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Researchers were asked to contribute new results addressing questions about the exposure and effects of mercury (Hg) in North American songbirds, a rapidly declining group of species that is the subject of enduring interest for millions of birdwatchers, the general public and conservation scientists. Important questions to be answered include: Is Hg causing or exacerbating songbird population declines? Which North American songbirds are at most risk and in which landscapes? Are there aspects of songbird natural history that pre-dispose them to risks of Hg exposure and effects, in particular, their migratory behavior? In all, 61 authors contributed 15 studies addressing aspects of these questions. Articles in this special issue address an array of topics including: (1) three studies on health effects in the laboratory using a domesticated songbird model species, the zebra finch; (2) three studies on changes in songbird exposure to Hg over time spans from less than a decade to more than a century; (3) five studies on landscape characteristics or management practices that cause the oft-noted spatial variation in Hg accumulation by resident songbirds, from the subarctic tundra to high-elevation tropical forests; (4) three papers examining the recently recognized role of migration behavior in predicting risk to songbirds from Hg; and (5) one paper on the potential pitfalls of using feather Hg concentration as a bioindicator for Hg exposure. In summary, although there are many questions still to be answered, it is clear that the effects of Hg are persistent long after exposure, Hg exposure of North American songbirds is not improving, predicting exposure requires a detailed understanding of ecosystem processes beyond simply the amount of Hg present at a site, migration behavior predisposes songbirds to risk of Hg exposure and effects, and carefully selecting appropriate bioindicator sites, species, and tissues is critical to any monitoring efforts.

For most birds, energy efficiency and conservation are paramount to balancing the competing demands of self-maintenance, reproduction, and other demanding life history stages. Yet the ability to maximize energy output for behaviors like predator escape and migration is often also critical. Environmental perturbations that affect energy metabolism may therefore have important consequences for fitness and survival. Methylmercury (MeHg) is a global pollutant that has wide-ranging impacts on physiological systems, but its effects on the metabolism of birds and other vertebrates are poorly understood. We investigated dose-dependent effects of dietary MeHg on the body composition, basal and peak metabolic rates (BMR, PMR), and respiratory quotients (RQ) of zebra finches (Taeniopygia guttata). Dietary exposure levels (0.0, 0.1, or 0.6 ppm wet weight) were intended to reflect a range of mercury concentrations found in invertebrate prey of songbirds in areas contaminated by atmospheric deposition or point-source pollution. We found adiposity increased with MeHg exposure. BMR also increased with exposure while PMR decreased, together resulting in reduced metabolic scope in both MeHg-exposed treatments. There were differences in RQ among treatments that suggested a compromised ability of exposed birds to rapidly metabolize carbohydrates during exercise in a hop-hover wheel. The elevated BMR of exposed birds may have been due to energetic costs of depurating MeHg, whereas the reduced PMR could have been due to reduced oxygen carrying capacity and/or reduced glycolytic capacity. Our results suggest that environmentally relevant mercury exposure is capable of compromising the ability of songbirds to both budget and rapidly exert energy.

Mercury exposure can disrupt development of the cerebellum, part of the brain essential for coordination of movement through a complex environment, including flight. In precocial birds, such as fowl, the cerebellum develops embryonically, and the chick is capable of leaving the nest within hours of hatching. However, most birds, including all songbirds, are altricial, and spend weeks in the nest between hatching and fledging. The objective of this study was to describe the normal development of the cerebellum in a model altricial songbird so as to determine the effect of exposure to mercury on cerebellar maturation. Adult zebra finch (Taeniopygia guttata) pairs were fed either a control diet, or a diet augmented with one of four treatment-levels of methylmercury (0.3–2.4 μg/g wet weight), and their offspring, the subjects of this study, were fed the same diet by parents. We documented, for the first time, the schedule of cerebellar development in an altricial bird, and compared stages of development among methylmercury-exposed groups. For all treatments of methylmercury, the age of completion of cellular migration was later than for control zebra finches, indicating a delay in cerebellar maturation. Displaced (heterotopic) Purkinje neurons, a pathology typical of methylmercury exposure in developing vertebrate brains, were more numerous in methylmercury-exposed birds, and persisted at least until the age of independence. Delays in maturation of the cerebellum could delay fledging in altricial bird species, with potential serious implications for the fitness of exposed individuals, as predation rates in the nest are often very high.

To examine whether sexually selected traits are particularly sensitive bioindicators of environmental toxicants, we assessed the effects of exposure to environmentally relevant dietary concentrations of the pollutant methylmercury on pigment coloration in zebra finches (Taeniopygia guttata). First, we tested whether effects of methylmercury on coloration were influenced by timing of exposure. Birds were either exposed developmentally (up to 114 days after hatching), as adults (after reaching sexual maturity), or for their entire life. Bill coloration, which is a carotenoid-based, sexually selected trait, was less red in males with lifetime exposure to methylmercury, compared to controls. Neither adult, nor developmental exposure influenced bill color in adult males, with the possible exception of early exposure of nestlings. Among females, where bill color is not under strong sexual selection, neither lifetime nor adult exposure to methylmercury affected bill color. For males and females, there was no effect of either lifetime or adult methylmercury exposure on coloration of back feathers, which is a non-sexually-dimorphic, melanin-based trait that is not likely the result of sexual selection. This study is a comprehensive experimental test of the proposal that sexually selected traits may be particularly useful bioindicators of the stress imposed by environmental toxins such as methylmercury.

Habitat loss and pollution are two of the greatest global threats to biodiversity. Due to their widespread prevalence, these threats often co-occur, yet their interactive effects on organisms remain poorly understood. Some reptiles are vulnerable to these threats because they have specific microclimate requirements for embryonic development and because pollutants are maternally transferred to their eggs; both incubation temperature and pollutants affect reptile sex determination. In aquatic turtles, females often select nest sites in recently planted agricultural fields but the impact of nesting in polluted agricultural habitats is not understood. We examined the influences of crop agriculture and mercury pollution on nest microclimate and offspring sex ratios of Chelydra serpentina . We hypothesized that crop growth in agricultural fields would shade and cool turtle nests, decrease moisture levels, increase male offspring production, and interact with maternally-derived mercury to impact sex determination. As predicted, nests shaded by crops had lower average temperatures (−2.5 °C) and moisture levels (−107 kPa) than control nests. In field and laboratory experiments, agricultural thermal regimens increased the proportion of male offspring in clutches and this effect was intensified in the presence of mercury. Global temperatures are expected to rise within the 21st century and to have a feminizing effect on reptiles with temperature-dependent sex determination. That prediction should be refined to incorporate how the cooling effect of some local habitat conditions (e.g., agricultural fields), and interactions between anthropogenic land-use and common pollutants, will interact with climate change to influence sex ratios of reptiles with temperature-dependent sex determination.

