Daniel A. Cristol

The Movement of Aquatic Mercury Through Terrestrial Food Webs

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.

Differential Migration Revisited

The annual migrations of birds are impressive phenomena that raise interesting physiological, evolutionary, and ecological questions. One facet of migration that has long intrigued biologists is the occurrence of differential migration, in which distance traveled differs between portions of a population. For example, in the eastern United States female Dark-eyed Juncos (Junco hyemalis) migrate farther into the winter range than males (Ketterson and Nolan, 1976), and among Snow Geese (Anser caerulescens) blue-morph and white-morph individuals separate longitudinally during migration (Cooke et al., 1975). (All common and scientific names follow Sibley and Monroe, 1990, and can be found in Tables I–III unless given in the text).

Interactions between chemical and climate stressors: A role for mechanistic toxicology in assessing climate change risks

Incorporation of global climate change (GCC) effects into assessments of chemical risk and injury requires integrated examinations of chemical and nonchemical stressors. Environmental variables altered by GCC (temperature, precipitation, salinity, pH) can influence the toxicokinetics of chemical absorption, distribution, metabolism, and excretion as well as toxicodynamic interactions between chemicals and target molecules. In addition, GCC challenges processes critical for coping with the external environment (water balance, thermoregulation, nutrition, and the immune, endocrine, and neurological systems), leaving organisms sensitive to even slight perturbations by chemicals when pushed to the limits of their physiological tolerance range. In simplest terms, GCC can make organisms more sensitive to chemical stressors, while alternatively, exposure to chemicals can make organisms more sensitive to GCC stressors. One challenge is to identify potential interactions between nonchemical and chemical stressors affecting key physiological processes in an organism. We employed adverse outcome pathways, constructs depicting linkages between mechanism-based molecular initiating events and impacts on individuals or populations, to assess how chemical- and climate-specific variables interact to lead to adverse outcomes. Case examples are presented for prospective scenarios, hypothesizing potential chemical–GCC interactions, and retrospective scenarios, proposing mechanisms for demonstrated chemical–climate interactions in natural populations. Understanding GCC interactions along adverse outcome pathways facilitates extrapolation between species or other levels of organization, development of hypotheses and focal areas for further research, and improved inputs for risk and resource injury assessments. Environ. Toxicol. Chem. 2013;32:32–48.

Mercury Exposure Affects the Reproductive Success of a Free-Living Terrestrial Songbird, the Carolina Wren (Thryothorus ludovicianus)

ABSTRACT. 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 mercury in tail feathers, and 0.11 µg g-1 mercury in eggs. This is the first field study to document the effect of specific adult songbird blood mercury concentrations on breeding performance; our results show that free-living songbirds can suffer negative reproductive effects at relatively low mercury concentrations.

Compromised immune competence in free-living tree swallows exposed to mercury

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.

Effect of prior residence on dominance status of dark-eyed juncos, Junco hyemalis

Abstract Both prior residence and age have been reported to influence rank in avian dominance hierarchies. The aim of this study was to determine the relative effects of these two factors on dominance status of dark-eyed juncos. Twenty-one mixed-age flocks of recently captured juncos (192 individuals) were observed in two similar aviary experiments, one conducted during autumn migration and the other after autumn migration had ended. In the experimental treatment, a group of adult juncos was held in a cage for 1 week and then introduced to an observation cage in which a group of young juncos had already been housed for 1 week, thereby giving the young birds a prior-residence advantage. In the control treatment, groups of each age were held in separate cages for 1 week and were then introduced simultaneously to a neutral observation cage. Dominance ranks of all individuals were determined through observation of aggressive interactions. When given prior residence, young birds clearly dominated adults. Because adults tended to dominante young in control flocks, these results demonstrate that prior residence can reverse agerelated dominance among flocks of juncos under semi-natural conditions. If these results apply to wild populations, they indicate that young juncos might gain in dominance status during winter if they timed their autumn migration so as to arrive on the wintering grounds before adults.

Feather growth influences blood mercury level of young songbirds

Dynamics of mercury in feathers and blood of free-living songbirds is poorly understood. Nestling eastern bluebirds (Sialia sialis) living along the mercury-contaminated South River (Virginia, USA) had blood mercury levels an order of magnitude lower than their parents (nestling: 0.09 +/- 0.06 mg/kg [mean +/- standard deviation], n = 156; adult: 1.21 +/- 0.57 mg/kg, n = 86). To test whether this low blood mercury was the result of mercury sequestration in rapidly growing feathers, we repeatedly sampled free-living juveniles throughout the period of feather growth and molt. Mean blood mercury concentrations increased to 0.52 +/- 0.36 mg/kg (n = 44) after the completion of feather growth. Some individuals had reached adult blood mercury levels within three months of leaving the nest, but levels dropped to 0.20 +/- 0.09 mg/kg (n = 11) once the autumn molt had begun. Most studies of mercury contamination in juvenile birds have focused on recently hatched young with thousands of rapidly growing feathers. However, the highest risk period for mercury intoxication in young birds may be during the vulnerable period after fledging, when feathers no longer serve as a buffer against dietary mercury. We found that nestling blood mercury levels were not indicative of the extent of contamination because a large portion of the ingested mercury ended up in feathers. The present study demonstrates unequivocally that in songbirds blood mercury level is influenced strongly by the growth and molt of feathers.

