Jenny Q. Ouyang

Within seasons and among years: When are corticosterone levels repeatable?

Hormones play a central role in integrating internal and external cues to help mediate life-history decisions as well as changes in behavior and physiology of individuals. Describing the consistency of endocrine traits within and among individuals is an important step for understanding whether hormonal traits are dependable predictors of phenotypes that selection could act upon. However, few long-term field studies have investigated the individual consistency of hormonal traits. Glucocorticoid hormones mediate homeostatic responses to environmental variation as well as stress responses to acute, unpredictable disturbances. We characterized the repeatability of plasma corticosterone concentrations in two species of free-living passerines across multiple years. We found repeatability in baseline corticosterone concentrations in both sexes of great tits (Parus major) and in female tree swallows (Tachycineta bicolor) within the breeding season but no repeatability of this trait among seasons or across years. Stress-induced levels of corticosterone were only assessed in great tits and were not repeatable in either sex. Our data suggest that in line with their function in mediating responses of individuals to longer-term and acute demands, both baseline and stress-induced plasma corticosterone concentrations are rather plastic traits. However, individuals may differ in their degree of trait plasticity and hence in behavioral and physiological responses to a variety of organismal challenges.

Corticosterone and brood abandonment in a passerine bird

Hormones regulate decision-making strategies, in particular by translating an individuals physiological state into decisions on major behavioural and life-history processes, such as reproduction. Corticosterone, a glucocorticoid hormone, has been gaining attention as a mediator of reproductive effort, and experimentally elevated corticosterone concentrations have been shown to disrupt reproduction in avian species. Here, we tested whether individual variation in corticosterone concentrations is related to the decision for brood abandonment in free-living great tits, Parus major. Because of harsh environmental conditions, many adults abandoned their first broods in 2010, enabling us to ask which physiological, environmental and individual characteristics increased the probability of nest desertion by both males and females. The best predictors of nest desertion were high stress-induced corticosterone levels in males and low average nestling mass. Furthermore, high stress-induced corticosterone levels in 2010 appeared to represent plastic responses to environmental conditions and reproductive investment: individual males that abandoned their nests in 2010 had higher stress-induced corticosterone concentrations and produced nestlings with lower average mass than in 2009, when nesting successfully. Females that abandoned their nests in 2010 had higher baseline corticosterone concentrations than in 2009, when nesting successfully. Also, males that renested after abandonment in 2010 had lower stress-induced corticosterone concentrations and nestlings with higher mass. Finally, pairs that abandoned but renested later in 2010 had similar fledgling success at the end of the season as those that did not abandon. These results indicate that an individuals reproductive decision is the result of a plastic modulation of the corticosterone stress response that influences reproductive decisions according to environmental conditions.

Small increases in corticosterone before the breeding season increase parental investment but not fitness in a wild passerine bird

Correlative evidence from field studies has suggested that baseline concentrations of corticosterone, the main avian glucocorticoid hormone, affect reproductive strategies in vertebrate species. Such a role is conceivable in light of corticosterones function as a metabolic hormone in regulating glucose and fat metabolism. From such correlational studies, however, the question has remained open whether glucocorticoid concentrations change in advance of reproductive activities or whether corticosterone concentrations vary passively as a consequence of the individuals reproductive investment and workload. To test such causal relationships, we manipulated corticosterone concentrations prior to the breeding season in adult great tits (Parus major) and quantified reproductive investment and success. Two weeks before egg-laying, we administered subcutaneous silastic implants filled with corticosterone that elevated circulating levels within the baseline range for approximately 30 days to adult males and females. Corticosterone manipulation did not affect lay date or yearly offspring production. However, reproductive behaviors were affected by corticosterone treatment: males fed their mates more often during incubation, and females increased incubation of eggs and brooding of nestlings compared to control individuals. Other behaviors during the nestling stage, when the implants were no longer effective, did not differ between the two treatment groups. Our findings do not support the view that baseline corticosterone concentrations, at least at the time of year when we administered implants, change reproductive strategies per se. The current data suggest that baseline corticosterone levels represent internal signals that causally mediate reproductive effort in individuals of a wild bird species. By increasing reproductive investment, baseline corticosterone concentrations may have functions during the breeding season that diverge from the suppressive effects of stress-induced concentrations.

Endocrine phenotype, reproductive success and survival in the great tit, Parus major

A central goal in evolutionary ecology is to characterize and identify selection patterns on the optimal phenotype in different environments. Physiological traits, such as hormonal responses, provide important mechanisms by which individuals can adapt to fluctuating environmental conditions. It is therefore expected that selection shapes hormonal traits, but the strength and the direction of selection on plastic hormonal signals are still under investigation. Here, we determined whether, and in which way, selection is acting on the hormones corticosterone and prolactin by characterizing endocrine phenotypes and their relationship with fitness in free‐living great tits, Parus major. We quantified variation in circulating concentrations of baseline and stress‐induced corticosterone and in prolactin during the prebreeding (March) and the breeding season (May) for two consecutive years, and correlated these with reproductive success (yearly fledgling number) and overwinter survival in female and male individuals. In both years, individuals with high baseline corticosterone concentrations in March had the highest yearly fledgling numbers; while in May, individuals with low baseline corticosterone had the highest yearly reproductive success. Likewise, individuals that displayed strong seasonal plasticity in baseline corticosterone concentrations (high in March and low in May) had the highest reproductive success in each year. Prolactin concentrations were not related to reproductive success, but were positively correlated to the proximity to lay. Between‐year plasticity in stress‐induced corticosterone concentrations of males was related to yearly variation in food abundance, but not to overall reproductive success. These findings suggest that seasonally alternating directional selection is operating on baseline corticosterone concentrations in both sexes. The observed between‐year consistency in selection patterns indicates that a one‐time hormone sample in a given season can allow the prediction of individual fitness.

