Mercury compromises stress hormones in tree swallow hatchlings.
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Wada, H, DA Cristol, FMA McNabb and WA Hopkins. 2009. Suppressed adrenocortical responses and thyroid hormone levels in birds near a mercury-contaminated river. Environmental Science and Technology 43(15):6031–6038. |
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| Jason Means/flickr |
New research in Virginia reports that levels of hormones key for growth and controlling stress were skewed in older tree swallow nestlings contaminated with mercury. The study finds some of the highest mercury levels ever measured in wild songbird nestlings. The researchers point to the birds' insect food as the source of contamination. The birds had higher levels of corticosterone stress hormones and lower thyroid hormones when compared to a less contaminated population. Younger nestlings did not show the effects.
Mercury can affect the health of fish-eating birds and mammals, but this is one of the first studies to show that birds eating lower on the food chain – insects rather than fish – have compromised hormone systems because of mercury contamination.
Context
Mercury is a familiar pollutant because of ongoing concerns over its widespread presence in the environment and its health effects on fish-eating wildlife and people.
Mercury occurs naturally in air, water, soils and rocks. Human activities – such as burning coal to produce electricity, incinerating garbage, and using the metal in some industries – also add to its concentrations in the environment.
Streams and rivers draining abandoned mines and old industrial sites in North America can have high and persistent levels of mercury. In addition, this pollutant is increasing in remote places because it is carried long distances by wind and water.
The inorganic form of mercury released in natural weathering or by human activities is converted into an organic form – methyl mercury – by bacteria that live in lakes or rivers. Concentrations of methyl mercury increase with each step up the aquatic food chain to the top predators. The metal can reach levels that are a million times higher in fish than in the water.
Abnormal behavior and reproductive and developmental effects can be seen in fish, birds and mammals (including humans) that eat fish with high levels of mercury. In mammals, mercury is a long-known and potent neurotoxin. Recently, mercury has been shown to affect behavior and decrease egg production, hatching success and immune system function in fish-eating birds, such as common loons (Burgess et al. 2008).
Little is known about how other wildlife – those that feed on insects and other lower levels of the food chain – are affected by mercury in their diets.
Tree swallows nest throughout much of North America in the summer months. They nest in hollow trees and bird houses that are near water and feed themselves and their young a diet of aquatic and terrestrial insects. The adults forage for food close to their young. When they nest near areas contaminated with metals or organic pollutants, the adult swallows and their offspring are exposed to them through their food and can become contaminated.
Mercuric sulfide – once used as a pigment in paints, rubber and plastics – was released into the South River, a tributary of the South Fork Shenandoah River, from 1929 until 1950 by an acetate fiber manufacturing plant in Waynesboro, Virginia. As a result, fish in the South River have high levels of mercury. There are currently consumption advisories for all species except stocked trout.
Previous studies have found that 18 different species of birds – including owls, kingfishers, chickadees, robins, sparrows and cardinals – living near this contaminated site had 2 to 27 times higher levels of mercury than reference birds. These higher amounts were due to the contaminated fish or insects the birds ate (Cristol et al. 2008). This study showed that aquatic insects move mercury from rivers into terrestrial ecosystems when birds feed upon the emerged adults (Cristol et al 2008).
What did they do?
Researchers captured 4 nestlings from each of 72 nests at sites that either were historically contaminated with mercury or were identified as less contaminated "reference sites."
The nestlings were sampled at three early stages of development: days 3-6 (when their eyes are opening), days 7-12 (when they are growing quickly), and days 13-17 (when weight gain slows and they prepare to fledge).
Mercury and thyroid and corticosterone hormones (growth and stress hormones, respectively) were measured in the blood collected from the birds.
One of the four birds was stressed by putting it in a bag for 30 minutes. Its ability to respond to stress was determined by comparing corticosterone in its blood before and after being put in the bag.
What did they find?
Hormone function in nestlings from the contaminated site was altered in two different and important ways.
Levels of total mercury in the blood of nestlings from the contaminated sites were 20 times higher (about 360 parts per billion (ppb)) than young birds from the reference sites (about 18 ppb). The adult tree swallows at the industrial and clean sites had blood mercury levels of 3,660 and 170 ppb, respectively. These blood mercury levels are among the highest ever observed for songbirds (Cristol et al. 2008).
The older nestlings from the contaminated sites had two times higher levels of corticosterone hormones in their blood before they were stressed by the researchers than those of the same age from the reference sites. This suggests that their hormone system is already under stress from mercury exposure.
The high-mercury nestlings did not respond as well to the stress as the reference birds. Corticosterone levels in their blood only increased 8-fold compared to a 14-fold increase in the reference animals. This indicates that they may be less able to activate their fight-or-flight response and avoid predators.
Thyroid hormone levels in the older nestlings were also affected (25 – 50 percent lower) by mercury exposure. These hormones regulate metabolism, temperature and growth in vertebrates.
What does it mean?
There are three important messages from this study.
The first is that the health of wildlife lower on the food chain – not just the top predators or fish-eaters - can also be affected by mercury. Other recent work has shown that tree swallows breeding for the first time in this area produced fewer fledglings because lower numbers of their eggs hatched (Brasso and Cristol 2008).
It raises the question about how widespread these kinds of effects may be in wildlife living near mercury-contaminated sites. Many of the other adult bird species at this site had blood mercury levels similar to or higher than those of the adult tree swallows (Cristol et al. 2008), suggesting that their offspring may also be exposed to high mercury and at risk for health effects.
The second finding is that it was only the older nestlings that showed effects from the mercury. This suggests that the birds may have ongoing health problems or even reduced survival after they fledge the nest.
Finally, the effects on hormones levels in swallows were seen at lower blood mercury levels than what is known to affect behavior and immune system function in young birds (Spalding et al. 2000) and what is currently considered to be safe for some species of birds (Scheuhammer et al. 2007). The endocrine system in birds may be more sensitive to mercury's effects.
Mercury contamination has traditionally been seen as an aquatic contamination problem, and research has focused mainly on understanding its effects on fish-eating birds and mammals. However, this study adds to growing evidence that mercury contamination is moving from the water to the land ecosystems where it is also affecting insect-eating birds (Franchesini et al. 2009). It is possible that insect-eating birds at other mercury-contaminated areas, like abandoned mine sites, may also have compromised endocrine systems.
ResourcesBrasso RL, and DA Cristol. 2008. Effects of mercury exposure on the reproductive success of tree swallows (Tachycineta bicolor). Ecotoxicology 17:133–141. Burgess NM and MW Meyer. 2008. Methylmercury exposure associated with reduced productivity in common loons. Ecotoxicology 17:83–91. Cristol DA, RL Brasso, AM Condon, RE Fovargue, SL Friedman, KK Hallinger, AP Monroe and AE White. 2008. The movement of aquatic mercury through terrestrial food webs. Science 320:335. Fish Consumption Advisories, Shenandoah River Basin. Virginia Department of Health, Franceschini, MD, OP Lane, DC Evers, JM Reed, B Hoskins and LM Romero. 2009. Mercury. US Environmental Protection Agency. Scheuhammer, AM, MW Meyer, MB Sandheinrich and MW Murray. 2007. Effects of environmental methylmercury on the health of wild birds, mammals, and fish. Ambio 36(1):12–18. Spalding MG, PC Frederick, HC McGill, SN Bouton and LR McDowell LR. 2000. Methylmercury accumulation in tissues and its effects on growth and appetite in captive great egrets. Journal of Wildlife Diseases 36(3):411-422. Tree swallows. Cornell Lab of Ornithology. |
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