates mass and size hierarchies within kestrel broods and we suggest that hierarchies in adrenocortical function among siblings may be one physiological mechanism by which these competitive hierarchies are maintained. [Copyright &y& Elsevier]

We investigated post-natal development of the adrenocortical stress-response system in captive American kestrels (Falco sparverius) by measurements of baseline and stress-induced levels of corticosterone at ages 10, 16, 22, and 28 days post-hatching. Baseline levels of corticosterone increased significantly during post-natal development and although chicks aged 10- and 16-days old exhibited comparable baseline corticosterone levels, those of 22-day-old chicks were significantly higher and those of 28-day-old chicks close to fledging were higher than all younger groups. Chicks in this study exhibited low levels of stress-induced corticosterone early in development and did not exhibit adult-type stress-induced levels of corticosterone until 22 days of age post-hatching. Finally, although baseline and stress-induced levels of 28-day-old birds were significantly higher than one-year-old adults, there was no relationship between baseline corticosterone concentrations and time to nest departure. The fact that baseline levels of corticosterone are low during early development and then increase during later development may be an adaptation to the negative effects of chronically elevated corticosterone levels and as previously noted in other studies may minimize these negative effects on rapid growth and development in young birds, potentially maximizing normal growth. The ability of even young kestrel chicks to elevate corticosterone levels in response to stress suggests that they may be able to physiologically cope with food shortages associated with unpredictable food resources which wild kestrels often face. [Copyright &y& Elsevier]

Nest predation is a primary cause of reproductive failure in birds; thus, predators apply strong selective pressure on nesting behaviour, especially risk assessment behaviours during predator encounters at nests. Prey's risk assessments are not static; rather, dynamic risk assessment theory predicts that prey assess risk in real-time and update it according to changes in cues posed by the predator(s). We used drone videography to film nest-flushing behaviours of common eiders, Somateria mollissima , in response to foraging polar bears, Ursus maritimus , on East Bay Island (Nunavut, Canada). We assessed how cue use influenced flushing behaviour and nest fate in a path analysis using 200 observations of 193 eiders in 2017. Our most supported model found that more direct angles of visual gaze and travel angle by polar bears resulted in conspicuous nest flushes by eiders (β = −0.236 ± 0.059), whereas the presence of herring gulls, Larus argentatus , resulted in more discrete flushes of hens walking from their nests (β = −0.181 ± 0.059). Shorter flush initiation distances between eiders and approaching bears resulted in greater nest predation by polar bears (β = −0.203 ± 0.076). We found no support that an eider's visibility from the nest influenced any component of flushing behaviour. We suggest that during encounters with bears, eiders are capable of assessing risk and making appropriate behavioural decisions to reduce the chances of nest loss. However, as the colony experienced heavy predation by bears in 2017, behavioural responses alone appear to be insufficient to mitigate polar bear predation at the population level. • We examined nesting seabirds' responses to polar bear predation. • We used drones to investigate polar bear foraging in a common eider colony. • Direct angles of gaze and travel by bears resulted in more conspicuous nest flushes. • However, when more gulls were present, eiders' flushes were more inconspicuous. [ABSTRACT FROM AUTHOR]

Highlights • We tested if CORT levels in an iridescent bird relates to plumage attributes. • CORT does not predict subsequent-year iridescent plumage colour in tree swallows. • Female feather characteristics do not predict same-year incubation CORT levels. • Female feather characteristics do not predict same-year breeding CORT levels. Abstract The production of high quality secondary sexual traits can be constrained by trade-offs in the allocation of energy and nutrients with other metabolic activities, and is mediated by physiological processes. In birds, the factors influencing male plumage quality have been well studied; however, factors affecting female plumage quality are poorly understood. Furthermore, it remains uncertain which physiological traits mediate the relationship between body condition and ornaments. In this three-year study of after-second-year female tree swallows (Tachycineta bicolor), we investigated (1) the relationship between baseline corticosterone near the end of the brood-rearing period (CORT BR) and feather colour characteristics (hue, saturation, brightness) the following year, and (2) the relationship between baseline corticosterone measured during incubation (CORT I) and brood rearing (CORT BR), and feather colour in the same year. To control for reproductive effort, we included reproductive parameters as covariates in all analyses. In this first study between CORT and the plumage colour characteristics of a species bearing iridescent feathers, we did not find any relationship between CORT BR and the colour of subsequently-produced feathers, nor did we find any relationship between CORT and the colour of feathers displayed during that breeding season. If CORT levels at the end of breeding carry over to influence the immediately subsequent moult period as we expect, our results generally indicate that structural plumage quality may not be as sensitive to circulating CORT levels compared to carotenoid-based colouration. Future studies, particularly those employing experimental manipulations of CORT during moult in species with iridescent traits, are necessary to fully determine the role glucocorticoids play in mediating the quality of secondary sexual characteristics. [ABSTRACT FROM AUTHOR]

