Mercury and drought along the lower Carson River, Nevada: IV. Snowy egret post-fledging dispersal, timing of migration and survival, 2002-2004.

This telemetry study is an extension of our 1997-2006 research on historical mercury contamination on snowy egrets (Egretta thula) up to ~ 20 days of age. Findings from initial studies at the mercury-contaminated Carson River colony at Lahontan Reservoir (LR) and a similar-sized reference (REF) colony on the Humboldt River included mercury-related physiological, biochemical, histopathological and reproductive effects up to ~20 days of age; with poor water years (2000-04), i.e., reduced prey availability, exacerbating effects. Herein, we compare timing of dispersal and migration at LR vs. REF, but the primary question now addressed is "whether survival of young mercury-exposed snowy egrets from LR would be further compromised beyond ~20 days of age? " Based upon telemetry signals until 90-110 days of age (including dead bird counts and survival rate estimates), we conclude that mercury did not further compromise survival. Dead bird counts and survival rate estimates included time in the colony when fed by adults, plus the critical period when young dispersed from the colony to forage independently. The extended drought during this 3-year study was most critical in 2002 when production of ~20d old egrets at LR was only 0.24 young/nest. In 2002, survival rates were low at both colonies and we documented the highest counts of dead egrets for both colonies. We suggest the losses in 2002 beyond 20 days of age were more a function of prey availability influenced by drought than exposure to mercury, especially at LR, because higher mercury concentrations, higher survival rates and fewer dead birds were documented at LR in 2003 when water conditions improved. Furthermore, total mercury (THg) in blood in 2003 was more than double 2002 (geometric mean, 3.39 vs 1.47µg/g wet weight (ww). This higher THg exposure at LR in 2003 was associated with a redistribution of parent and post-dispersal feeding activities upstream (where there was higher mercury from historic mining) related to slightly improved water levels. When comparing the 3-year telemetry findings based upon ~20d old young at LR (blood THg, geo. means 1.47, 3.39 and 1.89µg/g ww), we found no evidence that age at dispersal, Julian date at dispersal, timing of migration, or pre-migration survival (~20 to ~100 days post-hatch) were adversely affected by elevated mercury.

Mercury and drought along the lower Carson river, Nevada: III. Effects on blood and organ biochemistry and histopathology of snowy egrets and Black-crowned night-herons on Lahontan reservoir, 2002-2006.

A 10-year study (1997-2006) was conducted to evaluate reproduction and health of aquatic birds in the Carson River Basin of northwestern Nevada (on the U.S. Environmental Protection Agency Natural Priorities List) due to high mercury (Hg) concentrations from past mining activities. This part of the study evaluated physiological associations with blood Hg in young snowy egrets (Egretta thula) and black-crowned night-herons (Nycticorax nycticorax), and organ biochemistry and histopathological effects in snowy egrets on Lahontan Reservoir (LR) from the period 2002-2006. LR snowy egret geometric mean total Hg concentrations (microg/g ww) ranged from 1.5 to 4.8 for blood, 2.4 to 3.1 liver, 1.8 to 2.5 kidneys, 1.7 to 2.4 brain, and 20.5 to 36.4 feathers over these years. For night-herons, mean Hg for blood ranged from 1.6 to 7.4. Significant positive correlations were found between total Hg in blood and five plasma enzyme activities of snowy egrets suggesting hepatic stress. Histopathological findings revealed vacuolar changes in hepatocytes in LR snowy egrets as well as correlation of increased liver inflammation with increasing blood and tissue Hg. Hepatic oxidative effects were manifested by decreased hepatic total thiol concentration and glutathione reductase activity and elevated hepatic thiobarbituric acid-reactive subatances (TBARS), a measure of lipid peroxidation. However, other hepatic changes indicated compensatory mechanisms in response to oxidative stress, including decreased oxidized glutathione (GSSG) concentration and decreased ratio of GSSG to reduced glutathione. In young black-crowned night-herons, fewer correlations were apparent. In both species, positive correlations between blood total Hg and plasma uric acid and inorganic phosphorus were suggestive of renal stress, which was supported by histopathological findings. Both oxidative effects and adaptive responses to oxidative stress were apparent in kidneys and brain. Vacuolar change and inflammation in peripheral nerves were found to correlate with blood and tissue Hg. Hg-associated effects related to the immune system included alterations in specific white blood cells and lymphoid depletion in the bursa that were correlated with blood and tissue Hg. When the number of plasma variables that differed between young snowy egrets from the LR site and the reference site were compared between wet and drought years, over twice as many variables were affected during drought years. This resulted in many more variables correlating with blood total Hg during dry than during wet years, suggesting the combination of drought and Hg was more stressful than Hg alone. Drought may have exacerbated Hg-related effects as reported previously for overall productivity. This relationship was not evident in black-crowned night-herons, although data were more limited.

Acute oral toxicities of wildland fire control chemicals to birds.

Wildland fire control chemicals are released into the environment by aerial and ground applications to manage rangeland, grassland, and forest fires. Acute oral 24h median lethal dosages (LD50) for three fire retardants (Fire-Trol GTS-R, Phos-Chek D-75F, and Fire-Trol LCG-R) and two Class A fire suppressant foams (Silv-Ex and Phos-Chek WD881) were estimated for northern bobwhites, Colinus virginianus, American kestrels, Falco sparverius, and red-winged blackbirds, Agelaius phoeniceus. The LD50s of all chemicals for the bobwhites and red-winged blackbirds and for kestrels dosed with Phos-Chek WD881 and Silv-Ex were above the predetermined 2000mg chemical/kg body mass regulatory limit criteria for acute oral toxicity. The LD50s were not quantifiable for kestrels dosed with Fire-Trol GTS-R, Phos-Chek D-75F, and Fire-Trol LCG-R because of the number of birds which regurgitated the dosage. These chemicals appear to be of comparatively low order of acute oral toxicity to the avian species tested.