Choosing a high-quality mate contributes to increased reproductive success in birds. Females assess quality in males, in part, via condition-dependent signals such as songs and plumage. The production of these signals of quality can be disrupted by environmental stressors, including toxic pollutants such as mercury. Mercury affects song, plumage, bill color, and courtship behaviors in male birds, but the effect of these changes on female mate choice is unknown. By affecting the condition-dependent signals that females use to assess quality, mercury could alter males' attractiveness to females. We used mate choice of female Zebra Finches (Taeniopygia guttata) to determine if male attractiveness to females is affected by lifetime exposure to mercury. Males were either exposed to dietary mercury or left unexposed and then assessed by unexposed females in 3 types of mate preference tests: song-playback phonotaxis (preference for audio recordings of mercury-exposed or unexposed males' songs); 2-choice...

Mercury (Hg) is a ubiquitous environmental contaminant that affects avian reproduction and condition, in both aquatic and terrestrial species. Because Hg binds strongly to the keratin of growing feathers, molt is an important avenue for Hg elimination. We investigated the rate of depuration of Hg from songbird blood and organs (brain, kidney, liver, muscle) as a function of molt. We exposed 2 species of captive songbirds, European starling (Sturnus vulgaris) and zebra finch (Taeniopygia guttata), to environmentally relevant dietary Hg (1.2-1.5 mg/kg wet wt) for extended periods of time and then allowed them to depurate on a Hg-free diet for 20 wk. Depuration occurred either while birds were regrowing new feathers or when there was little to no molt occurring. Both species quickly eliminated Hg from blood and reduced Hg concentrations in other tissues by more than 90% over 20 wk. The rate of depuration was more rapid than previously reported in the literature, but depuration in small-bodied songbirds has rarely been studied. Molting birds depurated Hg from their blood more rapidly than nonmolting birds. The effect of molt on Hg retention in other tissues was more difficult to resolve, because by the time we sampled organ tissue, depuration was nearly complete. Our results confirm that molting expedites depuration and raises mechanistic questions about how songbirds depurate Hg stored in their tissues. Environ Toxicol Chem 2017;36:3120-3126. © 2017 SETAC.

Juvenile birds are generally less efficient foragers than adults, and this can lead to reduced survival or delayed reproduction. Feeding on hard-shelled nuts and mollusks after breaking them open by dropping is particularly challenging. To determine whether juvenile birds are as proficient as adults at this complex foraging task, we compared juvenile and adult Herring Gulls (Larus argentatus) feeding at low tide by dropping clams (Rangia cuneata) at a coastal estuary in Virginia, USA. Juvenile (first-winter) gulls were less successful than adult (2+ years old) birds at finding clams to drop. Moreover, the clams they selected were of a wider range of sizes, more often being difficult to handle or of lower nutritional value. Juveniles did, however, drop clams with the same degree of success as adult birds. Together these results suggest that learning to find and recognize profitable clams is the developmental bottleneck in this complex behavior.

Consumption of understory foliage by abundant ungulates can reshape forest structure and thus induce corresponding ecosystem changes. In forest songbirds, a negative response to deer browsing was documented for species that use understory foliage, although this association remains poorly understood at the community level. Such knowledge is especially important in eastern North America where deer populations have recently increased substantially. Our primary objective was to examine correlations between deer and songbirds in coastal Virginia (n = 92 sites, 2010–2013), a region with heavy forest fragmentation and abundant deer. The secondary objectives were to compare coastal surveys to an inland region (n = 99 sites, 2012) with fewer deer and lower fragmentation, and to explore links between fragmentation and deer. We predicted that densities of songbirds that use understory foliage (“deer-sensitive”) would correlate negatively with deer fecal pellets – our measure of relative deer use. The estimate of median pellets inland (0 ha−1, range 0–19,600 pellets ha−1) was significantly lower than on the coast (2,014 ha−1, range 0–28,193 ha−1). We found a negative correlation with deer-sensitive birds on the coast (rs = −0.35, P = 0.002), whereas inland, no correlation was evident. Coastal sites had approximately twice as much fragmented forest, which favors deer. Of 26 songbirds in our species-level analysis, we found negative correlations for three species, including one of high conservation concern. These results are consistent with the hypothesis that habitat modification by over-abundant deer promotes measurable changes in bird communities, with conservation implications for declining songbirds.

Models of interaction networks among species (i.e., network models) can be used to predict behavior of complex adaptive systems, such as ecosystems at the land-water interface. As theoretical and empirical understanding of ecological networks continues to grow, network models are increasingly used to quantify and predict ecosystem stability, metapopulation dynamics, disease spread, and social behavior. Here, we propose that ecological networks can also be used to synthesize many of the dynamics that regulate the movement of contaminants between aquatic and terrestrial systems. We discuss how network models can predict aquatic-terrestrial contaminant transport via food-web interactions, animal movement, and mutualistic interaction webs. In addition, we discuss how knowledge of social-ecological networks can help address contaminant exposure and risk to humans. To illustrate an application of network models in the field of contaminants and subsidies, we provide an empirical example of the responses of ecological networks to dam removal in a temperate river system. In this example, we observed that shifts in species composition (especially losses of larger-bodied, often predatory, sport fish) and subsequent changes to feeding relationships could reduce biomagnification of contaminants in the food web and thus reduce exposure to humans and other terrestrial consumers in the short term. In summary, we propose that ecological networks will be a valuable framework for understanding and forecasting temporally and spatially dynamic aquatic to terrestrial contaminant transport.