[Display omitted] • Blood samples from 106 common eiders from 6 colonies were analyzed (δ 13C, δ 15N, THg, prolactin). • A significant interaction was found between δ 13C, δ 15N and Hg on prolactin. • In the 3-way interaction, δ 15N positively correlated with incubating prolactin. • Low δ 13C and high Hg had a consistent interactive impact in combination with δ 15N. • This complex interaction may have downstream implications for reproductive success. Global climate change is causing abiotic shifts such as higher air and ocean temperatures, and disappearing sea ice in Arctic ecosystems. These changes influence Arctic-breeding seabird foraging ecology by altering prey availability and selection, affecting individual body condition, reproductive success, and exposure to contaminants such as mercury (Hg). The cumulative effects of alterations to foraging ecology and Hg exposure may interactively alter the secretion of key reproductive hormones such as prolactin (PRL), important for parental attachment to eggs and offspring and overall reproductive success. However, more research is needed to investigate the relationships between these potential links. Using data collected from 106 incubating female common eiders (Somateria mollissima) at six Arctic and sub-Arctic colonies, we examined whether the relationship between individual foraging ecology (assessed using δ 13C, δ 15N) and total Hg (THg) exposure predicted PRL levels. We found a significant, complex interaction between δ 13C, δ 15N and THg on PRL, suggesting that individuals cumulatively foraging at lower trophic levels, in phytoplankton-dominant environments, and with the highest THg levels had the most constant significant relationship PRL levels. Cumulatively, these three interactive variables resulted in lowered PRL. Overall, results demonstrate the potential downstream and cumulative implications of environmentally induced changes in foraging ecology, in combination with THg exposure, on hormones known to influence reproductive success in seabirds. These findings are notable in the context of continuing environmental and food web changes in Arctic systems, which may make seabird populations more susceptible to ongoing stressors. [ABSTRACT FROM AUTHOR]

Diel variation in baseline glucocorticoid (GC) secretion influences energetics and foraging behaviors. In temperate breeding, diurnal vertebrates, studies have shown that daily patterns of baseline GC secretion are influenced by environmental photoperiod, with baseline GCs peaking prior to sunrise to stimulate waking and foraging behaviors. Measures of physiological energy acquisition are also expected to peak in response to foraging activity, but their relationship to GC levels have not been well studied. In contrast to temperate breeding species, virtually nothing is known about diel GC and energetic metabolite secretion in Arctic breeding species, which experience almost constant photoperiods in spring and summer. Using a ten-year dataset, we examined the daily, 24-h pattern of baseline corticosterone (CORT) and triglyceride (TRIG) secretion in approximately 800 female pre-breeding Arctic-nesting common eiders ( Somateria mollissima ). We related these traits to environmental photoperiod and to tidal cycle. In contrast to temperate breeding species, we found that that neither time of day nor tidal trend predicted diel variation in CORT or TRIG secretion in Arctic-breeding eiders. Given the narrow window of opportunity for breeding in polar regions, we suggest that eiders must decouple their daily foraging activity from light and tidal cycles if they are to accrue sufficient energy for successful breeding. As CORT is known to influence foraging behavior, the absence of a distinct diel pattern of CORT secretion may therefore be an adaptation to optimize reproductive investment and likelihood for success in some polar-breeding species. [ABSTRACT FROM AUTHOR]

Abstract: Sexual differentiation of the brain has traditionally been thought to be driven by gonadal hormones, particularly testosterone (T). Recent studies in songbirds and other species have indicated that non-gonadal sex steroids may also be important. For example, dehydroepiandrosterone (DHEA) – a sex steroid precursor that can be synthesized in the adrenal glands and/or brain – can be converted into active sex steroids, such as 17β-estradiol (E2), within the brain. Here, we examine plasma DHEA and E2 levels in wild developing European starlings (Sturnus vulgaris), from hatch (P0) to fledging (P20). Blood samples were collected from either the brachial vein (n =143) or the jugular vein (n =129). In songbirds, jugular plasma is enriched with neurally-synthesized steroids and, therefore, jugular plasma is an indirect measure of the neural steroidal milieu. Interestingly, brachial DHEA levels were higher in males than females at P4. In contrast, jugular DHEA levels were higher in females than males at P0 and P10. Brachial E2 levels were higher in males than females at P6. Surprisingly, jugular E2 levels were not high and showed no sex differences. Also, we calculated the difference between brachial and jugular steroid levels. At several ages, jugular steroid levels were lower than brachial levels, particularly in males, suggesting greater neural metabolism of circulating DHEA and E2 in males than females. At a few ages, jugular steroid levels were higher than brachial levels, suggesting neural secretion of DHEA or E2 into the general circulation. Taken together, these data suggest that DHEA may play a role in brain sexual differentiation in songbirds. [Copyright &y& Elsevier]