Shorebirds at the Point Calimere Wildlife Sanctuary. © Govindarajan, M.

Wildlife biologists have found startling levels of toxic heavy metals in the feathers of 15 species of shorebirds in two important migratory bird sites in India1, prompting them to call for an urgent crackdown on polluting industries and human activities near these protected areas.

The zoologists found unusually high quantities of toxic metals zinc, nickel, cobalt, chromium, copper, lead and mercury in the feathers of shorebirds at the Ramsar site Point Calimere Wildlife Sanctuary (PWLS) and the Pichavaram Mangrove Forest (PMF), both in the southern state of Tamil Nadu. The non-biodegradable metals find their way into these sites from neighbouring salt production units, tanneries, battery making units, cement and dye production enterprises and chemical fertiliser factories. Besides, fertilisers and pesticides from nearby farmlands also flow into these bird havens.

Significant accumulation of these metals – known to threaten the breeding of various wild birds and potentially affecting the food chain – could not just jeopardise the local ecology but also adversely impact bird behaviour and migration patterns, according to the zoologists from Anbanathapuram Vahaira Charities College and Annamalai University in Tamil Nadu.

Both these protected natural habitats are stopover sites for migratory shorebirds on the Central Asian Flyway (CAF) and the East Asian Australasian Flyway (EAAF) routes.

Measuring toxins

Using non-invasive methods, the researchers analysed feathers of 15 species of shorebirds – black-winged stilt (Himantopus himantopus), common redshank (Tringa tetanus), common sandpiper (Actitis hypoleucos), curlew sandpiper (Calidris ferruginea), dunlin (Calidris alpina), Eurasian curlew (Numenius arquata), kentish plover (Charadrius alexandrines), lesser sandplover (Charadrius mongolus ), little-ringed plover (Charadrius dubius), little stint (Calidris minuta), marsh sandpiper (Tringa stagnatilis), painted stork (Mycteria leucocephala), spotted redshank (Tringa erythropus), Temminck's stint (Calidris temminckii) and wood sandpiper (Tringa glareola).

Earlier studies have examined heavy metal levels in tissues and organs of species, feathers, eggs, blood, eggshells liver and kidney, and in prey samples of migrant shorebirds. Feathers make for good bioindicators and ethical biomonitoring tools.

The zoologist picked up 12 dead carcasses of shorebird species from Point Calimere and 13 from Pichavaram. They also caught some other birds using mist nets and released them back into the wild after collecting their breast feathers for analysis. Researchers prefer breast feathers for such studies as the birds shed them lesser than the flight feathers. "All the seven metals detected from the feathers of the shorebirds were beyond permissible levels," says Marimuthu Govindarajan from the zoology department of Annamalai Uiversity.

Zinc deposits were the highest in dunlin, little-ringed plover, marsh sandpiper, and common sandpiper. Nickel, on the other hand, showed up highest in little ringed plover and common sandpiper, while cobalt, chromium and copper were found the most in little stint, marsh sandpiper and dunlin.

The black-winged stilt, common redshank, curlew sandpiper, Eurasian curlew, lesser sand-plover, Temminck’s stint, kentish plover, spotted redshank, and wood sandpiper feathers revealed the most mercury deposits. Lead was found the most in kentish plover, painted stork, spotted redshank, wood sandpiper, Eurasian Curlew, and lesser sand-plover.

Metals weigh heavy on birds

Heavy metals in excess could cause several diseases in shorebirds. High concentration of lead deposits in the tissues of bird feathers could destroy their thermoregulation, the growth of nestlings and affect the recognition of their siblings. Chromium is known to impact embryo development and hatching success of eggs in mallard ducks, while nickel affects pigment colours of feathers during moulting.

Zinc in higher concentration can affect reproduction and increase kidney toxicity. Scientists have earlier found that half of cockatiel populations could die if exposed to about 16 mg of zinc orally for a couple of weeks. The metal, in high levels, is also a known lethal poison for ducks and Nicobar pigeon. High volumes of mercury affects the breeding success of birds, and in quantities above 5 parts per million could adversely affect reproduction. Cobalt in excess can negatively affect metabolism in avian populations and a high concentration of copper can damage birds’ kidneys and impair reproduction, Govindarajan says.

He points out that over the years the density, diversity and species richness of many migratory shorebirds have drastically declined at these wintering grounds. “For instance, the endangered shorebird spoonbill sandpiper ( Calidris pygmaea ) was recorded in Point Calimere in 1980, 1985 and 2004 but has not been seen since then due to the degradation of the habitat and other ecological factors.” He says that the halophytic plants at Pichavaram show a higher concentration of metals than other mangrove species.

Questions on the extent of toxic pollutant load in birds’ bodies have bothered scientists for years, says Abdul Jamil Urfi, an associate professor in the department of environmental studies at the University of Delhi. “In many other parts of the world, studies have used droppings, feathers and carcasses to understand this. In India, there was little information on this subject and this study could lead the way.”

Migratory shorebirds use a linked chain of stopover sites during their annual migration and pass through a number of sites, lying in different countries, with varying levels of protection and toxins in these habitats. These toxins – pesticides and heavy metals – enter the food chain and make their way to top level carnivorous predators in lethal amounts.

An exact analysis of how these metals are affecting this population of birds will need deeper investigation, he says.

Conservation questions

The Point Calimere Wildlife Sanctuary in the Great Vedarnyam Swamp is flush with rainwater in the monsoons but dries into a small swamp by peak summer. The swamp area, separated from the Bay of Bengal and Palk Strait by narrow sandbanks, is also fed by sea mouths making it an important freshwater and saltwater foraging habitat for shorebirds. In the swamp, two industrial salt units – Chemical and Plastics Limited and Dharangadhra Chemical Works – spread over 1500 acres produce edible and industrial salts. Besides, several small scales salt extraction units function inside the sanctuary. Run-off agricultural water from adjoining farm lands laden with toxic chemicals pose a major threat to the swamp.

Pichavaram Mangrove Forest (PMF) is spread over 1100 hectares on the east coast of Tamil Nadu with 51 islets, a natural mangrove forest with the Vellar-Coleroon estuarine complex. The forest is lined with 1500 hectares of aquaculture farms, tanneries, manufacturing units making batteries, cement, dye, chemical fertilizers, and small scale industries, all of which push pollutants into the mangrove forest. Besides, a nearby fishing harbour and agricultural farm lands transport significant toxic metal loads into the forests via the Vellar and Kollidam rivers.

Subramaniam Muralidharan, an ecotoxicology expert at the Coimbatore-based Salim Ali Centre for Ornithology and Natural History says due to ignorance, many times farmers end up using poor quality insecticides and fertilisers available in the market. “This could end up harming the ecosystem around farmlands.”

Muralidharan says very little has been studied of the impact of contaminants on wildlife, particularly on birds. “Very few scientists work on these important issues, which have an obvious bearing on conservation,” he says.

Govindarajan says a thorough scientific understanding of the status of these fragile habitats will need a joined up strategy by ecologists, toxicologists and environmental chemists.

“The Ramsar convention and other international treaties that lay out mandatory conservation practices for migratory shorebirds could play a significant role in addressing this issue at a local level,” Urfi says. The state government will need to take urgent steps to regulate the spread of toxins not only within these protected areas but also outside them, he says.


1. Pandiyan, J. et al. Probing of heavy metals in the feathers of shorebirds of Central Asian Flyway wintering grounds. Sci. Rep. 10, 22118 (2020) doi: 10.1038/s41598-020-79029-z