Maternal transfer is a predominant route of methylmercury (MeHg) exposure to offspring. We reviewed and synthesized published and unpublished data on maternal transfer of MeHg in birds. Using paired samples of females' blood (n = 564) and their eggs (n = 1814) from 26 bird species in 6 taxonomic orders, we conducted a meta-analysis to evaluate whether maternal transfer of MeHg to eggs differed among species and caused differential toxicity risk to embryos. Total mercury (THg) concentrations in eggs increased with maternal blood THg concentrations; however, the proportion of THg transferred from females to their eggs differed among bird taxa and with maternal THg exposure. Specifically, a smaller proportion of maternal THg was transferred to eggs with increasing female THg concentrations. Additionally, the proportion of THg that was transferred to eggs at the same maternal blood THg concentration differed among taxonomic orders, with waterfowl (Anseriformes) transferring up to 382% more THg into their eggs than songbirds (Passeriformes). We provide equations to predict THg concentrations in eggs using female blood THg concentrations, and vice versa, which may help translate toxicity benchmarks across tissues and life stages. Our results indicate that toxicity risk of MeHg can vary among bird taxa due to differences in maternal transfer of MeHg to offspring.

Environmental stressors can negatively affect avian cognitive abilities, potentially reducing fitness, for example by altering response to predators, display to mates, or memory of locations of food. We expand on current knowledge by investigating the effects of dietary mercury, a ubiquitous environmental pollutant and known neurotoxin, on avian cognition. Zebra finches Taeniopygia guttata were dosed for their entire lives with sub-lethal levels of mercury, at the environmentally relevant dose of 1.2 parts per million. In our first study, we compared the dosed birds with controls of the same age using tests of three cognitive abilities: spatial memory, inhibitory control, and color association. In the spatial memory assay, birds were tested on their ability to learn and remember the location of hidden food in their cage. The inhibitory control assay measured their ability to ignore visible but inaccessible food in favor of a learned behavior that provided the same reward. Finally, the color association task tested each bird’s ability to associate a specific color with the presence of hidden food. Dietary mercury negatively affected spatial memory ability but not inhibitory control or color association. Our second study focused on three behavioral assays not tied to a specific skill or problem-solving: activity level, neophobia, and social dominance. Zebra finches exposed to dietary mercury throughout their lives were subordinate to, and more active than, control birds. We found no evidence that mercury exposure influenced our metric of neophobia. Together, these results suggest that sub-lethal exposure to environmental mercury selectively harms neurological pathways that control different cognitive abilities, with complex effects on behavior and fitness.

The trade-off between starvation and predation risk is of paramount importance to songbirds and many other small organisms. A contaminant with metabolic or neurological effects may hinder a bird's ability to manage these ecological risks, which are contingent on metabolic state and require cognitive assessments. Methylmercury (MeHg) is a ubiquitous pollutant that is associated with neurotoxicity, reproductive failure, altered behaviour and increased mortality in aquatic organisms. It was recently discovered that MeHg can enter terrestrial food webs and affect songbirds that eat contaminated invertebrates. Research on behavioural effects of environmentally relevant doses of MeHg in songbirds is a conservation priority as this pollutant is widespread, poorly regulated, increasing and understudied in terms of sublethal effects such as abnormal behaviours. To help close this knowledge gap, we examined how MeHg affects behavioural strategies in captive zebra finches, Taeniopygia guttata. We quantified the birds' responses to risk by measuring regulation of body mass, vigilance behaviour, willingness to move away from dense cover in search of food and reluctance to return to foraging after a disturbance. Dosed and undosed birds were placed in an experimental arena and were videorecorded over 3 days of increasing perception of predation risk. We found that MeHg-exposed birds, compared to control birds, (1) lost significantly more mass and (2) waited significantly longer to forage under the highest predation risk. Our results indicate that MeHg-exposed birds may react more strongly to threat of predation and thereby increase their risk of starvation. To our knowledge this is the first mechanistic study of how a pervasive pollutant may alter optimal decision making, and therefore potentially survival, in songbirds.

Long-term exposure to methyl-mercury has deleterious effects on avian reproduction. However, little is known about whether exposure to mercury solely during embryonic and juvenile development can have long-lasting effects on subsequent reproductive performance as adults. Birds that hatch on contaminated sites but disperse elsewhere will be exposed only during development. Hence, it is important to understand the reproductive consequences of avian exposure to methyl-mercury during early life. Accordingly, in this experiment, domesticated zebra finches ( Taeniopygia guttata) were exposed to dietary methyl-mercury (1.20 μg/g wet weight) from conception through independence (50 days post-hatching). Following maturity, developmentally exposed and control finches were paired within treatment groups and allowed to breed repeatedly for 8 months. Developmentally exposed pairs hatched 32% fewer eggs and produced 50% fewer independent juveniles despite transferring only traces of mercury into their offspring. This is the first example of mercury-related reproductive declines in birds not exposed to mercury during breeding. The magnitude of reproductive decline was similar to that of zebra finches exposed to methyl-mercury during the breeding process. Bird populations breeding in contaminated habitats may suffer from a 2-fold fitness cost of mercury exposure; adult exposure compromises parents' reproduction, while offspring exposure results in reduced reproduction in the next generation.

Mercury (Hg) exposure has been extensively studied in aquatic and piscivorous wildlife, but, historically, less attention has been directed towards terrestrial species. However, it has become apparent that aquatic Hg crosses ecosystem boundaries along with beneficial subsidies, thereby entering the terrestrial food chain. It is still not known how far from contaminated waterways Hg exposure remains a risk. We examined the spatial extent of exposure in terrestrial songbirds breeding in the floodplain along a 40-km stretch of Hg-contaminated river in Virginia, USA. Four songbird species were chosen that readily occupied artificial nest cavities placed at known distances from the river; Carolina chickadees (Poecile carolinensis), Carolina wrens (Thryothorus ludovicianus), eastern bluebirds (Sialia sialis), and house wrens (Troglodytes aedon). We examined Hg exposure at varying distances from the edge of the river for three endpoints: 1) adult bird blood, 2) nestling bird feather and 3) spider whole body (collected in pitfall traps and directly from the mouths of nestling birds). We used mixed effects models to understand 1) how total Hg (i.e., inorganic and organic, THg) concentrations differed between contaminated and reference sites and 2) how THg concentrations declined with distance away from the river on contaminated sites. Adult bird blood THg indicated that a species-by-distance interaction was significant, after accounting for site and year. Importantly, despite the decline with distance, we found that THg levels in some songbird species and their invertebrate prey remained elevated above reference levels for up to 400m away from the river. Our results predict a spatial extent of contamination that is an order of magnitude greater than similar studies investigating the lateral impact of other aquatically derived contaminants. To our knowledge, this study is the first to indicate that exposure to legacy aquatic Hg is possible for terrestrial wildlife across the entire floodplain, rather than being confined to narrow riparian corridors.

