Toxins are quenching the white herons that mate in the Everglades



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White herons in the Everglades are losing their sexual motivation because they are exposed to mercury through the fish they eat, a University of Florida study using more than 20 years of data has found.

The researchers observed that mercury contamination led to a 50% reduction in breeding attempts by birds, showing that heavy metal is affecting their breeding process much sooner than previously thought. Because most studies have focused on offspring-related parameters such as pup success, recent findings indicate that the full effects of mercury exposure among wading birds may be “systematically underestimated,” the study says.

“This study suggests that there are many birds that are just out of breeding season. It’s not that they’re starting and ending successfully; they’re not even trying to breed,” said Peter Frederick, one of the co-authors and professor at the university’s department of ecology and wildlife conservation. “We’ve probably underestimated the effects of mercury on reproduction, not only among large egrets but other animals as well.”

Heavy metals and other contaminants can disrupt hormones, which in turn can affect the courtship, propensity to reproduce, and even parental behavior of vertebrates, says the study that was published in the peer-reviewed scientific journal. Environmental sciences and technologies. In the case of the white herons in the Everglades, exposure to mercury occurs through fish, their main diet.

Mercury occurs naturally in the earth. It enters the atmosphere primarily through the burning of fossil fuels and mining. It can travel the world and eventually settles in the ocean and other bodies of water such as the Everglades. Once deposited, some microorganisms can transform mercury into methylmercury, a highly toxic form that accumulates in fish, crustaceans, and fish-eating animals, in a process known as bioaccumulation. The majority of human exposure to mercury results from the consumption of fish and shellfish contaminated with methylmercury.

The study used more than two decades of data on the number of great white egret nesting pairs in the Everglades. The researchers counted pairs nesting in around 130 white heron colonies, taking aerial and land surveys of nesting areas, taking photos and counting pairs in photos taken from 1994 to 2019.

The researchers compared the data from the nesting pair survey with information documenting the amount of mercury found in bird feathers and other factors such as fish density and water level.

“We know that things like food availability can affect breeding and reproduction, so we had to separate the effects of food availability from those of mercury,” said Jabi Zabala, the study’s lead author. Using statistical models, Zabala was able to isolate the effect of mercury on birds.

Although the study was conducted in the Everglades with white herons, this is a case study representing a phenomenon likely to occur in wildlife from the tropics to the poles.

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© 2020 Miami Herald
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