Mercury - the invisible contaminant that could be driving population declines among migratory shorebirds along the East Asian-Australasian Flyway

Over one-third of the sampled shorebirds along the East Asian-Australasian Flyway are facing Hg risk. Tringa genus in South China was at the highest risk. Feather Hg was best explained by feathers' moulting region, while habitat preference, diet, and forage stratum were less important.
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Figure 1. Shorebirds gather in huge flocks during spring migration, here in Yalu River, near the border between China and North Korea (photo credit: Qingquan Bai).

Migratory shorebirds  frequent the coastlines of continents and link countries with their spectacular migrations, in some cases spanning more than 10,000 kilometers1. Recently, many species along the East Asian-Australasian Flyway (EAAF) have experienced steep population declines, threatening the essential role they play in these vulnerable coastal habitats2. Unlike climate change3, habitat loss and hunting4, threats that have been shown to contribute to the observed population losses5, the impact of pollution has been largely overlooked.

Often described as the world’s most threatened but least studied migratory flyway, the EAAF stretches from Arctic Siberia and Alaska to southern Australia and New Zealand. It supports nearly 8 million shorebirds moving annually at a continental scale, and yet is at high risk of extensive pollutant contamination, particularly due to rampant industrial activities along the coasts of China, Japan, and the Korean Peninsula at the midpoint of the flyway. Mercury (Hg) is of great global concern among pollutants due to its long-range transmission as well as bioaccumulation and biomagnification documented in food webs around the world, which could lead to major health risks in humans and other wildlife6. Studies have shown that Hg can impair the health of migratory birds, manifested in the form of altered migratory behaviour7, impaired escape takeoff8 and reduced endurance flight performance9, and could thereby be a possible factor contributing to the dramatic population declines observed along the EAAF, which receives the highest atmospheric Hg emissions worldwide. Assessing the relationship between these two phenomena may be critical to halting ongoing shorebird population declines which are spreading throughout the world, providing guidelines for effective conservation strategy.

There's still much we don’t understand about Hg contamination along their migratory paths. For example, how much Hg do shorebirds encounter throughout their various life stages? What ecological risks do they face? And what factors can help predict their mercury exposure levels?

It's challenging to measure Hg exposure and assess the potential risks to shorebirds, which live across widely separated continents on a vast spatial scale. This difficulty arises from the challenge of gathering effective biological samples on these threatened birds. This is particularly true in remote breeding grounds, since it is hard to access and costly. Additionally, we must use non-invasive methods to ensure we don't harm their health or impair conservation efforts. Thus, a quick and non-destructive sampling method is recommended.

During the shedding of old feathers, birds grow new ones that may store a large amount of Hg, absorbed from surrounding body tissue. This makes feathers a good indicator10 of Hg bioaccumulation in birds. Researchers have found that feathers can effectively show the level of Hg at the site where the feathers are growing. Feathers (particularly flight feathers) can be collected easily at any time during bird migration. Thus, with careful planning, scientists have developed a reliable method to test for Hg contamination using feathers as a non-destructive biomonitoring tool.

Figure 2. Migratory shorebirds at their winter ground in the heavily disturbed mudflats of Mai Po, Hong Kong (photo credit: Katherine K-S. Leung).

With a standardised sampling protocol (using the 6th primary covert feathers), we assessed Hg contamination in nearly 1000 individuals across 33 migratory shorebird species on an intercontinental scale. Based on our findings, more than 1/3 of the birds showed Hg levels exceeding the safety benchmarks of toxicity. We found that certain species were at greater risk of mercury contamination. The Common Redshank Tringa totanus, often found in freshwater environments, was at greatest risk, particularly in their wintering grounds in South China.

Figure 3. A migratory Common Redshank Tringa totanus nesting at the Lalu wetland in Lhasa, the capital of the Tibet Autonomous Region in China (photo credit: Chi-Yeung Choi).

Overall, variation in feather Hg could be determined by the region where the bird moulted. Other factors like habitat preference (whether the birds lived near the coast or farther inland), what they ate (the proportion of invertebrates in their diet), and where they foraged (on the surface or below) also played a role. We found notably higher Hg levels in birds from South China (Mai Po and Leizhou), followed by Australia and the Yellow Sea, with low levels from temperate and Arctic breeding areas.

 

This study provides the first assessment of Hg exposure in these highly vulnerable migratory shorebirds along an entire flyway at an intercontinental scale. While Hg emissions are projected to decline due to policies such as the Minamata Convention, the relationship between atmospheric trends and Hg contamination in wildlife remains convoluted. Our study can aid in identifying biological hotspots (South China) and sentinel species (e.g., the Tringa genus) for Hg biomonitoring programs.

 

More research is needed on how Hg builds up and spreads through the diets of shorebirds, and how it affects their behaviour and chances of survival. Besides mercury, shorebirds encounter a variety of other pollutants on their migration routes. These toxic substances mix together in the contaminated areas where they feed, such as mudflats and river ecosystems, posing complex environmental threats.

 

Our research here fills a critical knowledge gap by establishing a baseline of Hg contamination in a poorly studied avian group, serving as a valuable reference for future monitoring endeavors. We must continue biomonitoring efforts and expand our research to include other pollutants to help protect these globally important shorebirds.

References

  1. Colwell, M. A. Shorebird ecology, conservation, and management. (Univ of California Press, 2010).
  2. Yong, D. L. et al. Migratory songbirds in the East Asian-Australasian Flyway: a review from a conservation perspective. Bird Conserv. Int. 25, 1–37 (2015).
  3. Van Gils, J. A. et al. Body shrinkage due to Arctic warming reduces red knot fitness in tropical wintering range. Science. 352, 819–821 (2016).
  4. Melville, D. S., Chen, Y. & Ma, Z. Shorebirds along the Yellow Sea coast of China face an uncertain future—a review of threats. Emu - Austral Ornithol. 116, 100–110 (2016).
  5. Ma, Y., Choi, C. Y., Thomas, A. & Gibson, L. Review of contaminant levels and effects in shorebirds: Knowledge gaps and conservation priorities. Ecotoxicol. Environ. Saf. 242, 113868 (2022).
  6. UN Environment. Global Mercury Assessment. http://www.unep.org/gc/gc22/Document/UNEP-GC22-INF3.pdf (2019).
  7. Bottini, C. L. J., Whiley, R. E., Branfireun, B. A. & MacDougall-Shackleton, S. A. Effects of methylmercury and food stress on migratory activity in song sparrows, Melospiza melodia. Horm. Behav. 146, 105261 (2022).
  8. Carlson, J. R., Cristol, D. & Swaddle, J. P. Dietary mercury exposure causes decreased escape takeoff flight performance and increased molt rate in European starlings (Sturnus vulgaris). Ecotoxicology 23, 1464–1473 (2014).
  9. Ma, Y., Perez, C. R., Branfireun, B. A. & Guglielmo, C. G. Dietary exposure to methylmercury affects flight endurance in a migratory songbird. Environ. Pollut. 234, 894–901 (2018).
  10. Appelquist, H., Asbirk, S. & Drabæk, I. Mercury monitoring: mercury stability in bird feathers. Mar. Pollut. Bull. 15, 22–24 (1984).

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