Dietary mercury exposure is associated with suppressed immune responsiveness in birds. This study examined the immune-responsiveness of domestic zebra finches (Taeniopygia guttata) experimentally exposed to mercury through their diet. We used the phytohemagglutinin (PHA) skin-swelling test to assay the effect of two modes of mercury exposure. Some finches received exposure to mercury only after reaching sexual maturity, while others were maintained on a mercury-dosed diet throughout life, including development. Each bird received one of five dietary concentrations of methylmercury cysteine (0.0, 0.3, 0.6, 1.2 or 2.4 ppm). In contrast to a study on wild songbirds at a mercury-contaminated site, we detected no relationship between mercury level and immunological response to PHA, regardless of mode of exposure. This result represents the first major difference found by our laboratory between wild birds exposed to environmental mercury and captive birds experimentally exposed to mercury.

The rate of anthropogenic habitat conversion often exceeds the rate of natural ecological and evolutionary processes, which sometimes creates mismatches between environmental cues and adaptive behaviors. In numerous species of aquatic turtles, nest site selection is primarily based on cues related to high solar exposure, which leads females to select sites where humans have disturbed habitat. These disturbed sites are often contaminated by pollutants, such as mercury. Despite the fact that anthropogenic disturbances often co-occur, few studies have examined the interactive influence of major global changes on animal development. Using Chelydra serpentina, we investigated the individual and interactive effects of crop agriculture and mercury pollution on hatch success and offspring phenotype. We hypothesized that following nesting, rapid crop growth would shade and cool nests in agricultural fields and subsequently negatively impact embryonic development. Agricultural and control nests were similar in temperature at the time of oviposition, but temperatures diverged as crops grew: agricultural nests averaged 2.5°C cooler than control nests over the course of incubation. In laboratory and field experiments, we found that turtles incubated under agricultural thermal regimens took longer to hatch, hatched at smaller body sizes, lost more mass, and had lower posthatching growth rates. Additionally, thermal conditions associated with agricultural land use interacted with mercury contamination to decrease hatching success. To our knowledge, this is the first documentation of negative interactive effects of mercury pollution and habitat quality on early vertebrate development and highlights the importance of examining the combined influence of anthropogenic global changes on organisms.

Mercury is an environmental contaminant that impairs avian reproduction, but the behavioral and physiological mechanisms underlying this effect are poorly understood. The objective of this study was to determine whether lifetime dietary exposure to mercury (1.2 µg/g wet weight in food) impacted avian parental behaviors, and how this might influence reproductive success. To distinguish between the direct effects of mercury on parents and offspring, we created four treatment groups of captive-bred zebra finches (Taeniopygia guttata), with control and mercury-exposed adults raising cross-fostered control or mercury-exposed eggs (from maternal transfer). Control parents were 23% more likely to fledge young than parents exposed to mercury, regardless of egg exposure. Mercury-exposed parents were less likely to initiate nests than controls and spent less time constructing them. Nests of mercury-exposed pairs were lighter, possibly due to an impaired ability to bring nest material into the nestbox. However, nest temperature, incubation behavior, and provisioning rate did not differ between parental treatments. Unexposed control eggs tended to have shorter incubation periods and higher hatching success than mercury-exposed eggs, but there was no effect of parental exposure on these parameters. We accidentally discovered that parent finches transfer some of their body burden of mercury to nestlings during feeding through secretion in the crop. These results suggest that, in mercury-exposed songbirds, pre-laying parental behaviors, combined with direct exposure of embryos to mercury, likely contribute to reduced reproductive success and should be considered in future studies. Further research is warranted in field settings, where parents are exposed to greater environmental challenges and subtle behavioral differences might have more serious consequences than were observed in captivity.

Songbirds exposed to methylmercury (MeHg) often exhibit reduced reproductive success and cognitive abilities. To better understand whether oxidative stress plays a role, we dosed zebra finches (Taeniopygia guttata) with a contaminated (1.2 ppm MeHg-cysteine) or control diet for their entire lives, including during development in the egg. Levels of antioxidant enzymes [superoxide dismutase (SOD1 and SOD2)], oxidative damage (4-hydroxynonenal; 4-HNE), and antioxidant transcription factors [nuclear factor (erythroid-derived 2)-like 2; Nrf2] were measured in the liver and pectoralis muscle of adults. MeHg treatment did not affect levels of 4-HNE or liver SOD2 or Nrf2. Birds in the MeHg treatment differed significantly from controls in pectoralis SOD1 and Nrf2, and tended to differ in liver SOD1 and pectoralis SOD2; however, we detected no overall pattern of effect of MeHg on oxidative status in dosed finches. We suspect that this is a consequence of the differential survival of MeHg-tolerant birds.

Mercury is a ubiquitous environmental contaminant known to accumulate in, and negatively affect, fish-eating and oceanic bird species, and recently demonstrated to impact some terrestrial songbirds to a comparable extent. It can bioaccumulate to concentrations of1 μg/g in tissues of prey organisms such as fish and insects. At high enough concentrations, exposure to mercury is lethal to birds. However, environmental exposures are usually far below the lethal concentrations established by dosing studies.The objective of this review is to better understand the effects of sublethal exposure to mercury in birds. We restricted our survey of the literature to studies with at least some exposures5 μg/g. The majority of sublethal effects were subtle and some studies of similar endpoints reached different conclusions. Strong support exists in the literature for the conclusion that mercury exposure reduces reproductive output, compromises immune function, and causes avoidance of high-energy behaviors. For some endpoints, notably certain measures of reproductive success, endocrine and neurological function, and body condition, there is weak or contradictory evidence of adverse effects and further study is required. There was no evidence that environmentally relevant mercury exposure affects longevity, but several of the sublethal effects identified likely do result in fitness reductions that could adversely impact populations. Overall, 72% of field studies and 91% of laboratory studies found evidence of deleterious effects of mercury on some endpoint, and thus we can conclude that mercury is harmful to birds, and the many effects on reproduction indicate that bird population declines may already be resulting from environmental mercury pollution.

