Brilliant birds and the mystery of the mutating mangroves

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As dusk falls in the Caroni Swamp on the west coast of Trinidad, hundreds of scarlet ibises fly in from their feeding grounds to roost in the mangrove trees that grow in the dark, brackish water. The birds settle in the trees like glowing embers.

For generations, local people hunted these birds for their meat and used their feathers for Carnival costumes. By the 1860s, colonial records warned that “a fierce war” had been waged on the ibis, and that it would soon be rare. Yet it is thanks to a hunter that the bird has a haven today.

In the 1930s, Simon Nanan started guiding fishermen and duck hunters into the maze of waterways in the Caroni Swamp – some 6000 hectares of marshes, mudflats and mangrove forest. Many of Nanan’s clients marvelled at the scarlet ibis, so he began offering tours to see the birds and these proved more popular than his hunting trips. And so, ecotourism in Trinidad was born.

In time, Nanan convinced the colonial authorities to create a sanctuary to protect the scarlet ibis. The bird now shares its umbrella of security with a rich variety of wildlife that includes silky anteaters, crab-eating raccoons, spectacled caimans and nearly 200 bird species. The Caroni Swamp also provides key services to local people. It protects the coastline from storm surges, provides a nursery for fish and shellfish, and supports a thriving tourism industry.

When Trinidad gained its independence in 1962, it made the scarlet ibis its national bird and banned hunting of the species. But old habits die hard and poaching has continued to the present day. Serving and eating the bird has apparently become a status symbol. In response, the government this week increased protection for the species.

Anyone caught harming, trading or possessing the species now faces a fine of 100,000 Trinidad and Tobago dollars (about US$15,000) and two years in prison. The penalties apply to possession of even a single feather. What few people realise is that those scarlet feathers have another story to tell.

In the late 1990s, researchers noticed harmful genetic mutations in mangrove trees in areas where the ibises roosted. They showed that the sediments beneath the trees had abnormally high levels of mercury, as did the feathers of the scarlet ibises — unlike those of some other birds in the swamp.

The researchers noted that, unlike the other birds, the scarlet ibises travelled to wetlands in Venezuela each year, where they fed on crustaceans and other animals. They concluded that mercury used by gold miners far inland had entered the country’s rivers and made its way into the food chain. Feeding on contaminated animals is what led the mercury to build up in the bodies of the scarlet ibises, they surmised.

Back in Trinidad, moulted feathers accumulating around the roost over decades appear to have polluted the sediments and resulted in the genetic mutations in the mangrove trees. This 20-year old scientific detective story takes on new relevance given the massive increase in mining and illegal use of mercury underway in Venezuela.

If Trinidad’s new penalties do not deter poachers and consumers of ibis meat, perhaps the risk of exposure to mercury will.

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Reference: Klekowski, E.J. et al. 1999. An association of mangrove mutation, scarlet ibis, and mercury contamination in Trinidad, West Indies. Environmental Pollution 105: 185-189. https://doi.org/10.1016/S0269-7491(99)00028-7

Photo credits: Bottom — Top left — Charles J Sharp / Wikimedia Commons; Top right — Fernando Flores / Creative Commons; Aaron Maizlish / Creative Commons

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