From North Texas to Florida, a high-pitched voice is strangely missing from the chatter of wintering birds.

Peter Assmann

Dedicated birder Peter Assmann says rusty blackbirds are more likely to be seen around creeks or flooded areas and where large numbers of red-winged blackbirds are found.

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The rusty blackbird, a winter visitor to Dallas-Fort Worth, has suffered one of North America's steepest and least understood declines. Since 1970, scientists say, its numbers have plunged 85 percent to 99 percent.

Experts have a lineup of suspects, including habitat changes, disease, climate change and mercury pollution. But they have no proof of what has pushed Euphagus carolinus toward an ecological brink here and across the continent.

"It's like Murder on the Orient Express," said Russell Greenberg, head of the Smithsonian Migratory Bird Center at the National Zoo in Washington, D.C. "There are seven people who might have done it, and maybe they all did it."

The search for answers is under way across the bird's winter range, including the low, wet, woody places of North Texas where, from mid-December to early March, a few rusties show up.

Amateur scientists, working in sun and ice, are seeking them, gathering clues to solve a mystery.

"That's a bird I'm always excited about finding," said East Dallas resident Chris Runk.

Since 2000, he's logged his bird sightings at White Rock Lake, especially at the former fish hatchery on the lake's southwestern shore.

Other blackbirds, starlings and grackles gather by the thousands each winter. Rusties are scarce enough now that just a few make a crowd.

"In the hatchery, which is an absolutely ideal environment for them, with everything they need, we've had as many as almost a dozen," Runk said. "Let's say an average of about six a winter."

Many common North American migratory birds have lost 60 percent or more of their numbers since 1967, according to the National Audubon Society.

They include North Texas species such as the northern bobwhite, northern pintail and eastern meadowlark, each down more than 70 percent.

Even so, the rusty's plight stands out.

"This loss is way out of proportion to other blackbirds," said the Smithsonian's Greenberg.

Some species adapt easily to farms, suburbs and malls, but others have a tough time if their particular habitats become scarcer.

The rusty isn't the pickiest, but it does prefer a watery home. It breeds in summer in wetlands in Canada and Alaska.

Rusties seek similar places when they head south for the winter, favoring wetlands from the Atlantic to North Texas. Their biggest winter concentration appears to be in Mississippi.

"They don't require any pristine forest or anything like that," said John C. Arvin, research coordinator of the Gulf Coast Bird Observatory in Lake Jackson, Texas.

"They do quite well in sort of a patchwork of woods and farms and towns. There's no obvious reason why they've gone into this precipitous decline."

Very few rusty blackbirds show up west of Fort Worth, according to eBird.org, an online bird observation site. The land is simply too dry.

Dedicated birder Peter Assmann has found rusties in damp spots in North Texas.

"You're more likely to see them in areas where there's a creek or a flooded field, flooded woodlands, where there's also large numbers of red-winged blackbirds," Assmann said.

"And what I've found is that horse farms in this area seem to attract them – maybe a lot of fallen grain, I guess."

A dry year means fewer sightings. Even considering weather, Assmann said, "it does seem that they've been hard to find in the last few years."

The National Audubon Society's Christmas Bird Count, conducted annually for more than a century, documents how hard it is to spot rusties in North Texas, where they were never too abundant.

In only three counts since 1963 has Dallas County seen more than a few dozen. In 26 winters, searchers found none.

Experts calculate not only the total seen but also the number seen per search party per hour, a measurement of how easy it is to find them. That allows the tracking of trends even as numbers of searchers fluctuate.

In Texas, the trend is grim: numbers off a cliff since 1960. "The bird's population has fallen through the floor in a real abrupt manner," Arvin said. "No one knows the reason why."

Habitat loss is an unproven possibility. The Boreal Forest, the great northern woodland that rings the globe across northern Canada, Alaska and Russia, has seen logging and development, but vast breeding areas remain.

Wetland loss in the winter range has been dramatic, but scientists can't lay all the blame there.

Other explanations include climate-related habitat changes. Northern latitudes, where the bird breeds, are perhaps most vulnerable; still, there's no proof that an altered climate is responsible.

Mercury from coal-burning power plants is also possible. Researchers have found high levels in red-winged blackbirds, which often share wetlands with rusties.

Mercury is a serious risk to wetland species and is also showing up in some upland ones. For people, the main threat is eating contaminated fish.

"There's very little real evidence of what the cause is," Greenberg said.

For birders in North Texas, the rapid loss of rusty blackbirds, while suggesting a conservation crisis, also has made a sighting something of a prize.

"We were very pleased to see it," said Assmann, "for that's becoming quite a scarce bird."