Mercury contamination from mining and fossil fuel combustion causes damage to humans and animals worldwide. Mercury exposure has been implicated in mammalian hearing impairment, but its effect on avian hearing is unknown. In this study, we examined whether lifetime dietary mercury exposure affected hearing in domestic zebra finches (Taeniopygia guttata) by studying their auditory brainstem responses (ABRs). Zebra finches exposed to mercury exhibited elevated hearing thresholds, decreased amplitudes, and longer latencies in the ABR, the first evidence of mercury-induced hearing impairment in birds. Birds are a more appropriate model for the human auditory spectrum than most mammals because of similarities in frequency discrimination, vocal learning, and communication behavior. When mercury is considered in combination with other anthropogenic stressors such as noise pollution and habitat alteration, the hearing impairments we document here could substantially degrade avian auditory communication in wild birds.

In migratory birds, the ability to depart wintering grounds at the appropriate time is an important determinant of fitness. Understanding the regulation of this timing will be essential for predicting whether timing of bird migration keeps up with global climate change. We examined whether the timing of the late-winter molt, in which white-throated sparrows (Zonotrichia albicollis) replace head and body feathers in advance of mating, may constrain the timing of northward migratory departure. In an observational study, we found a significant correlation between timing of molt and the date on which free-living male white-throated sparrows disappeared from our study site during migration. The following year, we tested whether experimentally manipulating molt date by advancing photoperiod during temporary captivity would subsequently advance disappearance date once the birds were released. Sparrows that were experimentally induced to molt early disappeared from the wintering site before controls. However, the captive control birds also molted and disappeared from the site earlier than free-living controls, suggesting that the diet during captivity had played a role. In the third winter we completed the study by advancing or delaying molt using only dietary manipulation. Together, these results show that the ability to molt early in spring is related to early disappearance from the wintering site. Early molt likely has carry-over effects on reproduction and the requirements of molt may prevent populations from adjusting migration timing in response to global climate change. J. Exp. Zool. 321A: 586–594, 2014. © 2014 Wiley Periodicals, Inc.

Because Belted Kingfishers (Megaceryle alcyon) eat a diet comprised primarily of fish they are a useful indicator species for aquatic contaminants such as mercury. Monitoring efforts generally compare nesting success or tissue contaminant concentrations from contaminated sites with reference sites. In contrast, this study examined subtler potential effects of mercury accumulation by quantifying plumage coloration (structural and melanin based) of nesting adult Belted Kingfishers and relating it to individual mercury concentrations. Mercury exposure was associated with increased brightness of plumage color consistent with the hypothesis that mercury slows the production of melanin. Clear sex differences in the chroma and hue of blue body feathers identified during this study suggest that Belted Kingfishers possesses cryptic dimorphism beyond the rufous “belt,” and thus mercury-induced alterations in blue plumage could reduce fitness.

Mercury can disrupt the endocrine systems of mammals and fish, but little is known about its effects on avian hormones. The authors employed an experimental manipulation to show that methylmercury suppresses the stress-induced corticosterone response in birds, an effect previously unreported in the literature. Corticosterone regulates many normal metabolic processes, such as the maintenance of proper blood glucose levels during stressful daily fasting; an inability to increase corticosterone levels in response to stressors renders a bird less able to face a wide array of environmental challenges. The authors studied reproductively mature zebra finches that had been exposed to 0.0 µg/g, 0.3 µg/g, 0.6 µg/g, 1.2 µg/g, or 2.4 µg/g (wet wt) dietary methylmercury throughout their life (i.e., from the egg onward). In contrast to some field studies, the present study found no significant change in baseline plasma corticosterone concentrations attributable to chronic methylmercury exposure. However, a comparison between the baseline corticosterone levels and levels after 30 min of handling stress revealed that the ability of birds to mount a stress response was reduced with increasing blood total mercury concentration. These results are consistent with adrenal corticoid disruption caused by chronic mercury exposure and mirror a similar study on free-living nestling songbirds exposed to environmental mercury. Environ Toxicol Chem 2014;33:1072–1076. © 2014 SETAC

Ecotoxicologists often implicitly assume that populations are homogenous entities in which all individuals have similar responses to a contaminant. However, genetically variable responses occur within populations. This variation can be visualized using dose-response curves of genetically related groups, similar to the way that evolutionary biologists construct reaction norms. We assessed the variation in reproductive success of full-sibling families of captive zebra finches (Taeniopygia guttata) experimentally exposed to methylmercury. We found significant variation among families in the effects of methylmercury on several reproductive parameters. This variation suggests that there may be strong responses to selection for resistant genotypes in contaminated areas. This has important implications for the evolution of tolerance as well as risk assessment and wildlife conservation efforts on sites with legacy contamination.

Little is known about effects of maternally transferred contaminants in snakes. The purpose of this study was to evaluate sublethal effects of maternally transferred mercury (Hg) on neonatal northern watersnakes (Nerodia sipedon). We captured 31 gravid females along a historically Hg-contaminated river. Following birth, we measured litter Hg concentrations and assessed locomotor performance, foraging ability (i.e., number of prey eaten, latency to first strike, strike efficiency, and handling time), and learning (i.e., change in foraging measures over time) in their offspring (n = 609). Mercury concentrations in offspring negatively correlated with motivation to feed and strike efficiency. Over time, strike efficiency and latency to strike decreased for all snakes in the study. However, offspring from contaminated areas maintained consistently lower efficiencies than reference individuals. This study is the first to examine sublethal behavioral effects of maternally transferred contaminants in snakes and suggests that maternally transferred Hg negatively affects offspring behavior.