Stressful colours: corticosterone concentrations in a free-living songbird vary with the spectral composition of experimental illumination

Organisms have evolved under natural daily light/dark cycles for millions of years. These cycles have been disturbed as night-time darkness is increasingly replaced by artificial illumination. Investigating the physiological consequences of free-living organisms in artificially lit environments is crucial to determine whether nocturnal lighting disrupts circadian rhythms, changes behaviour, reduces fitness and ultimately affects population numbers. We make use of a unique, large-scale network of replicated field sites which were experimentally illuminated at night using lampposts emanating either red, green, white or no light to test effect on stress hormone concentrations (corticosterone) in a songbird, the great tit (Parus major). Adults nesting in white-light transects had higher corticosterone concentrations than in the other treatments. We also found a significant interaction between distance to the closest lamppost and treatment type: individuals in red light had higher corticosterone levels when they nested closer to the lamppost than individuals nesting farther away, a decline not observed in the green or dark treatment. Individuals with high corticosterone levels had fewer fledglings, irrespective of treatment. These results show that artificial light can induce changes in individual hormonal phenotype. As these effects vary considerably with light spectrum, it opens the possibility to mitigate these effects by selecting street lighting of specific spectra.

Becoming more like your mate: hormonal similarity reduces divorce rates in a wild songbird

In animals with biparental care, maintaining a pair bond is of adaptive value because it increases reproductive success and reduces costs, such as energy and time, for finding a new mate. Hormones are important mediators of social behaviours as well as parental care, and endocrine mechanisms are therefore likely to be involved in the decision whether to stay with the same mate or separate after a breeding season. Because behavioural compatibility has been shown to increase fitness and hormones have been shown to regulate behavioural traits, here we examined whether the degree of endocrine similarity is also related to reproductive success and pair bond longevity. We used a 3-year study on free-living great tits, Parus major, to test whether mates had similar hormone levels during the parental phase. We tested specifically whether the metabolic hormone corticosterone was related to pair bond longevity and reproductive success. Baseline, but not stress-induced, corticosterone concentrations were highly correlated among members of a pair and became more similar among members of pairs that stayed together for multiple years. Pairs that increased their hormonal similarity within a season (from prebreeding to breeding) had the highest reproductive success. Pairs with more similar baseline corticosterone levels and higher reproductive success were also more likely to remain together after the breeding season. The results of this study suggest that pair bond longevity is related to endocrine similarity and reproductive success, and raise the possibility that hormonal mechanisms may be under sexual selection.

Analysis of the Optimal Duration of Behavioral Observations Based on an Automated Continuous Monitoring System in Tree Swallows (Tachycineta bicolor): Is One Hour Good Enough?

Studies of animal behavior often rely on human observation, which introduces a number of limitations on sampling. Recent developments in automated logging of behaviors make it possible to circumvent some of these problems. Once verified for efficacy and accuracy, these automated systems can be used to determine optimal sampling regimes for behavioral studies. Here, we used a radio-frequency identification (RFID) system to quantify parental effort in a bi-parental songbird species: the tree swallow (Tachycineta bicolor). We found that the accuracy of the RFID monitoring system was similar to that of video-recorded behavioral observations for quantifying parental visits. Using RFID monitoring, we also quantified the optimum duration of sampling periods for male and female parental effort by looking at the relationship between nest visit rates estimated from sampling periods with different durations and the total visit numbers for the day. The optimum sampling duration (the shortest observation time that explained the most variation in total daily visits per unit time) was 1h for both sexes. These results show that RFID and other automated technologies can be used to quantify behavior when human observation is constrained, and the information from these monitoring technologies can be useful for evaluating the efficacy of human observation methods.

Do wild great tits avoid exposure to light at night

Studies of wild populations have provided important insights into the effects of artificial light at night on organisms, populations and ecosystems. However, in most studies the exact amount of light at night individuals are exposed to remains unknown. Individuals can potentially control their nighttime light exposure by seeking dark spots within illuminated areas. This uncertainty makes it difficult to attribute effects to a direct effect of light at night, or to indirect effects, e.g., via an effect of light at night on food availability. In this study, we aim to quantify the nocturnal light exposure of wild birds in a previously dark forest-edge habitat, experimentally illuminated with three different colors of street lighting, in comparison to a dark control. During two consecutive breeding seasons, we deployed male great tits (Parus major) with a light logger measuring light intensity every five minutes over a 24h period. We found that three males from pairs breeding in brightly illuminated nest boxes close to green and red lamp posts, were not exposed to more artificial light at night than males from pairs breeding further away. This suggests, based on our limited sample size, that these males could have been avoiding light at night by choosing a roosting place with a reduced light intensity. Therefore, effects of light at night previously reported for this species in our experimental set-up might be indirect. In contrast to urban areas where light is omnipresent, bird species in non-urban areas may evade exposure to nocturnal artificial light, thereby avoiding direct consequences of light at night.