A saltmarsh sparrow peeks out from behind saltmarsh cordgrass. Credit: Courtesy of Brian Harris / U.S. Fish and Wildlife Service

Pollution and sea-level rise are putting Maine’s most unique birds in jeopardy.

Tidal marsh songbird offspring are less likely to survive long enough to leave the nest when the mother bird has high levels of mercury in its blood, according to  new research from the University of Maine. Even so, the birds, which have already seen extreme declines over the past two decades, are more sensitive to sea level rise.

The tidal marsh is a unique habitat, situated at the border of land and sea. The birds that live there — particularly the native saltmarsh sparrow and its closely related sister species, the Acadian Nelson’s sparrow — have uniquely adapted to survive there.

“They’re one of relatively few terrestrial or land species that live in tidal marsh,” Kate Ruskin, a UMaine lecturer in ecology and environmental sciences who led the study.

The birds are also in danger. The saltmarsh sparrow, a native Maine species, is slated to be considered for Endangered Species protection in 2024 and predicted to be extinct by 2060. Previous studies from the University of Maine showed that saltmarsh sparrow populations are declining 9 percent annually across the northeastern U.S., while Acadian Nelson’s sparrows are declining 4.2 percent annually.

“Those numbers are pretty huge,” Ruskin said. “I remember thinking it wasn’t quite that bad when I heard it, but from year one to year two, that’s from 100,000 to 91,000 [birds], and by year 10, that’s down to 39,000 birds.”

Scientists thought that this may be because tidal marshes are also breeding grounds for mercury, which can impede bird reproduction. Mercury floats through the air over to Maine from coal fired power plants from even as far away as the Midwest, but it takes the special bacteria like the kinds found in tidal marsh to convert mercury into a form organisms can consume.

Ruskin and her fellow researchers set out to find out if mercury was to blame for the decline in these special sparrows. Over the course of three breeding seasons, the researchers sampled more than 100 birds to determine whether mercury was accumulating in their blood and, if it was, whether that correlated to their offspring not making it out of the nest.

The study yielded a number of interesting results. First, though the birds from Maine had the lowest total mercury concentrations in their blood, Ruskin said the variability between plots that were only a few miles away from each other was vast.

“It’s variable on a small spatial scale and that’s a good takeaway for conservation,” Ruskin said. “That suggests small conservation action can make a difference.”

The levels of mercury in the birds’ blood also changed so much over the course of the years that the researchers determined that the toxic substance wasn’t bioaccumulating, or staying in the birds’ systems to build up over time.

However, when the birds did have high levels of mercury in their blood, the researchers saw that the baby birds were less likely to live long enough to leave the nest.

A more pressing concern, though, was flooding caused by sea level rise in these sensitive intertidal areas. Nests were at least twice as sensitive – and at most, 17 times more sensitive – to flooding than they were to mercury exposure. The researchers even found that study plots where the mercury was highest had relatively high nest survival – and happened to have some of the lowest nest flooding levels.

Doug Hitchcox, staff naturalist at Maine Audubon, said that knowing which factors are contributing to the species decline – and how important they are compared with one another – is essential to saving them.

“While we know mercury levels in many of Maine’s birds, including our iconic Common Loon, can negatively affect nesting success, this study shows that these sparrows nesting in tidal marshes are at a greater threat to increasing sea level rise from climate change, a threat that we need to act as soon as possible to slow,” Hitchcox said.

Losing the saltmarsh sparrow could send a ripple throughout the ecosystems of Maine in a way that will affect more than just birdwatchers. Hitchcox pointed to the fact that saltmarsh sparrows feed on pesky, biting greenhead flies during the summer.

“Without those sparrows, some of our favorite summer outdoor activities may become much less enjoyable,” Hitchcox said.

Ruskin hopes that her research will be useful — and hopeful — to conservationists looking to preserve the unique ecosystems of Maine.

“We found some negative effects from mercury but addressing that is not going to prevent them from going extinct this century,” Ruskin said. “Making sure they have a place to live where the nests aren’t getting flooded, that’s the key to survival in the next century.”