Although mercury (Hg) is a well-studied contaminant, knowledge about Hg accumulation in snakes is limited. The authors evaluated Hg bioaccumulation within and among four snake species (northern watersnakes, Nerodia sipedon; queen snakes, Regina septemvittata; common garter snakes, Thamnophis sirtalis; and rat snakes, Elaphe obsoleta [Pantherophis alleghaniensis]) from a contaminated site on the South River (Waynesboro, VA, USA) and two nearby reference sites. Total Hg (THg) concentrations in northern watersnake tail tissue at the contaminated site ranged from 2.25 to 13.84 mg/kg dry weight (mean: 4.85 ± 0.29), or 11 to 19 times higher than reference sites. Blood THg concentrations (0.03–7.04 mg/kg wet wt; mean: 2.24 ± 0.42) were strongly correlated with tail concentrations and were the highest yet reported in a snake species. Within watersnakes, nitrogen stable isotope values indicated ontogenetic trophic shifts that correlated with THg bioaccumulation, suggesting that diet plays a substantial role in Hg exposure. Female watersnakes had higher mean THg concentrations (5.67 ± 0.46 mg/kg) than males (4.93 ± 0.49 mg/kg), but no significant differences between sexes were observed after correcting for body size. Interspecific comparisons identified differences in THg concentrations among snake species, with more aquatic species (watersnakes and queen snakes) accumulating higher mean concentrations (5.60 ± 0.40 and 4.59 ± 0.38 mg/kg in tail tissue, respectively) than the more terrestrial species, garter snakes and rat snakes (1.28 ± 0.32 and 0.26 ± 0.09 mg/kg, respectively). The results of the present study warrant further investigation of potential adverse effects and will aid in prioritizing conservation efforts. Environ. Toxicol. Chem. 2013;32:1178–1186. © 2013 SETAC

Mercury is a ubiquitous environmental contaminant known to accumulate in, and negatively affect, fish-eating and oceanic bird species, and recently demonstrated to impact some terrestrial songbirds to a comparable extent. It can bioaccumulate to concentrations of >1 μg/g in tissues of prey organisms such as fish and insects. At high enough concentrations, exposure to mercury is lethal to birds. However, environmental exposures are usually far below the lethal concentrations established by dosing studies.

Exposure to mercury in humans, other mammals, and birds is primarily dietary, with mercury in the methylated form and bound to cysteine in the tissues of prey items. Yet dosing studies are generally carried out using methylmercury chloride. Here we tested whether the accumulation of total mercury in zebra finch blood, egg, muscle, liver, kidney or brain differed depending on whether dietary mercury was complexed with chloride or cysteine. We found no effect of form of mercury on tissue accumulation. Some previous studies have found lower accumulation of mercury in tissues of animals fed complexed mercury. Much remains to be understood about what happens to ingested mercury once it enters the intestines, but our results suggest that dietary studies using methylmercury chloride in birds will produce similar tissue accumulation levels to those using methylmercury cysteine.

The pollutant methylmercury accumulates within lean tissues of birds and other animals. Migrating birds catabolize substantial amounts of lean tissue during flight which may mobilize methylmercury and increase circulating levels of this neurotoxin. As a model for a migrating songbird, we fasted zebra finches (Taeniopygia guttata) that had been dosed with 0.0, 0.1, and 0.6 parts per million (ppm) dietary methylmercury and measured changes in blood total mercury concentrations (THg) in relation to reductions in lean mass. Birds lost 6–16% of their lean mass during the fast, and THg increased an average of 12% and 11% in the 0.1 and 0.6 ppm treatments, respectively. Trace amounts of THg in the 0.0 ppm control group also increased as a result of fasting, but remained extremely low. THg increased 0.4 ppm for each gram of lean mass catabolized in the higher dose birds. Our findings indicate that methylmercury is mobilized from lean tissues during protein catabolism and results in acute increases in circulating concentrations. This is a previously undocumented potential threat to wild migratory birds, which may experience greater surges in circulating methylmercury than demonstrated here as a result of their greater reductions in lean mass.

Nest boxes are commonly erected in managed landscapes such as backyards, golf courses and parks, attracting cavity-nesting birds to habitats they would not otherwise occupy. Although studies have shown that habitat around natural nests influences postfledging survival, this relationship has not been examined for artificial nest cavities, which humans may site at inappropriate locations. In Williamsburg, Virginia, we compared vegetation surrounding nest boxes which produced eastern bluebird fledglings that were predated by hawks within 10 days of leaving the nest to vegetation surrounding nest boxes producing surviving fledglings. We found a significant difference between PC scores for forest denseness around nest boxes that produced surviving versus predated fledglings; fledglings that survived tended to come from nest boxes with high forest and canopy cover, close trees and little grass.

Many bodies of water around the world are contaminated with mercury from historic industrial and mining activities or ongoing atmospheric deposition, resulting in numerous fish consumption advisories. However, concerns about mercury have only rarely led to consumption advisories on waterfowl. In contrast with fish, waterfowl frequently disperse long distances to new watersheds, so hunters and wildlife managers do not know whether waterfowl at a pristine site have spent time at a contaminated site elsewhere. We sampled tissue mercury concentrations of mallards (Anas platyrhynchos), wood ducks (Aix sponsa), and Canada geese (Branta canadensis) at a site contaminated with mercury, during the breeding and hunting seasons. We found that many mallards had bioaccumulated mercury to levels that had the potential to produce reproductive effects and exceeded consumption advisories set for fish by regulatory agencies, whereas this was true for only a few wood ducks and Canada geese. We also documented that mercury-exposed waterfowl from this contaminated site were harvested by hunters as far as 1,054 km away. Our results suggest the need for more proactive sampling of waterfowl for mercury, and likely other bioaccumulating contaminants, in order to allow hunters to make more informed choices about consumption of their harvest. © 2012 The Wildlife Society.

Mercury is a potent neurotoxin affecting birds and other wildlife worldwide. Bald eagles ( Haliaeetus leucocephalus ) are vulnerable to mercury bioaccumulation because they are high in the food web and associated with aquatic ecosystems prone to mercury methylation. Eagle populations, long endangered in the continental United States by contaminants and persecution, are recovering throughout their range. We used single adult eagle feathers collected near 83 occupied nests to show that mercury levels in the Chesapeake Bay eagle population are the lowest in North America (3.82 ± 5.15 μg/g dry weight). We then used 20 feathers from each of 20 salvaged eagles to calculate a confidence interval around the estimate based on single feathers from nesting eagles. Using an inexpensive and non-invasive method to assess mercury burdens we have demonstrated that few Chesapeake Bay bald eagles were above levels suspected of causing reproductive or survival effects in birds.