Mercury and drought along the lower Carson River, Nevada: II. Snowy egret and black-crowned night-heron reproduction on Lahontan Reservoir, 1997--2006.

Mercury concentrations in the floodplain of the Carson River Basin in northwestern Nevada are some of the highest ever reported in a natural system. Thus, a portion of the basin including Lahontan Reservoir was placed on the U.S. Environmental Protection Agency's Natural Priorities List for research and cleanup. Preliminary studies indicated that reproduction in piscivorous birds may be at risk. Therefore, a 10-year study (1997--2006) was conducted to evaluate reproduction of snowy egrets (Egretta thula) and black-crowned night-herons (Nycticorax nycticorax) nesting on Gull Island in Lahontan Reservoir. Special attention was given to the annual flow of the Carson River, the resultant fluctuation of this irrigation reservoir, and the annual exposure of snowy egrets and night-herons to methylmercury (MeHg). The dynamic character of the river due to flooding and drought (drought effect) influenced snowy egret and night-heron reproduction more so than did MeHg contamination of eggs. During an extended drought (2000--2004) in the middle of the study, snowy egret nests containing eggs with concentrations of MeHg (measured as total mercury [THg] approximately 100% MeHg) > or =0.80 microg THg/g, ww, all failed, but in 1997 and 2006 (wet years with general flooding), substantial numbers of young were produced (but fewer than at nests where eggs contained <0.80 microg/g). Thus, a variable reproductive threshold of tolerance to MeHg may be associated with habitat quality (food type and abundance). Clearly, drought was the most important factor affecting snowy egret annual productivity. In contrast to snowy egrets, night-herons generally had fewer nests meeting the 0.80 microg THg/g criterion, and those above the criterion were less sensitive to mercury than were snowy egrets. Furthermore, night-herons appeared more tolerant of drought conditions than snowy egrets because they nested earlier, selected more protected nesting sites, and had a more generalist diet that provided additional food options including terrestrial organisms, which also reduced exposure to MeHg. A putative biological effect threshold of 2.0 microg THg/g in whole blood for young of both species was evaluated, which was frequently exceeded, but with no evidence, while still in the colony, of an association with direct mortality. An evaluation of physiological associations with blood residues and post-fledging survival will be presented in future reports in this series.

Mercury exposure and effects on cavity-nesting birds from the Carson River, Nevada.

Mercury (Hg) concentrations were 15-40 times higher in the eggs and livers of tree swallows (Tachycineta bicolor) and house wrens (Troglodytes aedon) that nested along the Carson River at and below Dayton, Nevada than in the same species above the mining-impacted areas. Hg contamination was mainly the result of processing mills in the 1800s that used Hg to separate gold and silver from ore. The exposure pattern of tree swallows and house wrens along the Carson River was consistent with their trophic status (i.e., lower levels in liver tissue of aquatic insectivores than in piscivorous birds nesting nearby). Even though they are aquatic insectivores, tree swallows and house wrens were exposed to the same amount of Hg as piscivores in the Florida Everglades; this indicated the extreme level of Hg contamination in the Carson River. Only 70-74% of the eggs hatched. This was less than the nationwide average for these two species that generally hatch > or =85% of eggs. Although the sample size was small, Hg might be impacting reproductive end points in cavity-nesting birds from the Carson River. Other trace elements were present at background concentrations.

Nineteenth century mercury: hazard to wading birds and cormorants of the Carson River, Nevada.

Contemporary mercury interest relates to atmospheric deposition, contaminated fish stocks and exposed fish-eating wildlife. The focus is on methylmercury (MeHg) even though most contamination is of inorganic (IoHg) origin. However, IoHg is readily methylated in aquatic systems to become more hazardous to vertebrates. In response to a classic episode of historical (1859-1890) IoHg contamination, we studied fish-eating birds nesting along the lower Carson River, Nevada. Adult double-crested cormorants (Phalacrocorax auritus), snowy egrets (Egretta thula) and black-crowned night-herons (Nycticorax nycticorax) contained very high concentrations of total mercury (THg) in their livers (geo. means 134.8 microg/g wet weight (ww), 43.7 and 13.5, respectively) and kidneys (69.4, 11.1 and 6.1, respectively). Apparently tolerance of these concentrations was possible due to a threshold-dependent demethylation coupled with sequestration of resultant IoHg. Demethylation and sequestration processes also appeared to have reduced the amount of MeHg redistributed to eggs. However, the relatively short time spent by adults in the contaminated area before egg laying was also a factor in lower than expected concentrations of mercury in eggs. Most eggs (100% MeHg) had concentrations below 0.80 microg/g ww, the putative threshold concentration where reproductive problems may be expected; there was no conclusive evidence of mercury-related depressed hatchability. After hatching, the young birds were fed diets by their parents averaging 0.36-1.18 microgMeHg/g ww through fledging. During this four to six week period, accumulated mercury concentrations in the organs of the fledglings were much lower than found in adults, but evidence was detected of toxicity to their immune (spleen, thymus, bursa), detoxicating (liver, kidneys) and nervous systems. Several indications of oxidative stress were also noted in the fledglings and were most apparent in young cormorants containing highest concentrations of mercury. This stress was evidenced by increased thiobarbituric acid-reactive substances, low activities of enzymes related to glutathione metabolism and low levels of reduced thiols, plus an increase in the ratio of oxidized to reduced glutathione. At lower concentrations of mercury, as was found in young egrets, we observed elevated activities of protective hepatic enzymes, which could help reduce oxidative stress. Immune deficiencies and neurological impairment of fledglings may affect survivability when confronted with the stresses of learning to forage and the ability to complete their first migration.