Spring Arrival, Aggression and Testosterone in Female Red-Winged Blackbirds (Agelaius phoeniceus)

Female-female aggression among polygynously breeding Red-winged Blackbirds (Agelaius phoeniceus) has been reported by many researchers (e.g. Nero 1956, Roberts and Searcy 1988), but there is little agreement as to its function. A female Red-winged Blackbird might behave aggressively towards other females to deter or delay the settlement of potential competitors (Hurly and Robertson 1985, but see Searcy 1988), or to defend nest sites or subterritories from other females sharing her mates territory (Hurly and Robertson 1984, but see Searcy 1986). These putative functions of female-female aggression are not mutually exclusive, and aggression could also serve additional functions, such as protection against intraspecific brood parasitism. Our study documents temporal variation in femalefemale aggression in a population of Red-winged Blackbirds to determine whether aggressive behavior was associated more closely with female settlement or with initiation of nesting. If aggressive behavior by established females coincides with the peak of arrivals of unmated competitors, then it may serve to deter prospecting females from attempting to pair with males. However, if aggression occurs primarily during the initiation of clutches it may function in the defense of nest sites or nest-related resources. Also, since exogenous testosterone has been shown to increase aggressive displays in free-living female Red-winged Blackbirds (Searcy 1988), we examined the seasonal profile of plasma testosterone concentration (hereafter referred to as testosterone) and looked for correlations between temporal peaks in testosterone and timing of female aggression and arrival. Although other functions cannot be ruled out in a correlative study such as ours, describing the timing and endocrine basis of female-female aggression is a necessary step towards understanding the role of this behavior in a polygynous mating system. Methods.-The study site was a cattail (Typha sp.) marsh at the north end of Yellowwood Lake, a 54-ha lake in Yellowwood State Forest, Brown Co., Indiana. Red-winged Blackbirds have been studied and color

Mercury Reduces Avian Reproductive Success and Imposes Selection: An Experimental Study with Adult- or Lifetime-Exposure in Zebra Finch

Mercury is a global pollutant that biomagnifies in food webs, placing wildlife at risk of reduced reproductive fitness and survival. Songbirds are the most diverse branch of the avian evolutionary tree; many are suffering persistent and serious population declines and we know that songbirds are frequently exposed to mercury pollution. Our objective was to determine the effects of environmentally relevant doses of mercury on reproductive success of songbirds exposed throughout their lives or only as adults. The two modes of exposure simulated philopatric species versus dispersive species, and are particularly relevant because of the heightened mercury-sensitivity of developing nervous systems. We performed a dosing study with dietary methylmercury in a model songbird species, the zebra finch (Taeniopygia guttata), at doses from 0.3 – 2.4 parts per million. Birds were exposed to mercury either as adults only or throughout their lives. All doses of mercury reduced reproductive success, with the lowest dose reducing the number of independent offspring produced in one year by 16% and the highest dose, representing approximately half the lethal dose for this species, causing a 50% reduction. While mercury did not affect clutch size or survivorship, it had the most consistent effect on the proportion of chicks that fledged from the nest, regardless of mode of exposure. Among birds exposed as adults, mercury caused a steep increase in the latency to re-nest after loss of a clutch. Birds exposed for their entire lifetimes, which were necessarily the offspring of dosed parents, had up to 50% lower reproductive success than adult-exposed birds at low doses of methylmercury, but increased reproductive success at high doses, suggesting selection for mercury tolerance at the highest level of exposure. Our results indicate that mercury levels in prey items at contaminated sites pose a significant threat to populations of songbirds through reduced reproductive success.

Birdsong Differs between Mercury-Polluted and Reference Sites

ABSTRACT. Mercury is a toxic heavy metal that can cause obvious physiological and reproductive problems in animals. Very little is known, however, about its subtle behavioral effects. We examined whether birds that inhabited mercury-contaminated sites exhibited differences in singing behavior compared with birds at uncontaminated reference sites nearby. We recorded the songs of 3 oscines, the Carolina Wren (Thryothorus ludovicianus), House Wren (Troglodytes aedon), and Song Sparrow (Melospiza melodia), and 1 suboscine, the Eastern Phoebe (Sayornis phoebe). Spectrographic analysis revealed that songs of oscines living on contaminated sites contained a lower diversity of note types and were sung at lower tonal frequencies than songs of birds on reference sites. Additionally, both species of wren tended to sing shorter songs. By contrast, the songs of Eastern Phoebes did not differ between contaminated and reference sites, suggesting that mercury may affect singing behavior only in species that learn their songs. Such alterations in song could have important implications for the fitness of songbirds in polluted areas. Our results highlight the importance of considering behaviors in evaluations of contaminant effects.