Mercury is a ubiquitous environmental pollutant that can negatively impact physiology and behavior of vertebrates, causing sub-lethal changes in condition and reducing fitness. Here we examine its effect on offspring sex ratio. Previous studies demonstrate the ability of environmental contaminants to skew sex ratios in wild populations toward the production of females, and research in humans has demonstrated a decrease in male births following mercury exposure. We therefore hypothesized that female birds inhabiting the floodplain of a mercury-contaminated river would produce broods more biased towards the production of females relative to birds from uncontaminated areas. We examined complete broods of three species: the aquatic-feeding belted kingfisher Megaceryle alcyon, the terrestrial-feeding eastern bluebird Sialia sialis, and the tree swallow Tachycineta bicolor, which feeds from both aquatic and terrestrial sources. Nestling sex ratios were shifted toward the production of females in all three species on mercury-contaminated sites when compared to uncontaminated reference sites. These results may be explained by endocrine disruption or the Trivers–Willard theory of sex allocation. Our study is the first to examine the impact of mercury on offspring sex ratios in birds, and therefore contributes to our understanding of the potential for this persistent biomagnifying contaminant to alter fitness and effective population size in wildlife.

Bioaccumulation of contaminants can occur across ecosystem boundaries via transport by emergent aquatic insects. In the South River, Virginia, USA, aquatic mercury has contaminated songbirds nesting in adjacent riparian forests. Spiders contribute the majority of mercury to these songbirds' diets. We tested the hypothesis that massive annual mayfly emergences provide a vector for mercury from river sediments to the Lycosid spiders most frequently eaten by contaminated songbirds. We designed mayfly-specific PCR primers that amplified mtDNA from 76% of adult mayflies collected at this site. By combining this approach with an Agilent 2100 electrophoresis system, we created a highly sensitive test for mayfly predation by Lycosids, commonly known as wolf spiders. In laboratory spider feeding trials, mayfly DNA could be detected up to 192h post-ingestion; however, we detected no mayfly predation in a sample of 110 wolf spiders collected at the site during mayfly emergence. We suggest that mayfly predation is not an important mechanism for dietary transfer of mercury to wolf spiders and their avian predators at the South River. Instead, floodplain soil should be considered as a potential proximate source for mercury in the terrestrial food web.

Mercury (Hg) is a persistent environmental contaminant found in many freshwater and marine ecosystems. Historical Hg contamination in rivers can impact the surrounding terrestrial ecosystem, but there is little known about how far downstream this contamination persists. In 2009, we sampled terrestrial forest songbirds at five floodplain sites up to 137 km downstream of an historical source of Hg along the South and South Fork Shenandoah Rivers (Virginia, USA). We found that blood total Hg concentrations remained elevated over the entire sampling area and there was little evidence of decline with distance. While it is well known that Hg is a pervasive and long-lasting aquatic contaminant, it has only been recently recognized that it also biomagnifies effectively in floodplain forest food webs. This study extends the area of concern for terrestrial habitats near contaminated rivers for more than 100 km downstream from a waterborne Hg point source.

Despite mounting evidence of mercury accumulation in terrestrial ecosystems, few data exist on how environmental mercury exposure affects reproductive success in free-living songbirds. From 2007 through 2010, we monitored reproductive success of Carolina Wrens (Thryothorus ludovicianus) breeding along the forest floodplain of two mercury-contaminated rivers in Virginia. Using an information-theoretic approach, we found a 34% reduction in nesting success of Carolina Wrens on mercury-contaminated sites when compared with reference sites. Blood mercury concentration of the attending female was a strong predictor of nest success. Birds nesting on contaminated sites were 3× more likely to abandon their nests than birds on uncontaminated reference sites. We report a range of effects concentrations associated with various levels of reproductive impairment; for example, a 10% reduction in nest success corresponded with 0.7 µg g-1 mercury in the blood, 2.4 µg g-1 mercury in body feathers, 3.0 µg g-1 merc...

Understanding the role of green space in urban—suburban landscapes is becoming critical for bird conservation because of rampant habitat loss and conversion. Although not natural habitat, golf courses could play a role in bird conservation if they support breeding populations of some native species, yet scientists remain skeptical. In 2003–2009, we measured reproduction of Eastern Bluebirds (Siala sialis) in Virginia on golf courses and surrounding reference habitats, of the type that would have been present had golf courses not been developed on these sites (e.g., recreational parks, cemeteries, agriculture land, and college campus). We monitored >650 nest boxes and 2,255 nest attempts (n = 1,363 golf course, n = 892 reference site). We used an information-theoretic modeling approach to evaluate whether conditions on golf courses affected timing of breeding, investment, or nest productivity compared with nearby reference sites. We found that Eastern Bluebirds breeding on golf courses reproduced...

Golf courses ostensibly offer green space in urbanized areas, but it is unclear how suitable these human-modified habitats are for wildlife populations. Golf courses are home to a variety of wildlife, but in particular they have been the focus of research on avian responses to urbanization. Although numerous reproductive and diversity studies have been conducted on birds of golf courses, no research exists on postfledging survival in this created landscape. In 2008 and 2009, we estimated survival of eastern bluebird (Sialia sialis) fledglings using radio telemetry on golf course and other developed sites in Williamsburg, Virginia. We used nest survival models in Program MARK set in an information theoretic framework to assess whether the golf course habitat predicted mortality along with other previously studied variables, such as fledgling age, year, site, body condition, fledging date, and transmitter weight. We found no evidence that inhabiting a golf course increased mortality during the fledgling period, but we did find support for both fledgling age and fledging date as predictors of survival. Mortality decreased for older fledglings and those that fledged later in the season. Cause-specific postfledging survival rates did not differ among sites. Fledgling bluebirds did, however, move into habitat that was significantly more forested and less grassy than their natal habitat. For managers of wildlife on golf courses and other urbanized sites, our study is the first to show that placing nest boxes in manicured habitat may attract birds to areas without suitable habitat for fledglings. 2011 The Wildlife Society.

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.

