Hormones are central regulators of organismal function and flexibility that mediate a diversity of phenotypic traits from early development through senescence. Yet despite these important roles, basic questions about how and why hormone systems vary within and across species remain unanswered. Here we describe HormoneBase, a database of circulating steroid hormone levels and their variation across vertebrates. This database aims to provide all available data on the mean, variation, and range of plasma glucocorticoids (both baseline and stress-induced) and androgens in free-living and un-manipulated adult vertebrates. HormoneBase (www.HormoneBase.org) currently includes >6,580 entries from 476 species, reported in 648 publications from 1967 to 2015, and unpublished datasets. Entries are associated with data on the species and population, sex, year and month of study, geographic coordinates, life history stage, method and latency of hormone sampling, and analysis technique. This novel resource could be used for analyses of the function and evolution of hormone systems, and the relationships between hormonal variation and a variety of processes including phenotypic variation, fitness, and species distributions.

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Animals must balance various costs and benefits when deciding when to breed. The costs and benefits of breeding at different times have received much attention, but most studies have been limited to investigating short‐term season‐to‐season fitness effects. However, breeding early, versus late, in a season may influence lifetime fitness over many years, trading off in complex ways across the breeder’s lifespan. In this study, we examined the complete life histories of 867 female tree swallows (Tachycineta bicolor) breeding in Ithaca, New York, between 2002 and 2016. Earlier breeders outperformed later breeders in short‐term measures of reproductive output and offspring quality. Though there were weak indications that females paid long‐term future survival costs for breeding early, lifetime fledgling output was markedly higher overall in early‐breeding birds. Importantly, older females breeding later in the season did not experience compensating life history advantages that suggested an alternative equal‐fitness breeding strategy. Rather, most or all of the swallows appear to be breeding as early as they can, and differences in lay dates appear to be determined primarily by differences in individual quality or condition. Lay date had a significant repeatability across breeding attempts by the same female, and the first lay date of females fledged in our population was strongly influenced by the first lay date of their mothers, indicating the potential for ongoing selection on lay date. By examining performance over the entire lifespan of a large number of individuals, we were able to clarify the relationship between timing of breeding and fitness and gain new insight into the sources of variability in this important life history trait.

Artificial light at night (hereafter ‘ALAN’) affects 88% of the land area in Europe and almost half of the land area in the US, with even rural areas exposed to lights from agricultural and industrial buildings. To date, there have been few studies that assess the impacts of ALAN on both wildlife behavior and physiology. However, ALAN may alter energy expenditure and/or stress physiology during the breeding period, potentially reducing reproductive success and resulting in conservation implications. Here, we experimentally exposed adult female and nestling tree swallows (Tachycineta bicolor) to ALAN. We then measured the effects of ALAN compared to control conditions on parental behavior (provisioning rate), nestling physiology (corticosterone levels), and reproductive success (likelihood of all eggs hatching and all nestlings fledging per nest). Our results showed that ALAN-exposed females provisioned their nestlings at lower rates than control females. Although relatively weak, our results also suggested that ALAN-exposed nestlings had reduced baseline and increased stress-induced corticosterone compared to control nestlings. ALAN-exposed nestlings also showed greater negative feedback of circulating corticosterone. We found no support for our prediction that ALAN would reduce nestling body condition. Finally, we found some support for a negative effect of ALAN on the likelihood that all eggs hatched in a given nest, but not the likelihood that all nestlings fledged. Therefore, while it is possible that the behavioral and physiological changes found here result in long-term consequences, our results also suggest that direct ALAN exposure alone may not have substantially large or negative effects on tree swallows. Exposure regimes for free-living birds, such as exposure to a combination of anthropogenic disturbances (i.e. ALAN and noise pollution) or direct and indirect effects of ALAN (i.e. effects on physiology due to direct light exposure and alterations in food availability), may produce different results than those found here.

The risk of predation directly affects physiology, behavior, and fitness of wild birds. Social interactions with conspecifics may affect how individuals respond to stressors such as predators. Strong social connections could help individuals recover from a stressful experience; however, competitive interactions also have the potential to exacerbate stress. Few studies have investigated the interaction between environmental stressors and the social landscape in wild bird populations. Here, we experimentally simulated predation attempts on breeding female tree swallows (Tachicyneta bicolor). At the same time, we manipulated female breast plumage color, a key social signal. Simulated predation events on tree swallows negatively affected their nestlings condition, telomere lengths, and fledging success. However, the effects of experimental manipulations were timing-dependent: simulated predation during the early nestling period was more detrimental than predation during incubation. Contrary to our expectations, manipulation of the social environment did not affect the response of tree swallows to simulated predation. However, manipulating female plumage during the nestling period did affect nestling size, indicating an effect of the social environment on reproductive success. Our data demonstrate that transient stressors on breeding female birds can have carry-over effects on their nestlings, some of which may be long-lasting.