Mercury has contaminated rivers worldwide, with health consequences for aquatic organisms and humans who consume them. Researchers have focused on aquatic birds as sentinels for mercury. However, trophic transfer between adjacent ecosystems could lead to the export of aquatic mercury to terrestrial habitats. Along a mercury-contaminated river in Virginia, United States, terrestrial birds had significantly elevated levels of mercury in their blood, similar to their aquatic-feeding counterparts. Diet analysis revealed that spiders delivered much of the dietary mercury. We conclude that aquatic mercury pollution can move into terrestrial habitats, where it biomagnifies to levels in songbirds that may cause adverse effects. Rivers contaminated with mercury may pose a threat to the many bird species that feed on predatory invertebrates in adjacent riparian habitats.

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repeatable within or across years. Overall, our results suggest that taking into account the potential for individual differences in glucocorticoid trait expression in one context to influence optimal endocrine expression in other contexts could be important for understanding the evolution of endocrine systems.

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Mercury is a pervasive environmental contaminant and a well-documented immunosuppressor. However, little is known about the effects of mercury contamination on health of free-living vertebrate populations. The South River in Virginia, USA was heavily contaminated with industrial mercury from 1929 to 1950, and recent studies have documented high levels of circulating mercury in riparian songbirds breeding below the site of contamination. Here we used two standardized immune assays, mitogen-induced swelling in response to phytohaemagglutinin (PHA) and antibody response to sheep red blood cells (SRBCs), to test for effects of mercury toxicity on the immune system of female tree swallows (Tachycineta bicolor) which feed on terrestrial and aquatic insects along the contaminated waterway. We found that females breeding at mercury-contaminated sites mounted significantly weaker PHA-induced swelling responses than those at reference sites in both years of study. However, among females on the contaminated sites, individual bloodstream mercury concentration did not predict the extent of mitogen-induced swelling. We did not detect any differences between reference and contaminated females in the strength of antibody responses to SRBCs, but sample sizes for this assay were significantly smaller. Overall, our results suggest that mercury toxicity can exert sub-lethal immunosuppression in free-living, insectivorous songbirds. The potential fitness consequences of the detected differences in immunocompetence caused by mercury toxicity warrant further study.

Phenotypic flexibility is a central way that organisms cope with challenging and changing environments. As endocrine signals mediate many phenotypic traits, heritable variation in hormone levels, or their context-dependent flexibility, could present an important target for selection. Several studies have estimated the heritability of circulating glucocorticoid levels under acute stress conditions, but little is known about the potential for either baseline hormone levels or rapid endocrine flexibility to evolve. Here, we assessed the potential for selection to operate on the elevation (circulating hormone levels) and flexibility of glucocorticoid reaction norms to acute restraint stress. Multivariate animal models revealed low but significant heritability in baseline (h = 0.13-0.14) and stress-induced glucocorticoids (h = 0.18), and moderate heritability in glucocorticoid flexibility in response to acute stress (h = 0.38) in free-living juvenile tree swallows (Tachycineta bicolor; n = 408). Baseline glucocorticoids were not genetically correlated with either stress-induced glucocorticoids or glucocorticoid flexibility. These findings indicate that baseline glucocorticoids and the acute stress response are distinct traits that can be independently shaped by selection. Microevolutionary changes that influence the expression or flexibility of these endocrine mediators of phenotype may be an important way that populations adapt to changing environments and novel threats.

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Mercury is a heavy metal that has contaminated countless ecosystems throughout the world. A large body of literature has documented reproductive, physiological, and behavioral impairments associated with mercury exposure in laboratory settings, but whether and how such effects are manifest in free-living populations remains poorly understood. The purpose of this study was to evaluate whether tree swallow (Tachycineta bicolor) breeding success at a site with high mercury exposure varied with ambient temperature or precipitation at various points in the breeding cycle. Tree swallows nesting along the South River had significantly elevated blood total mercury (mean ± SE: 3.03 ± 0.15 μg/g) compared to swallows breeding on reference sites (mean ± SE: 0.16 ± 0.005 μg/g). These high levels of mercury were associated with reduced hatching and fledging success, and contaminated birds produced approximately one less fledgling per nest than their reference counterparts. The magnitude of this difference was weather-dependent: unusually high ambient temperatures encountered early in the nestling period were associated with reduced reproductive output in contaminated, but not in reference, birds. In contrast, little effect of mercury on success of nestlings was observed when temperatures were cooler, and precipitation also had no detectable interaction with mercury. These results provide insight into mechanisms underlying reproductive effects of mercury. In addition, these findings underscore the importance of considering variable environmental conditions when assessing effects of contaminants on free-living wildlife. In particular, projections about the effects of global climate change on ecotoxicological impacts must take into account the kinds of weather-mediated effect demonstrated here.