Recent evidence suggests that mercury exposure has negative effects on the health of songbirds, and species that forage in wetlands may be at a greater risk of bioaccumulation of mercury than are those of other habitats. We examined mercury concentrations in blood and feathers from the wetland obligate and rapidly declining Rusty Blackbird (Euphagus carolinus) from five regions across North America: three wintering areas in the contiguous United States and breeding areas in the western boreal forests of Alaska and the Acadian forests of northeastern North America. In blood, mercury concentrations in Rusty Blackbirds from the Acadian forest (geometric mean 0.94 µg g-1; n = 59) were >3× than in those from Alaska (0.26 µg g-1; 107). Wintering birds had blood mercury levels approximately an order of magnitude lower than those of breeding birds (0.07 µg g-1; 332). In feathers, mercury concentrations in samples from the Acadian forests exceeded published minimum levels for adverse effects on birds (8....

Mercury has become a ubiquitous contaminant in food chains worldwide. A large body of literature detailing bioaccumulation and effects on birds has revealed the potential for mercury to adversely impact avian physiology and reproduction. However, the extent to which these effects impair survival remains poorly understood. The objective of this study was to determine whether mercury exposure was associated with reduced annual survivorship in tree swallows (Tachycineta bicolor) breeding at a site with legacy industrial contamination. From 2005 to 2008, we captured and marked 932 adult swallows. We used Cormack-Jolly-Seber models and an information-theoretic approach to test our hypotheses that adult survival varied by sex, breeding location, and cumulative individual mercury exposure. Blood mercury was significantly elevated on contaminated sites (2005-2007 combined mean ± SE: 2.84 ± 0.09 μg/g; reference: 0.17 ± 0.01 μg/g). Model-averaged estimates of female apparent survival ranged from 0.483 to 0.488 on reference sites and 0.473 to 0.477 on contaminated sites. For males, apparent survival ranged from 0.451 to 0.457 on reference sites and 0.444 to 0.448 on contaminated sites. Thus, we observed approximately a 1% difference in survival between mercury-contaminated and reference sites. Such a small difference is unlikely to impact population viability in this short-lived species; however, some songbirds accumulate mercury to a greater degree than tree swallows and do not possess the migratory behavior that removes swallows to less contaminated areas for the majority of the year. Identifying whether such species are at risk of suffering biologically significant reductions in survival should become a focus of future research.

A moribund 5-year-old female northern river otter (Lontra canadensis) was found on the bank of a river known to be extensively contaminated with mercury. It exhibited severe ataxia and scleral injection, made no attempt to flee, and died shortly thereafter of drowning. Tissue mercury levels were among the highest ever reported for a free-living terrestrial mammal: kidney, 353 microg/g; liver, 221 microg/g; muscle, 121 microg/g; brain (three replicates from cerebellum), 142, 151, 151 microg/g (all dry weights); and fur, 183 ug/g (fresh weight). Histopathologic findings including severe, diffuse, chronic glomerulosclerosis and moderate interstitial fibrosis were the presumptive cause of clinical signs and death. This is one of a few reports to document the death of a free-living mammal from presumed mercury poisoning.

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.

Some species of birds store food, often hoarding several hundreds of seeds over a period of just a few weeks. Field and laboratory studies have demonstrated that food-storing species have an impressive memory and an enlarged region of the brain, the hippocampal region. Lesion experiments have shown that the hippocampus is important in accurate retrieval of stored food. Taken together, these results have led to the hypothesis that the enlarged hippocampus is associated with the memory requirements of retrieving stored food. In this review, we discuss four areas of study: comparative studies of the brain, comparative studies of behaviour, developmental plasticity and seasonal changes in food storing and the hippocampus.

Double brooding is the initiation of a second clutch of eggs after successfully raising young from the first clutch. Migratory birds that nest in temperate North America are often single-brooded, but there is widespread intra- and interspecific variation. Tree Swallows (Tachycineta bicolor), which are becoming a model species in biology, are typically classified as a single-brooded species. We documented 18 cases of double brooding in a population of Tree Swallows recently established in the Shenandoah Valley, Virginia. Double brooding may be underreported in this species or may be increasing as a result of climate change or range expansion. If double brooding is as common elsewhere as it was in our study population, it could significantly alter estimates of seasonal or lifetime reproductive success in this widely studied bird.

Permits to destroy wetlands often require the creation of the same type of wetland elsewhere. An assumption underlying this practice is that such created wetlands will replace the ecological functions lost when the developed wetland was destroyed. Part of this ecological function is providing habitat for wildlife, including, in coastal areas, a suite of bird species tied to salt marshes for some portion of their life cycle. We tested the hypothesis that created wetlands provide habitat for the avian communities lost when wetlands are destroyed by comparing the breeding and wintering birds on 11 small created salt marshes with those on 11 natural reference salt marshes that were carefully matched for size and surrounding land cover. We found that, during the breeding season, created salt marshes had lower avian abundance and richness than reference salt marshes. In particular, wetland-dependent species were poorly represented on created wetlands. On the other hand, bird use outside of the breeding season and use by an important salt marsh obligate species, the clapper rail (Rallus longirostris), did not differ. Created wetlands that we surveyed failed to completely replicate the bird and plant communities that we observed on nearby natural reference salt marshes, raising the question of whether current mitigation policies that encourage wetland creation should continue without further investigation into the success of such wetlands at recreating wildlife habitat.

An experimental tree swallow population was established in the headwaters of the Shenandoah River, Virginia, USA to assess the accumulation and effects of mercury contamination on birds that eat emergent aquatic insects. One tributary, the South River, was contaminated with mercury before 1950. Reproductive success of swallows nesting within 50 m of this river was compared to that of three uncontaminated reference tributaries in 2005 and 2006. Female swallows on the contaminated stretch of river had significantly elevated blood and feather total mercury (blood: 3.56 +/- 2.41 ppm ww vs. 0.17 +/- 0.15 ppm reference; feather: 13.55 +/- 6.94 ppm vs. 2.34 +/- 0.87 ppm reference), possibly the highest ever reported for an insectivorous songbird. Insects collected by the swallows to be fed to nestlings averaged 0.97 +/- 1.11 ppm dw total mercury, significantly higher than on reference sites. Swallows in the contaminated area produced fewer fledglings than those in reference areas. The effect of mercury contamination on productivity was detectable only for young females in the contaminated area that were breeding for the first time in 2006, a segment of the population that may already have been stressed by inexperience. Tree swallows served as practical and effective biomonitors for mercury levels and effects and have great potential as proxy biomonitors for more logistically challenging birds such as loons or eagles.